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+# MVVM
+
+
+Rendering or re-rendering the UI based on state – a collection of arrays, objects, or nested objects – is complex, but critical to application performance. In ArkUI, the Model-View-View Model (MVVM) pattern is leveraged for state management. The state management module functions as the view model to bind data to views. When data is changed, the views are directly updated.
+
+
+- Model: stores data and related logic. It represents data transferred between components or other related business logic. It is responsible for processing raw data.
+
+- View: typically represents the UI rendered by \@Component decorated components in ArkUI.
+
+- ViewModel: holds data of custom components stored in state variables, LocalStorage, and AppStorage.
+  - A custom component renders the UI by executing its **build()** method or \@Builder decorated method. In other words, the view model can render views.
+  - The view changes the view model through an event handler, that is, the change of the view model is driven by events. The view model provides the \@Watch callback method to listen for the change of state data.
+  - Any change of the view model must be synchronized back to the model to ensure the consistency between the view model and model, that is, the consistency of the application data.
+  - The view model structure should always be designed to adapt to the build and update of custom components. It is for this purpose that the model and view model are separated.
+
+
+A number of issues with UI construction and update arise from a poor view model design, which does not well support the rendering of custom components, or does not have a view model as a mediator, resulting in the custom component being forcibly adapted to the model. For example, a data model where an application directly reads data from the SQL database into the memory cannot directly adapt to the rendering of custom components. In this scenario, the view model adaptation must be considered during application development.
+
+
+![en-us_image_0000001653986573](figures/en-us_image_0000001653986573.png)
+
+
+In the preceding example involving the SQL database, the application should be designed as follows:
+
+
+- Model: responsible for efficient database operations.
+
+- View model: responsible for efficient UI update based on the ArkUI state management feature.
+
+- Converters/Adapters: responsible for conversion between the model and view model.
+  - Converters/Adapters can convert the model initially read from the database to create and initialize the view model.
+  - In the application scenario, the UI changes the view model through the event handler, and the converters/adapters need to synchronize the updated data of the view model back to the model.
+
+
+Compared with the Model-View (MV) pattern, which forcibly fitting the UI to the SQL database in this example, the MVVM pattern is more complex. The payback is a better UI performance with simplified UI design and implementation, thanks to its isolation of the view model layer.
+
+
+## View Model Data Sources
+
+
+The view model composes data from top-level multiple sources, such as variables decorated by \@State and \@Provide, LocalStorage, and AppStorage. Other decorators synchronize data with these data sources. The decorator to use depends on the extent to which the state needs to be shared between custom components. The following decorators are sorted in ascending order by sharing scope:
+
+
+- \@State: component-level sharing, implemented through the named parameter mechanism. It is sharing between the parent component and the child component by specifying parameters, for example, **CompA: ({ aProp: this.aProp })**.
+
+- \@Provide: component-level sharing, which is multi-level data sharing implemented by binding with \@Consume through a key. No parameter passing is involved during the sharing.
+
+- LocalStorage: page-level sharing, implemented by sharing LocalStorage instances in the current component tree through \@Entry.
+
+- AppStorage: application-level sharing, which is sharing of global application-wide UI state bound with the application process.
+
+
+### State Data Sharing from \@State Decorated Variables to One or More Child Components
+
+
+A one-way and two-way data synchronization relationship can be set up from an \@State decorated variable to an \@Prop, \@Link, or \@ObjectLink decorated variable. For details, see [\@State Decorator](arkts-state.md).
+
+
+1. Use the \@State decorated variable **testNum** in the **Parent** root node as the view model data item. Pass **testNum** to the child components **LinkChild** and **Sibling**.
+
+   ```ts
+   // xxx.ets
+   @Entry
+   @Component
+   struct Parent {
+     @State @Watch("testNumChange1") testNum: number = 1;
+   
+     testNumChange1(propName: string): void {
+       console.log(`Parent: testNumChange value ${this.testNum}`)
+     }
+   
+     build() {
+       Column() {
+         LinkChild({ testNum: $testNum })
+         Sibling({ testNum: $testNum })
+       }
+     }
+   }
+   ```
+
+2. In **LinkChild** and **Sibling**, use \@Link to set up a two-way data synchronization with the data source of the **Parent** component. In this example, **LinkLinkChild** and **PropLinkChild** are created in **LinkChild**.
+
+   ```ts
+   @Component
+   struct Sibling {
+     @Link @Watch("testNumChange") testNum: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`Sibling: testNumChange value ${this.testNum}`);
+     }
+   
+     build() {
+       Text(`Sibling: ${this.testNum}`)
+     }
+   }
+   
+   @Component
+   struct LinkChild {
+     @Link @Watch("testNumChange") testNum: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`LinkChild: testNumChange value ${this.testNum}`);
+     }
+   
+     build() {
+       Column() {
+         Button('incr testNum')
+           .onClick(() => {
+             console.log(`LinkChild: before value change value ${this.testNum}`);
+             this.testNum = this.testNum + 1
+             console.log(`LinkChild: after value change value ${this.testNum}`);
+           })
+         Text(`LinkChild: ${this.testNum}`)
+         LinkLinkChild({ testNumGrand: $testNum })
+         PropLinkChild({ testNumGrand: this.testNum })
+       }
+       .height(200).width(200)
+     }
+   }
+   ```
+
+3. Declare **LinkLinkChild** and **PropLinkChild** as follows. Use \@Prop in **PropLinkChild** to set up a one-way data synchronization with the data source of the **Parent** component.
