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要在后台捕获并处理按键，AccessibilityService是个好办法。当然其局限性也很明显，其一，AccessibilityService和其他Service最大的一个区别就是，必须在Setting->Accessibity Setting中获得用户的许可。而且，一旦启动，他的管理就在于操作系统，你无法在中途将其退出（不用的时候占着资源，挺烦的）。其二，对于用户而言，在后台弄这么一个有某方面监控大权的app，也不是一个很好的体验。

言归正传，下面我们来看源码说明。

1。Notification

我这里为了让用户明确看到程序在后台，特地挂了个Notification, （accessIntent.setAction("com.android.vending");这句是为了避免explicit报错）。

accessIntent = new Intent(MainActivity.this, KeyAccessibilityService.class);accessIntent.setAction(Settings.ACTION_ACCESSIBILITY_SETTINGS);accessIntent.setAction("com.android.vending");

如果你不想要那个Notification显示在手机前台，可以这么写

accessIntent = new Intent();accessIntent.setAction(Settings.ACTION_ACCESSIBILITY_SETTINGS);accessIntent.setAction("com.android.vending");

2。避免不需要响应时拦截按键信息

由于是常驻内存，所以为了避免不需要响应时拦截按键信息，定义

public static boolean isAccessAlive = false; 当需要拦截时，把isAccessAlive设置成true，当不需要拦截时，设置成false就不影响系统的音量调节了（我这里以音量键为例写的）。

3。使用时需要设置AccessibityService

我用的华为手机，安装完后，其设置是在

设置-》高级设置-》辅助功能-》keyActionService 这个keyActionService就是我们的监听程序。其他手机可能不一样，Android模拟机中是setting->accessibility。

这个显示的界面，其xml文件就是res->xml->accessibility_service_config.xml。

运行时debug显示捕捉到的按键信息如下，

源码

如下，

布局activity_main.java

MainActivity.java

package com.spacesoftwares.spacecapture;import android.content.BroadcastReceiver;import android.content.Context;import android.content.Intent;import android.provider.Settings;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.util.Log;import android.view.View;import android.widget.Button;import com.spacesoftwares.spacecapture.service.KeyAccessibilityService;public class MainActivity extends AppCompatActivity {    private Button mBtnWhite, mBtnStopWhite, mBtnExit; // mBtnBlack, mBtnStopBack,    private final static String TAG = MainActivity.class.getSimpleName();    private Intent accessIntent;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mBtnWhite = findViewById(R.id.btn_white);        mBtnStopWhite = findViewById(R.id.btn_stop_white);        mBtnExit = findViewById(R.id.btn_exit);        setListener();    }    @Override    protected void onDestroy() {        KeyAccessibilityService.isAccessAlive = false;        super.onDestroy();    }    class ExitReceiver extends BroadcastReceiver {        @Override        public void onReceive(Context context, Intent intent) {            MainActivity.this.finish();        }    }    private void setListener(){        OnClick onClick = new OnClick();        mBtnWhite.setOnClickListener(onClick);        mBtnStopWhite.setOnClickListener(onClick);        mBtnExit.setOnClickListener(onClick);    }    private class  OnClick implements  View.OnClickListener{        @Override        public void onClick(View view) {            int viewId = view.getId();            switch(viewId){                case R.id.btn_white:                    Log.i(TAG, "MAIN: btn_white");                    if(null == accessIntent) {                        // ---- method No. 1 ----                        //accessIntent = new Intent(Settings.ACTION_ACCESSIBILITY_SETTINGS);                        //accessIntent.setPackage("com.android.vending");                        // ---- method No. 2 ---- cannot get the key                        accessIntent = new Intent(MainActivity.this, KeyAccessibilityService.class);                        accessIntent.setAction(Settings.ACTION_ACCESSIBILITY_SETTINGS);                        accessIntent.setAction("com.android.vending");                    }                    startService(accessIntent);                    KeyAccessibilityService.isAccessAlive = true;                    break;                case R.id.btn_stop_white:                    Log.i(TAG, "MAIN: btn_stop_white");                    if(null != accessIntent)                        stopService(accessIntent);                    KeyAccessibilityService.isAccessAlive = false;                break;                case R.id.btn_exit:                    Log.i(TAG, "MAIN: btn_exit");                    if(null != accessIntent)                        stopService(accessIntent);                    finish();                    KeyAccessibilityService.isAccessAlive = false;                    System.exit(0);                    break;            }        }    }    /*@Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        switch(keyCode){            case KeyEvent.KEYCODE_VOLUME_UP:                isVolumeUpKeyPressed = true;                System.out.println("keycode_volume_up");                Log.i(TAG,"keycode_volume_up");                return true;            case KeyEvent.KEYCODE_POWER:                System.out.println("keycode_power");                Log.i(TAG,"keycode_power");                return true;            default:                return super.onKeyDown(keyCode, event);        }    }    @Override    public boolean onKeyLongPress(int keyCode, KeyEvent event) {        return super.onKeyLongPress(keyCode, event);    }*/}

