| type | string | Yes | Event type. The value **characteristicWrite** indicates a characteristic write request event.|
| type | string | Yes | Event type. The value is **characteristicWrite**, which indicates a characteristic write request event.|
| callback | Callback<[CharacteristicWriteRequest](#characteristicwriterequest)> | Yes | Callback invoked to return a characteristic write request from the GATT client. |
| type | string | Yes | Type of event to subscribe to. The value is **BLEMtuChange**, which indicates the MTU status changes. If this parameter is not set correctly, the callback cannot be registered.|
| callback | Callback<number> | Yes | Callback invoked to return the number of MTU bytes.|
| type | string | Yes | Type of event to unsubscribe from. The value is **BLEMtuChange**, which indicates the MTU status changes. If this parameter is not set correctly, the callback cannot be unregistered.|
| callback | Callback<number> | No | Callback for the MTU status changes. If this parameter is not set, this API unsubscribes from all callbacks corresponding to **type**.|
Implements the GATT client. Before using an API of this class, you must create a **GattClientDevice** instance using **createGattClientDevice(deviceId: string)**.
| type | string | Yes | Event type. The value is **BLEMtuChange**, which indicates a MTU status change event.|
| callback | Callback<number> | Yes | Callback invoked to return the MTU status, which can be connected or disconnected.|
| type | string | Yes | Type of event to subscribe to. The value is **BLEMtuChange**, which indicates the MTU status changes. If this parameter is not set correctly, the callback cannot be registered.|
| callback | Callback<number> | Yes | Callback invoked to return the number of MTU bytes.|
| type | string | Yes | Event type. The value is **BLEMtuChange**, which indicates a MTU status change event.|
| callback | Callback<number> | No | Callback for MTU status changes. If this parameter is not set, this API unsubscribes from all callbacks corresponding to **type**.|
| type | string | Yes | Type of event to unsubscribe from. The value is **BLEMtuChange**, which indicates the MTU status changes. If this parameter is not set correctly, the callback cannot be unregistered.|
| callback | Callback<number> | No | Callback for the MTU status changes. If this parameter is not set, this API unsubscribes from all callbacks corresponding to **type**.|