+
+   ```ts
+   @Component
+   struct LinkLinkChild {
+     @Link @Watch("testNumChange") testNumGrand: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`LinkLinkChild: testNumGrand value ${this.testNumGrand}`);
+     }
+   
+     build() {
+       Text(`LinkLinkChild: ${this.testNumGrand}`)
+     }
+   }
+   
+   
+   @Component
+   struct PropLinkChild {
+     @Prop @Watch("testNumChange") testNumGrand: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`PropLinkChild: testNumGrand value ${this.testNumGrand}`);
+     }
+   
+     build() {
+       Text(`PropLinkChild: ${this.testNumGrand}`)
+         .height(70)
+         .backgroundColor(Color.Red)
+         .onClick(() => {
+           this.testNumGrand += 1;
+         })
+     }
+   }
+   ```
+
+   ![en-us_image_0000001638250945](figures/en-us_image_0000001638250945.png)
+
+   When \@Link **testNum** in **LinkChild** changes:
+
+   1. The changes are first synchronized to its parent component **Parent**, and then from **Parent** to **Siling**.
+
+   2. Changes to \@Link **testNum** in **LinkChild** are also synchronized to child components **LinkLinkChild** and **PropLinkChild**.
+
+   The differences between the \@State decorator and \@Provide, LocalStorage, and AppStorage are as follows:
+
+   - When \@State is used, if you want to pass changes to a grandchild component, you must first pass the changes to the child component and then from the child component to the grandchild component.
+   - The changes can only be passed by specifying parameters of constructors, that is, through the named parameter mechanism CompA: ({ aProp: this.aProp }).
+
+   A complete code example is as follows:
+
+
+   ```ts
+   @Component
+   struct LinkLinkChild {
+     @Link @Watch("testNumChange") testNumGrand: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`LinkLinkChild: testNumGrand value ${this.testNumGrand}`);
+     }
+   
+     build() {
+       Text(`LinkLinkChild: ${this.testNumGrand}`)
+     }
+   }
+   
+   
+   @Component
+   struct PropLinkChild {
+     @Prop @Watch("testNumChange") testNumGrand: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`PropLinkChild: testNumGrand value ${this.testNumGrand}`);
+     }
+   
+     build() {
+       Text(`PropLinkChild: ${this.testNumGrand}`)
+         .height(70)
+         .backgroundColor(Color.Red)
+         .onClick(() => {
+           this.testNumGrand += 1;
+         })
+     }
+   }
+   
+   
+   @Component
+   struct Sibling {
+     @Link @Watch("testNumChange") testNum: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`Sibling: testNumChange value ${this.testNum}`);
+     }
+   
+     build() {
+       Text(`Sibling: ${this.testNum}`)
+     }
+   }
+   
+   @Component
+   struct LinkChild {
+     @Link @Watch("testNumChange") testNum: number;
+   
+     testNumChange(propName: string): void {
+       console.log(`LinkChild: testNumChange value ${this.testNum}`);
+     }
+   
+     build() {
+       Column() {
+         Button('incr testNum')
+           .onClick(() => {
+             console.log(`LinkChild: before value change value ${this.testNum}`);
+             this.testNum = this.testNum + 1
+             console.log(`LinkChild: after value change value ${this.testNum}`);
+           })
+         Text(`LinkChild: ${this.testNum}`)
+         LinkLinkChild({ testNumGrand: $testNum })
+         PropLinkChild({ testNumGrand: this.testNum })
+       }
+       .height(200).width(200)
+     }
+   }
+   
+   
+   @Entry
+   @Component
+   struct Parent {
+     @State @Watch("testNumChange1") testNum: number = 1;
+   
+     testNumChange1(propName: string): void {
+       console.log(`Parent: testNumChange value ${this.testNum}`)
+     }
+   
+     build() {
+       Column() {
+         LinkChild({ testNum: $testNum })
+         Sibling({ testNum: $testNum })
+       }
+     }
+   }
+   ```
+
+
+### State Data Sharing from \@Provide Decorated Variables to Descendant Components
+
+\@Provide decorated variables can share state data with any descendant component that uses \@Consume to create a two-way synchronization. For details, see [\@Provide and \@Consume Decorators](arkts-provide-and-consume.md).
+
+This \@Provide-\@Consume pattern is more convenient than the \@State-\@Link-\@Link pattern in terms of passing changes from a parent component to a grandchild component. It is suitable for sharing state data in a single page UI component tree.
+
+In the \@Provide-\@Consume pattern, changes are passed by binding \@Consume to \@Provide in the ancestor component through a key, instead of by specifying parameters in the constructor.
+
+The following example uses the \@Provide-\@Consume pattern to pass changes from a parent component to a grandchild component:
+
+
+```ts
+@Component
+struct LinkLinkChild {
+  @Consume @Watch("testNumChange") testNum: number;
+
+  testNumChange(propName: string): void {
+    console.log(`LinkLinkChild: testNum value ${this.testNum}`);
+  }
+
+  build() {
+    Text(`LinkLinkChild: ${this.testNum}`)
+  }
+}
+
+@Component
+struct PropLinkChild {
+  @Prop @Watch("testNumChange") testNumGrand: number;
+
+  testNumChange(propName: string): void {
+    console.log(`PropLinkChild: testNumGrand value ${this.testNumGrand}`);
+  }
+
+  build() {
+    Text(`PropLinkChild: ${this.testNumGrand}`)
+      .height(70)
+      .backgroundColor(Color.Red)
+      .onClick(() => {
+        this.testNumGrand += 1;
+      })
+  }
+}
+
+@Component
+struct Sibling {
+  @Consume @Watch("testNumChange") testNum: number;
+
+  testNumChange(propName: string): void {
+    console.log(`Sibling: testNumChange value ${this.testNum}`);
+  }
+
+  build() {
+    Text(`Sibling: ${this.testNum}`)
+  }
+}
+
+@Component
+struct LinkChild {
+  @Consume @Watch("testNumChange") testNum: number;
+
+  testNumChange(propName: string): void {
+    console.log(`LinkChild: testNumChange value ${this.testNum}`);
+  }
+
+  build() {
+    Column() {
+      Button('incr testNum')
+        .onClick(() => {
+          console.log(`LinkChild: before value change value ${this.testNum}`);
+          this.testNum = this.testNum + 1
+          console.log(`LinkChild: after value change value ${this.testNum}`);
+        })
+      Text(`LinkChild: ${this.testNum}`)
+      LinkLinkChild({ /* empty */ })
+      PropLinkChild({ testNumGrand: this.testNum })
+    }
+    .height(200).width(200)
+  }
+}
+
+@Entry
+@Component
+struct Parent {
+  @Provide @Watch("testNumChange1") testNum: number = 1;
+
+  testNumChange1(propName: string): void {
+    console.log(`Parent: testNumChange value ${this.testNum}`)
+  }
+
+  build() {
+    Column() {
+      LinkChild({ /* empty */ })
+      Sibling({ /* empty */ })
+    }
+  }
+}
+```
+
+
+### One- or Two-Way Synchronization for Properties in LocalStorage Instances
+
+You can use \@LocalStorageLink to set up a one-way synchronization for a property in a LocalStorage instance, or use \@LocalStorageProp to set up a two-way synchronization. A LocalStorage instance can be regarded as a map of the \@State decorated variables. For details, see [LocalStorage](arkts-localstorage.md).
