# Implementing Page Redirection and Data Transmission

This section describes how to implement page redirection and data transmission between pages:


1. Page redirection: Click a food item on the food list page to go to the food details page. Click the back button on the food details page to go back to the food list page.

2. Data transmission between pages: After you click a food item, **FoodDetail** receives data from the previous page and renders the corresponding food details page.


## Page Redirection

The declarative UI paradigm provides two mechanisms for page redirection:

1. **Navigator**: encapsulates the page routing capability. After the page target is specified, all child components in the page target have the routing capability.

2. Router APIs: called to implement various operations of page routing. You'll need to import **router** before calling the router APIs.

The procedure below uses these two mechanisms for redirection between the page list page and food details page.

1. Click **FoodListItem**. The **FoodDetail** page is displayed. Create a **Navigator** component in **FoodListItem** to enable its child components to have the routing function. The target page is **'pages/FoodDetail'**.
   ```ts
   @Component
   struct FoodListItem {
     private foodItem: FoodData
     build() {
       Navigator({ target: 'pages/FoodDetail' }) {
         Flex({ justifyContent: FlexAlign.Start, alignItems: ItemAlign.Center }) {
           Image(this.foodItem.image)
             .objectFit(ImageFit.Contain)
             .height(40)
             .width(40)
             .backgroundColor('#FFf1f3f5')
             .margin({ right: 16 })
           Text(this.foodItem.name)
             .fontSize(14)
             .flexGrow(1)
           Text(this.foodItem.calories + ' kcal')
             .fontSize(14)
         }
         .height(64)
       }
       .margin({ right: 24, left:32 })
     }
   }
   ```

   ![en-us_image_0000001223127744](figures/en-us_image_0000001223127744.gif)

2. Click **FoodGridItem**. The **FoodDetail** page is displayed. Import the **router** module, and then call the **push** API of this module to push the **FoodDetail** page to the route stack to implement page redirection.
   ```ts
   import router from '@ohos.router'

   @Component
   struct FoodGridItem {
     private foodItem: FoodData
     build() {
       Column() {
         ......
       }
       .height(184)
       .width('100%')
       .onClick(() => {
         router.pushUrl({ url: 'pages/FoodDetail' })
       })
     }
   }
   ```

   ![en-us_image_0000001267607909](figures/en-us_image_0000001267607909.gif)

3. Add the icon for returning from the **FoodDetail** page to the food list page. Save the **Back.png** file to the **resources** > **base** > **media** directory. Create a custom component **PageTitle**, which contains the back icon and Food Detail text. Call the **router.back()** API of the router to display the top page of the route stack, that is, the upper-level page.
   ```ts
   // FoodDetail.ets
   import router from '@ohos.router'

   @Component
   struct PageTitle {
       build() {
           Flex({ alignItems: ItemAlign.Start }) {
               Image($r('app.media.Back'))
                   .width(21.8)
                   .height(19.6)
               Text('Food Detail')
                   .fontSize(21.8)
                   .margin({left: 17.4})
           }
           .height(61)
           .backgroundColor('#FFedf2f5')
           .padding({ top: 13, bottom: 15, left: 28.3 })
           .onClick(() => {
               router.back()
           })
       }
   }
   ```

4. Create the Stack component in the **FoodDetail** component, including the **FoodImageDisplay** and **PageTitle** child components. Set the alignment mode to **TopStart**.
   ```ts
   @Entry
   @Component
   struct FoodDetail {
     build() {
       Column() {
         Stack( { alignContent: Alignment.TopStart }) {
           FoodImageDisplay()
           PageTitle()
         }
         ContentTable()
       }
       .alignItems(HorizontalAlign.Center)
     }
   }
   ```

   ![en-us_image_0000001267767881](figures/en-us_image_0000001267767881.png)


## Data Transmission Between Pages

We have implemented the redirection and going back of the **FoodCategoryList** and **FoodDetail** pages. At this point, the tomato details page is displayed no matter which **FoodListItem**/**FoodGridItem** is clicked. This is because the data transmission between pages is not yet configured. To configure data transmission between pages, set the routing with parameters as follows:

1. Set the **params** attribute, which accepts the key-value object, in the **Navigator** of the **FoodListItem** component.
   ```ts
   // FoodList.ets
   @Component
   struct FoodListItem {
     private foodItem: FoodData
     build() {
       Navigator({ target: 'pages/FoodDetail' }) {
         ......
       }
       .params({ foodData: this.foodItem })
     }
   }
   ```

   The router API called by **FoodGridItem** also has supports redirection with parameters. The method of using the router API is similar to that of using the **Navigator**.

