一.intenService是什么
  IntenService是继承并处理异步线程的一个类，在IntenService里面有一个工作线程来处理耗时任务，启动IntenService和启动传统的Service一样，同时，当任务完成后，IntenService会自动停止，而不需要我们手动去控制或者stop()。另外，IntenService可以启动多次，而每一个耗时操作会以工作队列的方式在IntenService的onHandleIntent回调方法里执行。并且，每次只能执行一个工作线程，执行完第一个再执行第二个
二.intenService使用
IntentServiceActivity

public class IntentServiceActivity extends AppCompatActivity implements MyIntentService.UpdateUI {


    /**
     * 图片地址集合
     */
    private String url[] = {
            "http://pic6.huitu.com/res/20130116/84481_20130116142820494200_1.jpg",
            "http://sucai.qqjay.com/qqjayxiaowo/201210/26/1.jpg",
            "http://img02.tooopen.com/images/20140504/sy_60294738471.jpg",
            "http://img2.imgtn.bdimg.com/it/u=819201812,3553302270&fm=214&gp=0.jpg",
            "http://pic55.nipic.com/file/20141208/19462408_171130083000_2.jpg",
            "http://img05.tooopen.com/images/20150531/tooopen_sy_127457023651.jpg",
            "http://pic.58pic.com/58pic/17/41/38/88658PICNuP_1024.jpg"
    };
    private ImageView imageView;
    private Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            imageView.setImageBitmap((Bitmap) msg.obj);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_intent_service);
        imageView = findViewById(R.id.imageview);
        Intent intent = new Intent(this, MyIntentService.class);
        for (int i = 0; i < 7; i++) {
            intent.putExtra(MyIntentService.DOWNLOAD_URL, url[i]);
            intent.putExtra(MyIntentService.INDEX_FLAG, i);
            startService(intent);
        }
        MyIntentService.setUpdateUi(this);
    }

    @Override
    public void updateUi(Message message) {
        mHandler.sendMessageDelayed(message, message.what * 1000);
    }
}

MyIntentService

public class MyIntentService extends IntentService {
    public static final String DOWNLOAD_URL = "download_url";
    public static final String INDEX_FLAG = "index_flag";
    public static UpdateUI updateUI;


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

    @Override
    protected void onHandleIntent(@Nullable Intent intent) {

        Bitmap bitmap = downloadUrlBitmap(intent.getStringExtra(DOWNLOAD_URL));
        Message message = new Message();
        message.what = intent.getIntExtra(INDEX_FLAG, 0);
        message.obj = bitmap;
        if (updateUI != null) {
            updateUI.updateUi(message);
        }


    }

    public static void setUpdateUi(UpdateUI updateUi) {
        updateUI = updateUi;
    }

    private Bitmap downloadUrlBitmap(String urlString) {
        HttpURLConnection urlConnection = null;
        BufferedInputStream in = null;
        Bitmap bitmap = null;
        try {
            final URL url = new URL(urlString);
            urlConnection = (HttpURLConnection) url.openConnection();
            in = new BufferedInputStream(urlConnection.getInputStream(), 8 * 1024);
            bitmap = BitmapFactory.decodeStream(in);
        } catch (final IOException e) {
            e.printStackTrace();
        } finally {
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
            try {
                if (in != null) {
                    in.close();
                }
            } catch (final IOException e) {
                e.printStackTrace();
            }
        }
        return bitmap;
    }

    public interface UpdateUI {
        void updateUi(Message message);
    }
}

三.intenService源码分析
intenService源码也不是很多，先看看oncreate方法

  @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);
    }

先创建一个HandlerThread ，说明IntentService内部是利用HandlerThread进行消息传递的，然后创建了一个Handler（ServiceHandler），上一步创建了一个Looper，这个Looper是什么？Looper对象来自HandlerThread，ServiceHandler就变成了处理异步线程的执行类了。

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

 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);

        }
    }

主动调用 stopSelf(msg.arg1)这个方法，和stopSelf()方法不一样，没有参数会立即停止，等待所有消息处理完之后，终止任务。

protected abstract void onHandleIntent(@Nullable Intent intent);

它本质上就是封装了HandlerThread和Handler的异步框架。