android四大组件笔记之Service（二）

安卓的开发主要是围绕activity、broadcastReceiver、service、ContentProvider四大组件和基于view的子类组成，其中activity、broadcastReceiver、service都是通过它们的桥梁Intent创建和传递消息。通过Handler来执行消息、线程通讯的操作。而关于IPC，我觉得不得不说的就是binder。咱们这篇的主角是android的四大天王之一Service。

Service

概念：

Service可以说是一个在后台运行的Activity，它不是一个单独的进程，它需要应用告诉它要在干什么就可以了。它要实现和用户交互，需要通过通知栏或者是发送广播，UI去接收显示。它的应用十分广泛，尤其是在框架层。应用更多的是对系统服务的调用。

应用：

它用于处理一些不干扰用户使用的后台操作。如下载、网络获取、播放音乐。同时也可以绑定到宿主对象（如Activity）来使用。
ps:一般情况下，Service都是运行在UI线程的，所以不能执行耗时操作。通常都是在服务里开启子线程，进行自己需要的工作。

分类：

按启动方式：

• Context.startService()
  调用startService()会执行onCreate，再调用onStartCommand。多次调用startService()不会多次创建，但是会多次调用onStartCommand()，只有调用stopService()才会结束服务。
• Context.bindService()
  调用bindService()会执行onCreate，再调用onBind。多次调用不会多次创建。它是和调用者绑定到一起的，如果调用者退出了。就会调用unBindService()。当然调用者也可以主动调用unBIndService()。

按服务范围：

• 本地服务（LocalService）：用于程序内部。
• 远程服务（RemoteService）：用于android应用程序之间。

按运行类别：

• 前台服务
  调用 startForeground(int startId,Notification ntf) (android2.0以上 ) 或 setForeground(android2.0以下)使服务成为前台服务。提升进程优先级，避免被杀死。
  使用：在onStartCommand里面调用startForeground方面让服务运行在前台，在onDstroy调用stopForeground解除。
• 后台服务
  服务默认运行状态

生命周期

• context.startService()
  context.startService -> onCreate() -> onStartCommand() -> Service running -> context.stopService() -> onDestroy() -> service stop
• context.bindService()
  context.bindService() -> onCreate -> onBind() -> service running -> onUnbind() -> onDestroy() ->service stop

IntentService

我们先看下源码：

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.
     *
     * <p>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.
     *
     * <p>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);
}

首先我们大致看到了，有looper，有handler，有Thread，我们想到了什么？当然是android的消息机制。
还看到了，该类帮我们处理了Service的实现方法，他的子类需要实现一个onHandleIntent()。这下就清晰了。因为Service本身是不支持执行耗时操作的。要想执行耗时操作，必须新建线程。所以，IntentService的出现就解决这个问题，它继承于Service，封装了多线程，耗时操作在子线程中操作。

我们看先这句：

@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);
    }

熟悉android的消息机制的应该都会知道，当一个线程启动的时候他会创建一个looper对象，执行looper.loop(),用来循环获取MessageQueue队列的对象。当前的Looper对象是属于HandlerThread的，通过这句代码就可以得知ServiceHandler是也是运行在HandlerThread线程的。

mServiceHandler = new ServiceHandler(mServiceLooper);

我们在看看这段代码：

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);
        }
    }

说明onHandleMessage也是运行在HandleThread线程中。
所以我们在实现IntentService中onHandleIntent方法时，就可以直接进行耗时操作。