+
+LocalStorage objects can be shared on several pages of an ArkUI application. In this way, state can be shared across pages of an application using \@LocalStorageLink, \@LocalStorageProp, and LocalStorage.
+
+Below is an example.
+
+1. Create a LocalStorage instance and inject it into the root node through \@Entry(storage).
+
+2. When the \@LocalStorageLink("testNum") variable is initialized in the **Parent** component, the **testNum** property is created in the LocalStorage instance and the specified initial value is set to 1, that is, \@LocalStorageLink("testNum") testNum: number = 1.
+
+3. In the child components, bind \@LocalStorageLink or \@LocalStorageProp to the same property name key to pass data.
+
+The LocalStorage instance can be considered as a map of the \@State decorated variable, and the property name is the key in the map.
+
+The synchronization between \@LocalStorageLink and the corresponding property in LocalStorage is two-way, the same as that between \@State and \@Link.
+
+The following figure shows the flow of component state update.
+
+![en-us_image_0000001588450934](figures/en-us_image_0000001588450934.png)
+
+
+```ts
+@Component
+struct LinkLinkChild {
+  @LocalStorageLink("testNum") @Watch("testNumChange") testNum: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`LinkLinkChild: testNum value ${this.testNum}`);
+  }
+
+  build() {
+    Text(`LinkLinkChild: ${this.testNum}`)
+  }
+}
+
+@Component
+struct PropLinkChild {
+  @LocalStorageProp("testNum") @Watch("testNumChange") testNumGrand: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`PropLinkChild: testNumGrand value ${this.testNumGrand}`);
+  }
+
+  build() {
+    Text(`PropLinkChild: ${this.testNumGrand}`)
+      .height(70)
+      .backgroundColor(Color.Red)
+      .onClick(() => {
+        this.testNumGrand += 1;
+      })
+  }
+}
+
+@Component
+struct Sibling {
+  @LocalStorageLink("testNum") @Watch("testNumChange") testNum: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`Sibling: testNumChange value ${this.testNum}`);
+  }
+
+  build() {
+    Text(`Sibling: ${this.testNum}`)
+  }
+}
+
+@Component
+struct LinkChild {
+  @LocalStorageLink("testNum") @Watch("testNumChange") testNum: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`LinkChild: testNumChange value ${this.testNum}`);
+  }
+
+  build() {
+    Column() {
+      Button('incr testNum')
+        .onClick(() => {
+          console.log(`LinkChild: before value change value ${this.testNum}`);
+          this.testNum = this.testNum + 1
+          console.log(`LinkChild: after value change value ${this.testNum}`);
+        })
+      Text(`LinkChild: ${this.testNum}`)
+      LinkLinkChild({ /* empty */ })
+      PropLinkChild({ /* empty */ })
+    }
+    .height(200).width(200)
+  }
+}
+
+// Create a LocalStorage object to hold the data.
+const storage = new LocalStorage();
+@Entry(storage)
+@Component
+struct Parent {
+  @LocalStorageLink("testNum") @Watch("testNumChange1") testNum: number = 1;
+
+  testNumChange1(propName: string): void {
+    console.log(`Parent: testNumChange value ${this.testNum}`)
+  }
+
+  build() {
+    Column() {
+      LinkChild({ /* empty */ })
+      Sibling({ /* empty */ })
+    }
+  }
+}
+```
+
+
+### One- or Two-Way Synchronization for Properties in AppStorage
+
+AppStorage is a singleton object of LocalStorage. ArkUI creates this object when an application is started and uses \@StorageLink and \@StorageProp to share data across pages. The usage of AppStorage is similar to that of LocalStorage.
+
+You can also use PersistentStorage to persist specific properties in AppStorage to files on the local disk. In this way, \@StorageLink and \@StorageProp decorated properties can restore upon application re-start to the values as they were when the application was closed. For details, see [PersistentStorage](arkts-persiststorage.md).