   ```ts
   router.pushUrl({
     url: 'pages/FoodDetail',
     params: { foodData: this.foodItem }
   })
   ```

2. Import the **FoodData** class to the **FoodDetail** page and add the **foodItem** member variable to the **FoodDetail** component.
   ```ts
   // FoodDetail.ets
   import { FoodData } from '../model/FoodData'

   @Entry
   @Component
   struct FoodDetail {
     private foodItem: FoodData
     build() {
       ......
     }
   }
   ```

3. Obtain the value of **foodData**. Call **router.getParams()['foodData']** to obtain the data corresponding to **foodData** carried when the **FoodCategoryList** page is displayed.
   ```ts
   @Entry
   @Component
   struct FoodDetail {
     private foodItem: FoodData = router.getParams()['foodData']

     build() {
       ......
     }
   }
   ```

4. Re-build the components on the **FoodDetail** page. During building, the food information on the **FoodDetail** page is all directly declared constants. You need to use the passed **FoodData** data to assign a new value to the constants. The sample code is as follows:
   ```ts
   @Component
   struct PageTitle {
       build() {
           Flex({ alignItems: ItemAlign.Start }) {
               Image($r('app.media.Back'))
                   .width(21.8)
                   .height(19.6)
               Text('Food Detail')
                   .fontSize(21.8)
                   .margin({left: 17.4})
           }
           .height(61)
           .backgroundColor('#FFedf2f5')
           .padding({ top: 13, bottom: 15, left: 28.3 })
           .onClick(() => {
               router.back()
           })
       }
   }

   @Component
   struct FoodImageDisplay {
     private foodItem: FoodData
     build() {
       Stack({ alignContent: Alignment.BottomStart }) {
         Image(this.foodItem.image)
           .objectFit(ImageFit.Contain)
         Text(this.foodItem.name)
           .fontSize(26)
           .fontWeight(500)
           .margin({ left: 26, bottom: 17.4 })
       }
       .height(357)
       .backgroundColor('#FFedf2f5')
     }
   }

   @Component
   struct ContentTable {
     private foodItem: FoodData

     @Builder IngredientItem(title:string, name: string, value: string) {
       Flex() {
         Text(title)
           .fontSize(17.4)
           .fontWeight(FontWeight.Bold)
           .layoutWeight(1)
         Flex() {
           Text(name)
             .fontSize(17.4)
             .flexGrow(1)
           Text(value)
             .fontSize(17.4)
         }
         .layoutWeight(2)
       }
     }

     build() {
       Flex({ direction: FlexDirection.Column, justifyContent: FlexAlign.SpaceBetween, alignItems: ItemAlign.Start }) {
         this.IngredientItem('Calories', 'Calories', this.foodItem.calories + 'kcal')
         this.IngredientItem('Nutrition', 'Protein', this.foodItem.protein + 'g')
         this.IngredientItem('', 'Fat', this.foodItem.fat + 'g')
         this.IngredientItem('', 'Carbohydrates', this.foodItem.carbohydrates + 'g')
         this.IngredientItem('', 'VitaminC', this.foodItem.vitaminC + 'mg')
       }
       .height(280)
       .padding({ top: 30, right: 30, left: 30 })
     }
   }

   @Entry
   @Component
   struct FoodDetail {
     private foodItem: FoodData = router.getParams()['foodData']

     build() {
       Column() {
         Stack( { alignContent: Alignment.TopStart }) {
           FoodImageDisplay({ foodItem: this.foodItem })
           PageTitle()
         }
         ContentTable({ foodItem: this.foodItem })
       }
       .alignItems(HorizontalAlign.Center)
     }
   }
   ```