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Your application can listen for the broadcast intent, which the system broadcasts whenever the Bluetooth state changes. This broadcast contains the extra fields and , containing the new and old Bluetooth states, respectively. Possible values for these extra fields are STATE_TURNING_ON, STATE_ON, STATE_TURNING_OFF, and STATE_OFF. Listening for this broadcast can be useful if your app needs to detect runtime changes made to the Bluetooth state.

准备

IDE：

Android Studio 4.1.1Build #AI-201.8743.12.41.6953283, built on November 5, 2020Runtime version: 1.8.0_242-release-1644-b01 amd64VM: OpenJDK 64-Bit Server VM by JetBrains s.r.oWindows 10 10.0

接收蓝牙状态改变的广播

新建项目，选择 Empty Activity，在配置项目时，Minimum SDK 选择 API 16: Android 4.1 (Jelly Bean)。

编辑 src\main\AndroidManifest.xml 应用清单文件，声明使用 android.permission.BLUETOOTH 权限（第 4 行）：

编辑 MainActivity 文件：

package com.mk;import android.bluetooth.BluetoothAdapter;import android.content.BroadcastReceiver;import android.content.Context;import android.content.Intent;import android.content.IntentFilter;import android.os.Bundle;import android.widget.Toast;import androidx.appcompat.app.AppCompatActivity;public class MainActivity extends AppCompatActivity {       @Override    protected void onCreate(Bundle savedInstanceState) {           super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        registerReceiver(broadcastReceiver, new IntentFilter(BluetoothAdapter.ACTION_STATE_CHANGED));    }    @Override    protected void onDestroy() {           super.onDestroy();        unregisterReceiver(broadcastReceiver);    }    private final BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {           @Override        public void onReceive(Context context, Intent intent) {               String action = intent.getAction();            if (BluetoothAdapter.ACTION_STATE_CHANGED.equals(action)) {                   String currentState = null;                switch (intent.getIntExtra(BluetoothAdapter.EXTRA_STATE, -1)) {                       case BluetoothAdapter.STATE_TURNING_ON:                        currentState = "BluetoothAdapter.STATE_TURNING_ON";                        break;                    case BluetoothAdapter.STATE_ON:                        currentState = "BluetoothAdapter.STATE_ON";                        break;                    case BluetoothAdapter.STATE_TURNING_OFF:                        currentState = "BluetoothAdapter.STATE_TURNING_OFF";                        break;                    case BluetoothAdapter.STATE_OFF:                        currentState = "BluetoothAdapter.STATE_OFF";                        break;                    default:                        currentState = "UNKNOWN";                        break;                }                Toast.makeText(context, currentState, Toast.LENGTH_SHORT).show();//                String previousState = null;//                switch (intent.getIntExtra(BluetoothAdapter.EXTRA_PREVIOUS_STATE, -1)) {   //                    case BluetoothAdapter.STATE_TURNING_ON://                        previousState = "BluetoothAdapter.STATE_TURNING_ON";//                        break;//                    case BluetoothAdapter.STATE_ON://                        previousState = "BluetoothAdapter.STATE_ON";//                        break;//                    case BluetoothAdapter.STATE_TURNING_OFF://                        previousState = "BluetoothAdapter.STATE_TURNING_OFF";//                        break;//                    case BluetoothAdapter.STATE_OFF://                        previousState = "BluetoothAdapter.STATE_OFF";//                        break;//                    default://                        previousState = "UNKNOWN";//                        break;//                }////                Toast.makeText(context, previousState, Toast.LENGTH_SHORT).show();            }        }    };}

参考