+
+An example is as follows:
+
+
+```ts
+@Component
+struct LinkLinkChild {
+  @StorageLink("testNum") @Watch("testNumChange") testNum: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`LinkLinkChild: testNum value ${this.testNum}`);
+  }
+
+  build() {
+    Text(`LinkLinkChild: ${this.testNum}`)
+  }
+}
+
+@Component
+struct PropLinkChild {
+  @StorageProp("testNum") @Watch("testNumChange") testNumGrand: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`PropLinkChild: testNumGrand value ${this.testNumGrand}`);
+  }
+
+  build() {
+    Text(`PropLinkChild: ${this.testNumGrand}`)
+      .height(70)
+      .backgroundColor(Color.Red)
+      .onClick(() => {
+        this.testNumGrand += 1;
+      })
+  }
+}
+
+@Component
+struct Sibling {
+  @StorageLink("testNum") @Watch("testNumChange") testNum: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`Sibling: testNumChange value ${this.testNum}`);
+  }
+
+  build() {
+    Text(`Sibling: ${this.testNum}`)
+  }
+}
+
+@Component
+struct LinkChild {
+  @StorageLink("testNum") @Watch("testNumChange") testNum: number = 1;
+
+  testNumChange(propName: string): void {
+    console.log(`LinkChild: testNumChange value ${this.testNum}`);
+  }
+
+  build() {
+    Column() {
+      Button('incr testNum')
+        .onClick(() => {
+          console.log(`LinkChild: before value change value ${this.testNum}`);
+          this.testNum = this.testNum + 1
+          console.log(`LinkChild: after value change value ${this.testNum}`);
+        })
+      Text(`LinkChild: ${this.testNum}`)
+      LinkLinkChild({ /* empty */
+      })
+      PropLinkChild({ /* empty */
+      })
+    }
+    .height(200).width(200)
+  }
+}
+
+
+@Entry
+@Component
+struct Parent {
+  @StorageLink("testNum") @Watch("testNumChange1") testNum: number = 1;
+
+  testNumChange1(propName: string): void {
+    console.log(`Parent: testNumChange value ${this.testNum}`)
+  }
+
+  build() {
+    Column() {
+      LinkChild({ /* empty */
+      })
+      Sibling({ /* empty */
+      })
+    }
+  }
+}
+```
+
+
+## View Model Nesting Scenario
+
+
+In most cases, view model data items are of complex types, such as arrays of objects, nested objects, or combinations of these types. In nested scenarios, you can use \@Observed and \@Prop or \@ObjectLink to observe changes.
+
+
+### \@Prop and \@ObjectLink Nested Data Structures
+
+When possible, design a separate \@Component decorator to render each array or object. In this case, an object array or nested object (which is an object whose property is an object) requires two \@Component decorators: one for rendering an external array/object, and the other for rendering a class object nested within the array/object. Variables decorated by \@Prop, \@Link, and \@ObjectLink can only observe changes at the first layer.
+
+- For a class:
+  - The value assignment change can be observed: this.obj=new ClassObj(...)
+  - The change of the object property can be observed: this.obj.a=new ClassA(...)
+  - Property changes at a deeper layer cannot be observed: this.obj.a.b = 47
+
+- For an array:
+  - The overall value assignment of the array can be observed: this.arr=[...]
+  - The deletion, insertion, and replacement of data items can be observed: this.arr[1] = new ClassA(); this.arr.pop(); this.arr.push (new ClassA(...))), this.arr.sort(...)
+  - Array changes at a deeper layer cannot be observed: this.arr[1].b = 47
+
+To observe changes of nested objects inside a class, use \@ObjectLink or \@Prop. \@ObjectLink is preferred, which initializes itself through a reference to an internal property of a nested object. \@Prop initializes itself through a deep copy of the nested object to implement one-way synchronization. The deep copy of \@Prop significantly outperforms the reference copy of \@ObjectLink.
+
+\@ObjectLink or \@Prop can be used to store nested objects of a class. This class must be decorated with the \@Observed decorator. Otherwise, property changes of the class do not trigger UI update or refresh. \@Observed implements a custom constructor for its decorated class. This constructor creates an instance of a class and uses the ES6 proxy wrapper (implemented by the ArkUI framework) to intercept all get and set operations on the decorated class property. "Set" observes the property value. When value assignment occurs, the ArkUI framework is notified of the update. "Get" collects which UI components depend on this state variable to minimize UI re-render.
+
+In the nesting scenario, use the \@Observed decorator as follows:
+
+- If the nested data is a class, directly decorate it with \@Observed.
+
+- If the nested data is an array, you can observe the array change in the following way:
+
+  ```ts
+  @Observed class ObservedArray<T> extends Array<T> {
+      constructor(args: any[]) {
+          super(...args);
+      }
+      /* otherwise empty */
+  }
+  ```
+
+  The view model is the outer class.
+
+
+  ```ts
+  class Outer {
+    innerArrayProp : ObservedArray<string>;
+    ...
+  }
+  ```
+
+
+### Differences Between \@Prop and \@ObjectLink in Nested Data Structures
+
+In the following example: 
+
+- The parent component **ViewB** renders \@State arrA: Array\<ClassA>. \@State can observe the allocation of new arrays, and insertion, deletion, and replacement of array items.
+
+- The child component **ViewA** renders each object of **ClassA**.
+
+- With \@ObjectLink a: ClassA:
+
+  - When \@Observed ClassA is used, the changes of **ClassA** objects nested in the array can be observed.
+
+  - When \@Observed ClassA is not used:
+    This.arrA[Math.floor(this.arrA.length/2)].c=10 in **ViewB** cannot be observed, and therefore the **ViewA** component will not be updated.
+
+    For the first and second array items in the array, both of them initialize two **ViewA** objects and render the same **ViewA** instance. When this.a.c += 1; is assigned to a property in **ViewA**, another **ViewA** initialized from the same **ClassA** is not re-rendered.
+
+![en-us_image_0000001588610894](figures/en-us_image_0000001588610894.png)
+
+
+```ts
+let NextID: number = 1;
+
+// Use the class decorator @Observed to decorate ClassA.
+@Observed
+class ClassA {
+  public id: number;
+  public c: number;
+
+  constructor(c: number) {
+    this.id = NextID++;
+    this.c = c;
+  }
+}
+
+@Component
+struct ViewA {
+  @ObjectLink a: ClassA;
+  label: string = "ViewA1";
+
+  build() {
+    Row() {
+      Button(`ViewA [${this.label}] this.a.c= ${this.a.c} +1`)
+        .onClick(() => {
+          // Change the object property.
