Android多线程(IntentService篇)

【齐天的博客】转载请注明出处（万分感谢！）：
https://blog.csdn.net/qijinglai/article/details/80747150

关联文章:
Android多线程(Handler篇)
Android多线程(AsyncTask篇)
Android多线程(HandlerThread篇)
Android多线程(IntentService篇)

前言

例如上传下载等操作原则上要尽可能的交给Service去做，原因就是上传等过程中用户可能会有将应用至于后台，那这时候Activity很有可能就被杀死了。如果担心Service被杀死还能通过startForeground提升优先级。
但在Service里需要开启线程才能进行耗时操作，自己管理Service与线程听起来就不像一个优雅的做法，此时就可以用到Android提供的一个类，IntentService。上一篇说到了HandlerThread，而HandlerThread的一个实例就是今天写到的IntentService。

使用

步骤

1. 创建MyIntentService继承IntentService，并在onHandlIntent()中实现耗时操作
2. Activity注册并启动广播监听耗时操作完成事件，如更新UI
3. Activity中启动MyIntentService
   示例代码如下:

public class MyIntentService extends IntentService {
    private static final String ACTION_UPLOAD_IMG = "com.qit.action.UPLOAD_IMAGE";
    public static final String EXTRA_IMG_PATH = "com.qit.extra.IMG_PATH";

    public static void startUploadImg(Context context, String path) {
        Intent intent = new Intent(context, MyIntentService.class);
        intent.setAction(ACTION_UPLOAD_IMG);
        intent.putExtra(EXTRA_IMG_PATH, path);
        context.startService(intent);
    }


    public MyIntentService() {
        super("MyIntentService");
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        if (intent != null) {
            final String action = intent.getAction();
            if (ACTION_UPLOAD_IMG.equals(action)) {
                final String path = intent.getStringExtra(EXTRA_IMG_PATH);
                handleUploadImg(path);
            }
        }
    }

    private void handleUploadImg(String path) {
        try {
            //模拟上传耗时
            Thread.sleep(((long) (Math.random() * 3000)));

            Intent intent = new Intent(MainActivity.UPLOAD_RESULT);
            intent.putExtra(EXTRA_IMG_PATH, path);
            sendBroadcast(intent);

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class MainActivity extends AppCompatActivity {
    public static final String UPLOAD_RESULT = "com.qit.UPLOAD_RESULT";

    private LinearLayout ll;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ll = (LinearLayout) findViewById(R.id.ll);
        findViewById(R.id.btn).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                addTask();
            }
        });
        IntentFilter filter = new IntentFilter();
        filter.addAction(UPLOAD_RESULT);
        registerReceiver(uploadImgReceiver, filter);
    }

    private BroadcastReceiver uploadImgReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            if (intent.getAction() == UPLOAD_RESULT) {
                String path = intent.getStringExtra(MyIntentService.EXTRA_IMG_PATH);

                handleResult(path);

            }

        }
    };

    private void handleResult(String path) {
        TextView tv = (TextView) ll.findViewWithTag(path);
        tv.setText(path + " upload success ~~~ ");
    }

    int i = 0;

    public void addTask() {
        //模拟路径
        String path = "/sdcard/imgs/" + (++i) + ".png";
        MyIntentService.startUploadImg(this, path);

        TextView tv = new TextView(this);
        ll.addView(tv);
        tv.setText(path + " is uploading ...");
        tv.setTag(path);
    }


    @Override
    protected void onDestroy() {
        super.onDestroy();
        unregisterReceiver(uploadImgReceiver);
    }
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/ll"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <Button
        android:id="@+id/btn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="点" />

LinearLayout>

记得IntentServic也是Service要注册

原理分析

源码不多全部拿过来看

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    private volatile ServiceHandler mServiceHandler;
    private String mName;
    private boolean mRedelivery;

    private final class ServiceHandler extends Handler {
        public ServiceHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            onHandleIntent((Intent)msg.obj);
            stopSelf(msg.arg1);
        }
    }

    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     *
     * @param name Used to name the worker thread, important only for debugging.
     */
    public IntentService(String name) {
        super();
        mName = name;
    }

    /**
     * Sets intent redelivery preferences.  Usually called from the constructor
     * with your preferred semantics.
     *
     * 
If enabled is true,
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_REDELIVER_INTENT}, so if this process dies before
     * {@link #onHandleIntent(Intent)} returns, the process will be restarted
     * and the intent redelivered.  If multiple Intents have been sent, only
     * the most recent one is guaranteed to be redelivered.
     *
     * 
If enabled is false (the default),
     * {@link #onStartCommand(Intent, int, int)} will return
     * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
     * dies along with it.
     */
    public void setIntentRedelivery(boolean enabled) {
        mRedelivery = enabled;
    }

    @Override
    public void onCreate() {
        // TODO: It would be nice to have an option to hold a partial wakelock
        // during processing, and to have a static startService(Context, Intent)
        // method that would launch the service & hand off a wakelock.

        super.onCreate();
        HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
        thread.start();

        mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

    @Override
    public void onStart(@Nullable Intent intent, int startId) {
        Message msg = mServiceHandler.obtainMessage();
        msg.arg1 = startId;
        msg.obj = intent;
        mServiceHandler.sendMessage(msg);
    }

    /**
     * You should not override this method for your IntentService. Instead,
     * override {@link #onHandleIntent}, which the system calls when the IntentService
     * receives a start request.
     * @see android.app.Service#onStartCommand
     */
    @Override
    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
        onStart(intent, startId);
        return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
    }

    @Override
    public void onDestroy() {
        mServiceLooper.quit();
    }

    /**
     * Unless you provide binding for your service, you don't need to implement this
     * method, because the default implementation returns null.
     * @see android.app.Service#onBind
     */
    @Override
    @Nullable
    public IBinder onBind(Intent intent) {
        return null;
    }

    /**
     * This method is invoked on the worker thread with a request to process.
     * Only one Intent is processed at a time, but the processing happens on a
     * worker thread that runs independently from other application logic.
     * So, if this code takes a long time, it will hold up other requests to
     * the same IntentService, but it will not hold up anything else.
     * When all requests have been handled, the IntentService stops itself,
     * so you should not call {@link #stopSelf}.
     *
     * @param intent The value passed to {@link
     *               android.content.Context#startService(Intent)}.
     *               This may be null if the service is being restarted after
     *               its process has gone away; see
     *               {@link android.app.Service#onStartCommand}
     *               for details.
     */
    @WorkerThread
    protected abstract void onHandleIntent(@Nullable Intent intent);
}

• onCreate：里面初始化了一个HandlerThread，关于HandlerThread可以看我上一篇文章 Android多线程(HandlerThread篇)
• onStartCommand：中回调了onStart
• onStart中通过mServiceHandler发送消息到该handler的handleMessage中去。
• handleMessage()：回调onHandleIntent(intent)后回调用 stopSelf(msg.arg1)，注意这个msg.arg1是个int值，相当于一个请求的唯一标识。每发送一个请求，会生成一个唯一的标识，然后将请求放入队列，当全部执行完成(最后一个请求也就相当于getLastStartId == startId)，或者当前发送的标识是最近发出的那一个（getLastStartId == startId），则会销毁我们的Service.如果传入的是-1则直接销毁。
• 当任务完成销毁Service回调onDestory，可以看到在onDestroy中释放了我们的Looper:mServiceLooper.quit()。

end。