| write(string eventName, EventType type, object keyValues, AsyncCallback\<void> callback): void | void | Logs application events in asynchronous mode. This method uses an asynchronous callback to return the result. |
| write(string eventName, EventType type, object keyValues): Promise\<void> | Promise\<void> | Logs application events in asynchronous mode. This method uses a promise to return the result. |
**Table 1** APIs for application event logging
When an asynchronous callback is used, the return value can be processed directly in the callback. When a promise is used, the return value can also be processed in the promise in a similar way. For details about the result codes, see [Event Verification Result Codes](#event-verification-result-codes).
| write(string eventName, EventType type, object keyValues, AsyncCallback\<void> callback): void | Logs application events in asynchronous mode. This API uses an asynchronous callback to return the result. |
| write(string eventName, EventType type, object keyValues): Promise\<void> | Logs application events in asynchronous mode. This API uses a promise to return the result. |
| write(AppEventInfo info, AsyncCallback\<void> callback): void | Logs application events by domain in asynchronous mode. This API uses an asynchronous callback to return the result.|
| write(AppEventInfo info): Promise\<void> | Logs application events by domain in asynchronous mode. This API uses a promise to return the result.|
**Table 2** APIs for event logging configuration
When an asynchronous callback is used, the return value can be processed directly in the callback.
| configure(ConfigOption config) | boolean | Sets the configuration options for application event logging.<br>The value **true** indicates that the operation is successful, and value **false** indicates the opposite. |
If a promise is used, the return value can also be processed in the promise in a similar way.
## Event Verification Result Codes
For details about the result codes, see [Event Verification Result Codes](#event-verification-result-codes).
| Result Code | Cause | Check Rule | Processing Method |
| 0 | None | Event verification is successful. | Event logging is normal. No action is required. |
| -1 | Invalid event name | The event name is not empty and contains a maximum of 48 characters.<br/>The event name consists of only the following characters: digits (0 to 9), letters (a to z), and underscore (\_).<br/>The event name does not start with a digit or underscore (_). | Ignore this event and do not perform logging. |
| -2 | Invalid event parameter type | The event name must be a string.<br/>The event type must be a number.<br/>The key value must be an object. | Ignore this event and do not perform logging. |
| -99 | Application event logging disabled | The application event logging function is disabled. | Ignore this event and do not perform logging. |
| -100 | Unknown error | None | Ignore this event and do not perform logging. |
| 1 | Invalid key name | The key name is not empty and contains a maximum of 16 characters.<br/>The key name consists of only the following characters: digits (0 to 9), letters (a to z), and underscore (\_).<br/>The key name does not start with a digit or underscore (\_).<br/>The key name does not end with an underscore (_). | Ignore the key-value pair and continue to perform logging. |
| 2 | Invalid key type | The key must be a string. | Ignore the key-value pair and continue to perform logging. |
| 3 | Invalid value type | The supported value types vary depending on the programming language:<br/>boolean, number, string, or Array [basic element] | Ignore the key-value pair and continue to perform logging. |
| 4 | Value too long | The value can contain a maximum of 8*1024 characters. | Ignore the key-value pair and continue to perform logging. |
| 5 | Excess key-value pairs | The number of key-value pairs must be less than or equal to 32. | Ignore the excess key-value pairs and continue to perform logging. |
| 6 | Excess elements in a list value | The number of elements in a list value must be less than or equal to 100. | Truncate the list with only the first 100 elements retained, and continue to perform logging. |
| 7 | Invalid list value | A list value can only be a basic element.<br/>The elements in a list value must be of the same type. | Ignore the key-value pair and continue to perform logging. |
| addWatcher(Watcher watcher): AppEventPackageHolder | Adds an event watcher.|
| removeWatcher(Watcher watcher): void | Removes an event watcher.|
1. Create a JS application project. In the displayed Project window, choose **entry > src > main** > **js** > **default** > **pages > index**, and double-click index.js. Add the code to log the initial application event after page loading. The sample code is as follows:
| 0 | N/A | Event verification is successful. | Event logging is normal. No action is required. |
| -1 | Invalid event name | The name is not empty and contains a maximum of 48 characters.<br>The name consists of only the following characters: digits (0 to 9), letters (a to z), and underscore \(_).<br>The name does not start with a digit or underscore \(_).| Ignore this event and do not perform logging. |