+          this.a.c += 1;
+        })
+    }
+  }
+}
+
+@Entry
+@Component
+struct ViewB {
+  @State arrA: ClassA[] = [new ClassA(0), new ClassA(0)];
+
+  build() {
+    Column() {
+      ForEach(this.arrA,
+        (item) => {
+          ViewA({ label: `#${item.id}`, a: item })
+        },
+        (item) => item.id.toString()
+      )
+
+      Divider().height(10)
+
+      if (this.arrA.length) {
+        ViewA({ label: `ViewA this.arrA[first]`, a: this.arrA[0] })
+        ViewA({ label: `ViewA this.arrA[last]`, a: this.arrA[this.arrA.length-1] })
+      }
+
+      Divider().height(10)
+
+      Button(`ViewB: reset array`)
+        .onClick(() => {
+          // Replace the entire array, which will be observed by @State this.arrA.
+          this.arrA = [new ClassA(0), new ClassA(0)];
+        })
+      Button(`array push`)
+        .onClick(() => {
+          // Insert data into the array, which will be observed by @State this.arrA.
+          this.arrA.push(new ClassA(0))
+        })
+      Button(`array shift`)
+        .onClick(() => {
+          // Remove data from the array, which will be observed by @State this.arrA.
+          this.arrA.shift()
+        })
+      Button(`ViewB: chg item property in middle`)
+        .onClick(() => {
+          // Replace an item in the array, which will be observed by @State this.arrA.
+          this.arrA[Math.floor(this.arrA.length / 2)] = new ClassA(11);
+        })
+      Button(`ViewB: chg item property in middle`)
+        .onClick(() => {
+          // Change property c of an item in the array, which will be observed by @ObjectLink in ViewA.
+          this.arrA[Math.floor(this.arrA.length / 2)].c = 10;
+        })
+    }
+  }
+}
+```
+
+In **ViewA**, replace \@ObjectLink with \@Prop.
+
+
+```ts
+@Component
+struct ViewA {
+
+  @Prop a: ClassA;
+  label : string = "ViewA1";
+
+  build() {
+     Row() {
+        Button(`ViewA [${this.label}] this.a.c= ${this.a.c} +1`)
+        .onClick(() => {
+            // change object property
+            this.a.c += 1;
+        })
+     }
+  }
+}
+```
+
+When \@ObjectLink is used, if you click the first or second item of the array, the following two **ViewA** instances change synchronously.
+
+Unlike \@ObjectLink, \@Prop sets up a one-way data synchronization. Clicking the button in **ViewA** triggers only the re-render of the button itself and is not propagated to other **ViewA** instances. **ClassA** in **ViewA** is only a copy, not an object of its parent component \@State arrA : Array&lt;ClassA&gt, nor a **ClassA** instance of any other **ViewA**. As a result, though on the surface, the array and **ViewA** have the same object passed in, two irrelevant objects are used for rendering on the UI.
+
+Note that there is another difference between \@Prop and \@ObjectLink: \@ObjectLink decorated variables are readable only and cannot be assigned values, whereas \@Prop decorated variables can be assigned values.
+
+- \@ObjectLink implements two-way data synchronization because it is initialized through a reference to the data source.
+
+- \@Prop implements one-way data synchronization and requires a deep copy of the data source.
+
+- To assign a new object to \@Prop is to overwrite the local value. However, for \@ObjectLink that implements two-way data synchronization, to assign a new object is to update the array item or class property in the data source, which is not possible in TypeScript/JavaScript.
+
+
+## Example
+
+
+The following example discusses the application design of nested view models, especially how a custom component renders a nested object. This scenario is common in real-world application development.
+
+
+Let's develop a phonebook application to implement the following features:
+
+
+- Display the phone numbers of contacts and the local device ("Me").
+
+- You can select a contact and edit its information, including the phone number and address.
+
+- When you update contact information, the changes are saved only after you click **Save Changes**.
+
+- You can click **Delete Contact** to delete a contact from the contacts list.
+
+
+In this example, the view model needs to include the following:
+
+
+- **AddressBook** (class)
+  - **me** (device): stores a **Person** class.
+  - **contacts**: stores a **Person** class array.
+
+
+The **AddressBook** class is declared as follows:
+
+
+
+```ts
+export class AddressBook {
+  me: Person;
+  contacts: ObservedArray<Person>;
+​
+  constructor(me: Person, contacts: Person[]) {
+    this.me = me;
+    this.contacts = new ObservedArray<Person>(contacts);
+  }
+}
+```
+
+
+- Person (class)
+  - name : string
+  - address : Address
+  - phones: ObservedArray&lt;string&gt;
+  - Address (class)
+    - street : string
+    - zip : number
+    - city : string
+
+
+The **Address** class is declared as follows:
+
+
+
+```ts
+@Observed
+export class Address {
+  street: string;
+  zip: number;
+  city: string;
+
+  constructor(street: string,
+              zip: number,
+              city: string) {
+    this.street = street;
+    this.zip = zip;
+    this.city = city;
+  }
+}
+```
+
+
+The **Person** class is declared as follows:
+
+
+
+```ts
+@Observed
+export class Person {
+  id_: string;
+  name: string;
+  address: Address;
+  phones: ObservedArray<string>;
+
+  constructor(name: string,
+              street: string,
+              zip: number,
+              city: string,
+              phones: string[]) {
+    this.id_ = `${nextId}`;
+    nextId++;
+    this.name = name;
+    this.address = new Address(street, zip, city);
+    this.phones = new ObservedArray<string>(phones);
+  }
+}
+```
+
+
+Note that **phones** is a nested property. To observe its change, you need to extend the array and decorate it with \@Observed. The **ObservedArray** class is declared as follows:
+
+
+
+```ts
+@Observed
+export class ObservedArray<T> extends Array<T> {
+  constructor(args?: any[]) {
+    console.log(`ObservedArray: ${JSON.stringify(args)} `)
+    if (Array.isArray(args)) {
+      super(...args);
+    } else {
+      super(args)
+    }
+  }
+}
+```
+
+
+- **selected**: reference to **Person**.