KeyAccessbilityService.java

package com.spacesoftwares.spacecapture.service;import android.accessibilityservice.AccessibilityService;import android.app.Notification;import android.app.NotificationChannel;import android.app.NotificationManager;import android.app.PendingIntent;import android.content.Context;import android.content.Intent;import android.graphics.Color;import android.os.Build;import android.support.v4.app.NotificationCompat;import android.util.Log;import android.view.KeyEvent;import android.view.accessibility.AccessibilityEvent;import com.spacesoftwares.spacecapture.MainActivity;import com.spacesoftwares.spacecapture.R;import java.util.Calendar;public class KeyAccessibilityService extends AccessibilityService {    public static boolean isAccessAlive = false;    private final static int FOREGROUND_ID = 1000;    private Notification mNotification;    private static final String TAG_KEY = "KEY_ACCESS";    @Override    protected boolean onKeyEvent(KeyEvent event) {        System.out.println("onKeyEvent");        Log.i(TAG_KEY, "onKeyEvent");        if(isAccessAlive) {            int key = event.getKeyCode();            switch (key) {                case KeyEvent.KEYCODE_VOLUME_DOWN:                    System.out.println("KEYCODE_VOLUME_DOWN");                    Log.i(TAG_KEY, "KEYCODE_VOLUME_DOWN");                    return true;                case KeyEvent.KEYCODE_VOLUME_UP:                    System.out.println("KEYCODE_VOLUME_UP");                    Log.i(TAG_KEY, "KEYCODE_VOLUME_UP");                    return true;                default:                    break;            }        }        return super.onKeyEvent(event);    }    @Override    public int onStartCommand(Intent intent, int flags, int startId) {        if(null != mNotification){            Log.i(TAG_KEY, "KeyService->onStartCommand->Notification exists");            return super.onStartCommand(intent, flags, startId);        }        Log.i(TAG_KEY, "KeyService->onStartCommand->Create Notification");        //NotificationManager manager = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);        //NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);        String NOTIFICATION_CHANNEL_ID = "my_channel_id_01";        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {            NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, "My Notifications", NotificationManager.IMPORTANCE_HIGH);            // Configure the notification channel.            notificationChannel.setDescription("Channel description");            notificationChannel.enableLights(true);            notificationChannel.setLightColor(Color.RED);            notificationChannel.setVibrationPattern(new long[]{0, 1000, 500, 1000});            notificationChannel.enableVibration(true);            NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);            notificationManager.createNotificationChannel(notificationChannel);        }        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID);        builder.setSmallIcon(R.mipmap.ic_launcher);        builder.setContentTitle("Foreground");        builder.setContentText("Text shown on notification bar");        builder.setContentInfo("Content Info");        builder.setWhen(System.currentTimeMillis());        Intent activityIntent = new Intent(this, MainActivity.class);        PendingIntent pendingIntent = PendingIntent.getActivity(this, 1, activityIntent, PendingIntent.FLAG_UPDATE_CURRENT);        builder.setContentIntent(pendingIntent);        mNotification = builder.build();        startForeground(FOREGROUND_ID, mNotification);        return super.onStartCommand(intent, flags, startId);    }    @Override    protected void onServiceConnected() {        System.out.println("KEYCODE_onServiceConnected");        Log.i(TAG_KEY, "KEYCODE_onServiceConnected");        super.onServiceConnected();    }    @Override    public void onAccessibilityEvent(AccessibilityEvent accessibilityEvent) {        System.out.println("KEYCODE_onAccessibilityEvent");        Log.i(TAG_KEY, "KEYCODE_onAccessibilityEvent");    }    @Override    public void onInterrupt() {    }}

AndroidManifest.xml

res->xml->accessibility_service_config.xml

完整的项目资源在这里下载，