Android進階之IntentService 源碼解析

先來介紹下intentService和Service的區別

IntentService繼承與Service與Service有着相同的生命週期和方法，區別就在於IntentService是google工程師看到我們經常在Service裏面做一些耗時的操作就幫我們封裝一下，如果不封裝自己使用Service+Thread+Handler或者Service + HandlerThread也能實現。也就是說與service不同的是，IntentService在執行onCreate操作的時候，內部開了一個線程，去你執行你的耗時操作。

基本使用方法

package com.sym.service;

import android.app.IntentService;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.IBinder;
import android.os.Message;

import com.zejian.handlerlooper.util.LogUtils;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

/**
 * Created by zejian
 * Time 17/3/18.
 * Description:
 */
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 static void setUpdateUI(UpdateUI updateUIInterface){
        updateUI=updateUIInterface;
    }

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

    /**
     * 實現異步任務的方法
     * @param intent Activity傳遞過來的Intent,數據封裝在intent中
     */
    @Override
    protected void onHandleIntent(Intent intent) {

        //在子線程中進行網絡請求
        Bitmap bitmap=downloadUrlBitmap(intent.getStringExtra(DOWNLOAD_URL));
        Message msg1 = new Message();
        msg1.what = intent.getIntExtra(INDEX_FLAG,0);
        msg1.obj =bitmap;
        //通知主線程去更新UI
        if(updateUI!=null){
            updateUI.updateUI(msg1);
        }
        //mUIHandler.sendMessageDelayed(msg1,1000);

        LogUtils.e("onHandleIntent");
    }
    @Override
    public void onCreate() {
        LogUtils.e("onCreate");
        super.onCreate();
    }

    @Override
    public void onStart(Intent intent, int startId) {
        super.onStart(intent, startId);
        LogUtils.e("onStart");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        LogUtils.e("onStartCommand");
        return super.onStartCommand(intent, flags, startId);

    }

    @Override
    public void onDestroy() {
        LogUtils.e("onDestroy");
        super.onDestroy();
    }

    @Override
    public IBinder onBind(Intent intent) {
        LogUtils.e("onBind");
        return super.onBind(intent);
    }


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


    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;
    }

}

佈局文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/image"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

使用實例

public class IntentServiceActivity extends Activity implements MyIntentService.UpdateUI{
    /**
     * 圖片地址集合
     */
    private String url[] = {
            "https://img-blog.csdn.net/20160903083245762",
            "https://img-blog.csdn.net/20160903083252184",
            "https://img-blog.csdn.net/20160903083257871",
            "https://img-blog.csdn.net/20160903083257871",
            "https://img-blog.csdn.net/20160903083311972",
            "https://img-blog.csdn.net/20160903083319668",
            "https://img-blog.csdn.net/20160903083326871"
    };

    private static ImageView imageView;
    private static final Handler mUIHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            imageView.setImageBitmap((Bitmap) msg.obj);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_intent_service);
        imageView = (ImageView) findViewById(R.id.image);

        Intent intent = new Intent(this,MyIntentService.class);
        for (int i=0;i<7;i++) {//循環啓動任務
            intent.putExtra(MyIntentService.DOWNLOAD_URL,url[i]);
            intent.putExtra(MyIntentService.INDEX_FLAG,i);
            startService(intent);
        }
        MyIntentService.setUpdateUI(this);
    }

    //必須通過Handler去更新，該方法爲異步方法，不可更新UI
    @Override
    public void updateUI(Message message) {
        mUIHandler.sendMessageDelayed(message,message.what * 1000);
    }
}

原理解析

讓我們先來看看IntentService的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);
}

發現了吧，它在Service內部創建了一個HandlerThread,並且創建了一個ServiceHandler對象並將該Handler對象和HandlerThread的內部Looper關聯了起來，這和我們在上一篇中Android 進階之HandlerThread中的使用方法是一樣的，如果不熟悉HandlerThread的使用的可以先看看這篇文章鞏固一下。

讓我們再來看看onStart() 和onStartCommand()方法

@Override
public void onStart(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(Intent intent, int flags, int startId) {
    onStart(intent, startId);
    return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
}

回顧一下Service的基本使用
startService(Intent intent) 開啓一個Service 並通過Intent來傳遞數據，查看源碼會發現google的官方文檔有這麼句話

 * <p>If you need your application to run on platform versions prior to API
     * level 5, you can use the following model to handle the older {@link #onStart}
     * callback in that case.  The <code>handleCommand</code> method is implemented by
     * you as appropriate:

在 api 5及之前的版本會調用onStart()方法在之後都是調用了onStartCommand()方法，所以上面的邏輯我們就梳理通了，如果有多任務的耗時操作（上面的圖片下載）我們寫了一個循環多次調用startService(Intent intent)方法將下載的url地址傳遞給IntentService 然後調用IntentService的OnStartCommand 方法，然後通過mServiceHandler 將任務加入到HandlerThread的消息隊列中。

查看下ServiceHandler源碼

 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);
        }
    }

發現我們就在onHandleIntent 方法中做耗時的操作。

附上IntentService源碼

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.app;

import android.annotation.WorkerThread;
import android.annotation.Nullable;
import android.content.Intent;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.IBinder;
import android.os.Looper;
import android.os.Message;

/**
 * IntentService is a base class for {@link Service}s that handle asynchronous
 * requests (expressed as {@link Intent}s) on demand.  Clients send requests
 * through {@link android.content.Context#startService(Intent)} calls; the
 * service is started as needed, handles each Intent in turn using a worker
 * thread, and stops itself when it runs out of work.
 *
 * <p>This "work queue processor" pattern is commonly used to offload tasks
 * from an application's main thread.  The IntentService class exists to
 * simplify this pattern and take care of the mechanics.  To use it, extend
 * IntentService and implement {@link #onHandleIntent(Intent)}.  IntentService
 * will receive the Intents, launch a worker thread, and stop the service as
 * appropriate.
 *
 * <p>All requests are handled on a single worker thread -- they may take as
 * long as necessary (and will not block the application's main loop), but
 * only one request will be processed at a time.
 *
 * <div class="special reference">
 * <h3>Developer Guides</h3>
 * <p>For a detailed discussion about how to create services, read the
 * <a href="{@docRoot}guide/components/services.html">Services</a> developer
 * guide.</p>
 * </div>
 *
 * @see android.os.AsyncTask
 */
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);
}