+
+
+The update process is as follows:
+
+
+1. Initialize all data in the root node **PageEntry**, and establish two-way data synchronization between **me** and **contacts** and its child component **AddressBookView**. The default value of **selectedPerson** is **me**. Note that **selectedPerson** is not data in the **PageEntry** data source, but a reference to a **Person** object in the data source.
+   **PageEntry** and **AddressBookView** are declared as follows:
+
+
+   ```ts
+   @Component
+   struct AddressBookView {
+   
+       @ObjectLink me : Person;
+       @ObjectLink contacts : ObservedArray<Person>;
+       @State selectedPerson: Person = undefined;
+   
+       aboutToAppear() {
+           this.selectedPerson = this.me;
+       }
+   
+       build() {
+           Flex({ direction: FlexDirection.Column, justifyContent: FlexAlign.Start}) {
+               Text("Me:")
+               PersonView({person: this.me, phones: this.me.phones, selectedPerson: this.$selectedPerson})
+   
+               Divider().height(8)
+   
+               ForEach(this.contacts,
+                   contact => {
+                       PersonView({person: contact, phones: contact.phones, selectedPerson: this.$selectedPerson})
+                   },
+                   contact => contact.id_
+               )
+   
+               Divider().height(8)
+   
+               Text("Edit:")
+               PersonEditView({ selectedPerson: this.$selectedPerson, name: this.selectedPerson.name, address: this.selectedPerson.address, phones: this.selectedPerson.phones })
+           }
+               .borderStyle(BorderStyle.Solid).borderWidth(5).borderColor(0xAFEEEE).borderRadius(5)
+       }
+   }
+   
+   @Entry
+   @Component
+   struct PageEntry {
+     @Provide addrBook: AddressBook = new AddressBook(
+       new Person("Gigi", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********", "18*********"]),
+       [
+         new Person("Oly", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********"]),
+         new Person("Sam", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********"]),
+         new Person("Vivi", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********"]),
+       ]);
+   
+     build() {
+       Column() {
+         AddressBookView({ me: this.addrBook.me, contacts: this.addrBook.contacts, selectedPerson: this.addrBook.me })
+       }
+     }
+   }
+   ```
+
+2. **PersonView** is the view that shows a contact name and preferred phone number in the phonebook. When a user selects a contact (person), that contact is highlighted and needs to be synchronized back to the **selectedPerson** of the parent component **AddressBookView**. In this case, two-way data synchronization needs to be established through \@Link.
+   **PersonView** is declared as follows:
+
+
+   ```ts
+   // Display the contact name and preferred phone number.
+   // To update the phone number, @ObjectLink person and @ObjectLink phones are required.
+   // this.person.phones[0] cannot be used to display the preferred phone number because @ObjectLink person only proxies the Person property and the changes inside the array cannot be observed.
+   // Trigger the onClick event to update selectedPerson.
+   @Component
+   struct PersonView {
+   
+       @ObjectLink person : Person;
+       @ObjectLink phones :  ObservedArray<string>;
+   
+       @Link selectedPerson : Person;
+   
+       build() {
+           Flex({ direction: FlexDirection.Row, justifyContent: FlexAlign.SpaceBetween }) {
+             Text(this.person.name)
+             if (this.phones.length) {
+               Text(this.phones[0])
+             }
+           }
+           .height(55)
+           .backgroundColor(this.selectedPerson.name == this.person.name ? "#ffa0a0" : "#ffffff")
+           .onClick(() => {
+               this.selectedPerson = this.person;
+           })
+       }
+   }
+   ```
+
+3. The information about the selected contact (person) is displayed in the **PersonEditView** object. The data synchronization for the **PersonEditView** can be implemented as follows:
+
+   - When the user's keyboard input is received in the Edit state through the **Input.onChange** callback event, the change should be reflected in the current **PersonEditView**, but does not need to be synchronized to the data source before **Save Changes** is clicked. In this case, \@Prop is used to make a deep copy of the information about the current contact (person).
+
+   - **PersonEditView** establishes through \@Link **seletedPerson: Person** two-way data synchronization with **selectedPerson** of **AddressBookView**. When a user clicks **Save Changes**, the change to \@Prop is assigned to \@Link **seletedPerson: Person**. In this way, the data is synchronized back to the data source.
+
+   - In **PersonEditView**, \@Consume **addrBook: AddressBook** is used to set up two-way synchronization with the root node **PageEntry**. When a user deletes a contact on the **PersonEditView** page, the deletion is directly synchronized to **PageEntry**, which then instructs **AddressBookView** to update the contracts list page. **PersonEditView** is declared as follows:
+
+     ```ts
+     // Render the information about the contact (person).
+     // The @Prop decorated variable makes a deep copy from the parent component AddressBookView and retains the changes locally. The changes of TextInput apply only to the local copy.
+     // Click Save Changes to copy all data to @Link through @Prop and synchronize the data to other components.