| -2 | Invalid event parameter type | The event name must be a string.<br>The event type must be a number.<br>The event parameter must be an object.| Ignore this event and do not perform logging. |
| -4 | Invalid event domain name | The name is not empty and contains a maximum of 32 characters.<br>The name consists of only the following characters: digits (0 to 9), letters (a to z), and underscore \(_).<br>The name does not start with a digit or underscore \(_).| Ignore this event and do not perform logging. |
| -99 | Application event logging disabled | Application event logging is disabled. | Ignore this event and do not perform logging. |
| -100 | Unknown error | None. | Ignore this event and do not perform logging. |
| 1 | Invalid key name | The name is not empty and contains a maximum of 16 characters.<br>The name consists of only the following characters: digits (0 to 9), letters (a to z), and underscore \(_).<br>The name does not start with a digit or underscore \(_).<br>The name does not end with an underscore \(_).| Ignore the key-value pair and continue to perform logging. |
| 2 | Invalid key type | The key must be a string. | Ignore the key-value pair and continue to perform logging. |
| 3 | Invalid value type | The supported value types vary depending on the programming language:<br>boolean, number, string, or Array [basic element]| Ignore the key-value pair and continue to perform logging. |
| 4 | Invalid length for values of the string type| For a value of the string type, the maximum length is 8*1024 characters. | Truncate the value with the first 8*1024 characters retained, and continue to perform logging.|
| 5 | Excess key-value pairs | The number of key-value pairs must be less than or equal to 32. | Ignore the excess key-value pairs and continue to perform logging. |
| 6 | Invalid number of elements in values of the array type | For a value of the array type, the number of elements must be less than or equal to 100. | Truncate the array with the first 100 elements retained, and continue to perform logging. |
| 7 | Invalid parameters in values of the array type | For a value of the array type, all the parameters must be of the same type, which can only be boolean, number, or string.| Ignore the key-value pair and continue to perform logging. |
## Development Procedure
The following uses a one-time event watcher as an example to illustrate the development procedure.
1. Create an eTS application project. In the displayed **Project** window, choose **entry** > **src** > **main** > **ets** > **pages** > **index.ets**, and double-click **index.ets**. Then, add three buttons to simulate the process of watching for application events. Wherein, button 1 is used to invoke application event logging, button 2 to add an event watcher that automatically triggers a callback, and button 3 to remove the watcher. The complete sample code is as follows:
// 4. Set the maximum size of the event file storage directory. The default value is 10M.
hiAppEvent.configure({
maxStorage:'100M'
});
Button("3 removeWatcherTest").onClick(()=>{
hiAppEvent.removeWatcher({
name:"watcher1"
})
})
}
.width('100%')
}
.height('100%')
}
}
```
2. Tap the run button on the application UI to run the project.
2. Touch the run button on the IDE to run the project.
3. Touch button 1 on the application UI to start application event logging. If the logging is successful, you'll see the following message in the **Log** window:
```
success to write event: 0
```
4. On the application UI, touch button 2 to add an event watcher, and touch button 1 for multiple times to perform application event logging. If any callback trigger condition (event count, event data size, and timeout duration) is met, the event watcher will invoke a callback and the event package obtained through the callback will be displayed in the **Log** window.
5. On the application UI, touch button 3 to remove the event watcher. Then, touch button 1 for multiple times to perform application event logging. In such a case, there will be no log information about the callback invoked by the event watcher.
## Samples
The following sample is provided to help you better understand how to develop the application event logging feature:
HiAppEvent provides event logging APIs for applications to log the fault, statistical, security, and user behavior events reported during running. Based on event information, you will be able to analyze the running status of your application.
The HiAppEvent module of OpenHarmony can be used to develop application event services and provide functions related to application events, including flushing application events to a disk and querying historical application event data.
The HiAppEvent module can be used to develop application event-related functions, including flushing application events to a disk and querying historical application events.
## Basic Concepts
-**Logging**
**Logging**
Logs changes caused by user operations to provide service data for development, product, and O&M analysis.
A function that logs changes caused by user operations to provide service data for development, product, and O&M analysis.
