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import android.bluetooth.BluetoothAdapter; 

【BluetoothAdapter.java】

@SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)

public static final String
            ACTION_STATE_CHANGED = "android.bluetooth.adapter.action.STATE_CHANGED";//当前action动作

    /**
     * Used as an int extra field in {@link #ACTION_STATE_CHANGED}
     * intents to request the current power state. Possible values are:
     * {@link #STATE_OFF},
     * {@link #STATE_TURNING_ON},
     * {@link #STATE_ON},
     * {@link #STATE_TURNING_OFF},
     */

//intent.getIntExtra 参数
    public static final String EXTRA_STATE = "android.bluetooth.adapter.extra.STATE";//当前状态

    public static final String EXTRA_PREVIOUS_STATE =
            "android.bluetooth.adapter.extra.PREVIOUS_STATE";//上一个状态

public static final int STATE_OFF = 10;

 public static final int STATE_TURNING_ON = 11;
  
public static final int STATE_ON = 12;
 
public static final int STATE_TURNING_OFF = 13;
 

  BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
                boolean bluetoothEnable = bluetoothAdapter == null ? false : bluetoothAdapter.isEnabled();
                if (bluetoothEnable) {//判断蓝牙是否开启
                    //处理逻辑
                }

 

 监听蓝牙广播：

IntentFilter bt = new IntentFilter();
        bt.addAction(BluetoothAdapter.ACTION_STATE_CHANGED);
        mContext.registerReceiver(mReceiver, bt);

    private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();
            if (BluetoothAdapter.ACTION_STATE_CHANGED.equals(action)) {
                int state = intent.getIntExtra(BluetoothAdapter.EXTRA_STATE, BluetoothAdapter.ERROR);//监听当前蓝牙状态

                switch (state) {
                    case BluetoothAdapter.STATE_OFF:
                         //添加其他处理逻辑，比如亮灭led灯，密码弹窗，控制可用禁用等
                        break;
                    case BluetoothAdapter.STATE_ON:
                        //添加其他处理逻辑
                        break;

                   case BluetoothAdapter.STATE_TURNING_OFF:
                        break;

                     case BluetoothAdapter.STATE_TURNING_ON:
                        break;
                }
            }
        }
    }; 

最好的监听地方是状态栏上：

/vendor/mediatek/proprietary/packages/apps/SystemUI/src/com/android/systemui/statusbar/phone/StatusBar.java 的start()方法里面注册广播

其实，对应如此，NFC也可以这样！NfcAdapter