+     @Component
+     struct PersonEditView {
+     
+         @Consume addrBook : AddressBook;
+     
+         /* Reference pointing to selectedPerson in the parent component. */
+         @Link selectedPerson: Person;
+     
+         /* Make changes on the local copy until you click Save Changes. */
+         @Prop name: string;
+         @Prop address : Address;
+         @Prop phones : ObservedArray<string>;
+     
+         selectedPersonIndex() : number {
+             return this.addrBook.contacts.findIndex((person) => person.id_ == this.selectedPerson.id_);
+         }
+     
+         build() {
+             Column() {
+                 TextInput({ text: this.name})
+                     .onChange((value) => {
+                         this.name = value;
+                       })
+                 TextInput({text: this.address.street})
+                     .onChange((value) => {
+                         this.address.street = value;
+                     })
+     
+                 TextInput({text: this.address.city})
+                     .onChange((value) => {
+                         this.address.city = value;
+                     })
+     
+                 TextInput({text: this.address.zip.toString()})
+                     .onChange((value) => {
+                         const result = parseInt(value);
+                         this.address.zip= isNaN(result) ? 0 : result;
+                     })
+     
+                 if(this.phones.length>0) {
+                     ForEach(this.phones,
+                             (phone, index) => {
+                                 TextInput({text: phone})
+                                     .width(150)
+                                     .onChange((value) => {
+                                         console.log(`${index}. ${value} value has changed`)
+                                         this.phones[index] = value;
+                                     })
+                             },
+                             (phone, index) => `${index}-${phone}`
+                     ) 
+                 }
+     
+                 Flex({ direction: FlexDirection.Row, justifyContent: FlexAlign.SpaceBetween }) {
+                     Text("Save Changes")
+                         .onClick(() => {
+                             // Assign the updated value of the local copy to the reference pointing to selectedPerson in the parent component.
+                             // Avoid creating new objects. Modify existing properties instead.
+                             this.selectedPerson.name = this.name;
+                             this.selectedPerson.address.street = this.address.street
+                             this.selectedPerson.address.city   =  this.address.city
+                             this.selectedPerson.address.zip    = this.address.zip
+                             this.phones.forEach((phone : string, index : number) => { this.selectedPerson.phones[index] = phone } );
+                         })
+                     if (this.selectedPersonIndex()!=-1) {
+                         Text("Delete Contact")
+                             .onClick(() => {
+                                 let index = this.selectedPersonIndex();
+                                 console.log(`delete contact at index ${index}`);
+     
+                                 // Delete the current contact.
+                                 this.addrBook.contacts.splice(index, 1);
+     
+                                 // Delete the current selectedPerson. The selected contact is then changed to the contact immediately before the deleted contact.
+                                 index = (index < this.addrBook.contacts.length) ? index : index-1;
+     
+                                 // If all contracts are deleted, the **me** object is selected.
+                                 this.selectedPerson = (index>=0) ? this.addrBook.contacts[index] : this.addrBook.me;
+                             })
+                     }
+                 }
+     
+             }
+         }
+     }
+     ```
+
+     Pay attention to the following differences between \@ObjectLink and \@Link:
+
+     1. To implement two-way data synchronization with the parent component **PageView** in **AddressBookView**, you need to use \@ObjectLink, instead of \@Link, to decorate **me: Person** and **contacts: ObservedArray\<Person>**. The reasons are as follows:
+        - The type of the \@Link decorated variable must be the same as that of the data source, and \@Link can only observe the changes at the first layer.
+        - \@ObjectLink allows for initialization from the property of the data source. It functions as a proxy for the properties of the \@Observed decorated class and can observe the changes of the properties of that class.
+     2. When the contact name (**Person.name**) or preferred phone number (**Person.phones[0]**) is updated, **PersonView** needs to be updated. As the update to **Person.phones[0]** occurs at the second layer, it cannot be observed if \@Link is used. In addition, \@Link requires its decorated variable be of the same type as the data source. Therefore, \@ObjectLink is required in **PersonView**, that is, \@ObjectLink **person: Person** and \@ObjectLink **phones: ObservedArray\<string>**.
+
+     ![en-us_image_0000001605293914](figures/en-us_image_0000001605293914.png)
+
+     Now you have a basic idea of how to build a view model. In the root node of an application, the view model may comprise a huge amount of nested data, which is more often the case. Yet, we can make reasonable separation of the data in the UI tree structure. We can adapt the view model data items to views so that the view at each layer contains relatively flat data, and you only need to observe changes at the current layer.
+
+     In this way, the UI re-render workload is minimized, leading to higher application performance.
+
+     The complete sample code is as follows:
+
+
+```ts
+
+ // ViewModel classes
+ let nextId = 0;
+ 
+ @Observed
+ export class ObservedArray<T> extends Array<T> {
+   constructor(args?: any[]) {
+     console.log(`ObservedArray: ${JSON.stringify(args)} `)
+     if (Array.isArray(args)) {
+       super(...args);
+     } else {
+       super(args)
+     }
+   }
+ }
+ 
+ @Observed
+ export class Address {
+   street: string;
+   zip: number;
+   city: string;
+ 
+   constructor(street: string,
+               zip: number,
+               city: string) {
+     this.street = street;
+     this.zip = zip;
+     this.city = city;
+   }
+ }
+ 
+ @Observed
+ export class Person {
+   id_: string;
+   name: string;
+   address: Address;
+   phones: ObservedArray<string>;
+ 
+   constructor(name: string,
+               street: string,
+               zip: number,
+               city: string,
+               phones: string[]) {
+     this.id_ = `${nextId}`;
+     nextId++;
+     this.name = name;
+     this.address = new Address(street, zip, city);
+     this.phones = new ObservedArray<string>(phones);
+   }
+ }
+
+ export class AddressBook {
+   me: Person;
+   contacts: ObservedArray<Person>;
+ 
+   constructor(me: Person, contacts: Person[]) {
+     this.me = me;
+     this.contacts = new ObservedArray<Person>(contacts);
+   }
+ }
+ 
+ // Render the name of the Person object and the first phone number in the @Observed array <string>.
+ // To update the phone number, @ObjectLink person and @ObjectLink phones are required.
+ // this.person.phones cannot be used. Otherwise, changes to items inside the array will not be observed.
