Services初步

有编程经验的朋友一听名字service就应该知道是提供的某种服务，而且是后台长时运行的，不提供ui，同样这也是android提供的又一个重要的应用组件。

service的两种表现形式

Started

应用组件(如activity)调用startService()，service被启动，一旦启动，通常，service在后台一直运行，不会和应用有任何交互，操作完成后，service应该自己停止。

Bound

应用组件(如activity)调用bindService()，service被绑定，一旦绑定，通常，service提供了允许组件和service有数据交互的客户端-服务器接口，只有组件绑定了service的情况下，service才会运行。

注意：service是运行在主线程的，如果有耗时操作，需要开线程来做。很方便的是android提供了一些类来帮助大家使用，比如下面介绍的IntentService(不能处理并发任务)

继承IntentService

需要实现的接口

构造函数：需要调用父类IntentService(string) 参数是队列名

onHandleIntent(Intent intent)

所做的工作：

1. 创建一个默认线程，此工作线程传递intent到onStartCommand，这样和主线程分离。
2. 创建一个工作队列，一次传递一个intent到onHandleIntent()，不用担心多线程并发问题。
3. 当所有请求被处理完毕，会自动停止service，不用手动调用stopSelf
4.提供默认onBind()实现
5.提供默认onStartCommand()，发送intent到工作队列，再到onHandleIntent()。

继承Service

public class HelloService extends Service {
  private Looper mServiceLooper;
  private ServiceHandler mServiceHandler;

  // Handler that receives messages from the thread
  private final class ServiceHandler extends Handler {
      public ServiceHandler(Looper looper) {
          super(looper);
      }
      @Override
      public void handleMessage(Message msg) {
          // Normally we would do some work here, like download a file.
          // For our sample, we just sleep for 5 seconds.
          long endTime = System.currentTimeMillis() + 5*1000;
          while (System.currentTimeMillis() < endTime) {
              synchronized (this) {
                  try {
                      wait(endTime - System.currentTimeMillis());
                  } catch (Exception e) {
                  }
              }
          }
          // Stop the service using the startId, so that we don't stop
          // the service in the middle of handling another job
          stopSelf(msg.arg1);
      }
  }

  @Override
  public void onCreate() {
    // Start up the thread running the service.  Note that we create a
    // separate thread because the service normally runs in the process's
    // main thread, which we don't want to block.  We also make it
    // background priority so CPU-intensive work will not disrupt our UI.
    HandlerThread thread = new HandlerThread("ServiceStartArguments",
            Process.THREAD_PRIORITY_BACKGROUND);
    thread.start();

    // Get the HandlerThread's Looper and use it for our Handler
    mServiceLooper = thread.getLooper();
    mServiceHandler = new ServiceHandler(mServiceLooper);
  }

  @Override
  public int onStartCommand(Intent intent, int flags, int startId) {
      Toast.makeText(this, "service starting", Toast.LENGTH_SHORT).show();

      // For each start request, send a message to start a job and deliver the
      // start ID so we know which request we're stopping when we finish the job
      Message msg = mServiceHandler.obtainMessage();
      msg.arg1 = startId;
      mServiceHandler.sendMessage(msg);

      // If we get killed, after returning from here, restart
      return START_STICKY;
  }

  @Override
  public IBinder onBind(Intent intent) {
      // We don't provide binding, so return null
      return null;
  }

  @Override
  public void onDestroy() {
    Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show();
  }
}

start a service

Intent intent = new Intent(this, HelloService.class); startService(intent);

startService(intent);