+ // Update selectedPerson in onClick in AddressBookView and PersonEditView.
+ @Component
+ struct PersonView {
+   @ObjectLink person: Person;
+   @ObjectLink phones: ObservedArray<string>;
+   @Link selectedPerson: Person;
+ 
+   build() {
+     Flex({ direction: FlexDirection.Row, justifyContent: FlexAlign.SpaceBetween }) {
+       Text(this.person.name)
+       if (this.phones.length) {
+         Text(this.phones[0])
+       }
+     }
+     .height(55)
+     .backgroundColor(this.selectedPerson.name == this.person.name ? "#ffa0a0" : "#ffffff")
+     .onClick(() => {
+       this.selectedPerson = this.person;
+     })
+   }
+ }
+ 
+ // Render the information about the contact (person).
+ // The @Prop decorated variable makes a deep copy from the parent component AddressBookView and retains the changes locally. The changes of TextInput apply only to the local copy.
+ // Click Save Changes to copy all data to @Link through @Prop and synchronize the data to other components.
+ @Component
+ struct PersonEditView {
+   @Consume addrBook: AddressBook;
+ 
+   /* Reference pointing to selectedPerson in the parent component. */
+   @Link selectedPerson: Person;
+ 
+   /* Make changes on the local copy until you click Save Changes. */
+   @Prop name: string;
+   @Prop address: Address;
+   @Prop phones: ObservedArray<string>;
+ 
+   selectedPersonIndex(): number {
+     return this.addrBook.contacts.findIndex((person) => person.id_ == this.selectedPerson.id_);
+   }
+ 
+   build() {
+     Column() {
+       TextInput({ text: this.name })
+         .onChange((value) => {
+           this.name = value;
+         })
+       TextInput({ text: this.address.street })
+         .onChange((value) => {
+           this.address.street = value;
+         })
+ 
+       TextInput({ text: this.address.city })
+         .onChange((value) => {
+           this.address.city = value;
+         })
+ 
+       TextInput({ text: this.address.zip.toString() })
+         .onChange((value) => {
+           const result = parseInt(value);
+           this.address.zip = isNaN(result) ? 0 : result;
+         })
+ 
+       if (this.phones.length > 0) {
+         ForEach(this.phones,
+           (phone, index) => {
+             TextInput({ text: phone })
+               .width(150)
+               .onChange((value) => {
+                 console.log(`${index}. ${value} value has changed`)
+                 this.phones[index] = value;
+               })
+           },
+           (phone, index) => `${index}-${phone}`
+         )
+       }
+ 
+       Flex({ direction: FlexDirection.Row, justifyContent: FlexAlign.SpaceBetween }) {
+         Text("Save Changes")
+           .onClick(() => {
+             // Assign the updated value of the local copy to the reference pointing to selectedPerson in the parent component.
+             // Avoid creating new objects. Modify existing properties instead.
+             this.selectedPerson.name = this.name;
+             this.selectedPerson.address.street = this.address.street
+             this.selectedPerson.address.city = this.address.city
+             this.selectedPerson.address.zip = this.address.zip
+             this.phones.forEach((phone: string, index: number) => {
+               this.selectedPerson.phones[index] = phone
+             });
+           })
+         if (this.selectedPersonIndex() != -1) {
+           Text("Delete Contact")
+             .onClick(() => {
+               let index = this.selectedPersonIndex();
+               console.log(`delete contact at index ${index}`);
+ 
+               // Delete the current contact.
+               this.addrBook.contacts.splice(index, 1);
+ 
+               // Delete the current selectedPerson. The selected contact is then changed to the contact immediately before the deleted contact.
+               index = (index < this.addrBook.contacts.length) ? index : index - 1;
+ 
+               // If all contracts are deleted, the **me** object is selected.
+               this.selectedPerson = (index >= 0) ? this.addrBook.contacts[index] : this.addrBook.me;
+             })
+         }
+       }
+ 
+     }
+   }
+ }
+ 
+ @Component
+ struct AddressBookView {
+   @ObjectLink me: Person;
+   @ObjectLink contacts: ObservedArray<Person>;
+   @State selectedPerson: Person = undefined;
+ 
+   aboutToAppear() {
+     this.selectedPerson = this.me;
+   }
+ 
+   build() {
+     Flex({ direction: FlexDirection.Column, justifyContent: FlexAlign.Start }) {
+       Text("Me:")
+       PersonView({ person: this.me, phones: this.me.phones, selectedPerson: this.$selectedPerson })
+ 
+       Divider().height(8)
+ 
+       ForEach(this.contacts,
+         contact => {
+           PersonView({ person: contact, phones: contact.phones, selectedPerson: this.$selectedPerson })
+         },
+         contact => contact.id_
+       )
+ 
+       Divider().height(8)
+ 
+       Text("Edit:")
+       PersonEditView({
+         selectedPerson: this.$selectedPerson,
+         name: this.selectedPerson.name,
+         address: this.selectedPerson.address,
+         phones: this.selectedPerson.phones
+       })
+     }
+     .borderStyle(BorderStyle.Solid).borderWidth(5).borderColor(0xAFEEEE).borderRadius(5)
+   }
+ }
+ 
+ @Entry
+ @Component
+ struct PageEntry {
+   @Provide addrBook: AddressBook = new AddressBook(
+     new Person("Gigi", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********", "18*********"]),
+     [
+       new Person("Oly", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********"]),
+       new Person("Sam", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********"]),
+       new Person("Vivi", "Itamerenkatu 9", 180, "Helsinki", ["18*********", "18*********"]),
+     ]);
+ 
+   build() {
+     Column() {
+       AddressBookView({ me: this.addrBook.me, contacts: this.addrBook.contacts, selectedPerson: this.addrBook.me })
+     }
+   }
+ }
+```
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