Android开发学习之路--Service之初体验

    android最后一个组件便是service了，终于学习到最后一个组件了，从年前的开发环境的搭建，到现在学到最后一个组件花了三周的时间，期间记录的点点滴滴，照着书本学习编写的代码都受益匪浅，这里要感谢第一行代码这本书。三个星期除了三十和初一没有学习，其余时间坚持每天学习一个知识点，总算慢慢地学习了个大概，接下去继续学习其他的东西。说了这么多还是开始学习Service组件吧。

    Service从字面上理解就是服务的意思，也就是为应用程序提供服务，比如我们在播放一首歌的时候，我又想把它下载下来，那么我们就可以把他交给Service去处理。播放歌曲是在后台运行的，那么它也是个服务，还是来个例子吧。先新建工程ServiceTest。编写布局如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:layout_margin="10dp"
    android:padding="10dp"
    tools:context="com.example.jared.servicetest.MainActivity">

    <Button
        android:id="@+id/startService"
        android:text="启动Service"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

    <Button
        android:id="@+id/stopService"
        android:text="停止Service"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

</LinearLayout>

    这里实现了两个按钮启动和停止Service。接着新建一个MyService类，继承Service，代码如下：

package com.example.jared.servicetest;

import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.support.annotation.Nullable;
import android.util.Log;
import android.widget.Toast;

/**
 * Created by jared on 16/2/18.
 */
public class MyService extends Service{

    private static final String TAG = "MyService";

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        Log.d(TAG, "onCreate is called");
        Toast.makeText(MyService.this, "onCreate is called",Toast.LENGTH_SHORT).show();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.d(TAG, "onStartCommand is called");
        Toast.makeText(MyService.this, "onStartCommand is called",Toast.LENGTH_SHORT).show();
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy is called");
        Toast.makeText(MyService.this, "onDestroy is called",Toast.LENGTH_SHORT).show();
    }
}

    这里重写了onCreate方法，onStartCommand方法和onDestroy方法，这里在各个方法里加了些调试信息，接着我们再实现MainActivity的代码：

package com.example.jared.servicetest;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    private Button mStartServiceBtn;
    private Button mStopServiceBtn;

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

        mStartServiceBtn = (Button)findViewById(R.id.startService);
        mStopServiceBtn = (Button)findViewById(R.id.stopService);

        mStartServiceBtn.setOnClickListener(new myOnClickListener());
        mStopServiceBtn.setOnClickListener(new myOnClickListener());
    }

    private class myOnClickListener implements View.OnClickListener {
        @Override
        public void onClick(View view) {
            switch (view.getId()) {
                case R.id.startService:
                    Intent mStartIntent = new Intent(getApplicationContext(), MyService.class);
                    startService(mStartIntent);
                    break;
                case R.id.stopService:
                    Intent mSopIntent = new Intent(getApplicationContext(), MyService.class);
                    stopService(mSopIntent);
                    break;
                default:
                    break;
            }
        }
    }
}

    这里当按start的时候启动service，按stop的按钮的时候停止服务。先来运行下看下效果，先启动Service吧，效果如下：

         

    从上可知先调用了onCreate方法，然后调用onStartCommand方法。然后我们看下在运行着的服务，打开手机的setting下的apps里的running：

    从图可知ServiceTest已经在了，然后这个时候我们再点击启动Service的话，是不会再调用onCreate方法的，只会调用onStartCommand方法，只有stop了Service才可以，那我们先stop掉Service，效果如下：

    如图调用了onDestroy方法。接着再启动Service的话，就和上述一样了。

    上面代码中还有个方法那就是onBind方法了，从字面可以理解到这个方法是把Activity和Service绑定了，那样就可以通过Activity控制Service了。下面实现下代码，修改layout如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:layout_margin="10dp"
    android:padding="10dp"
    tools:context="com.example.jared.servicetest.MainActivity">

    <Button
        android:id="@+id/startService"
        android:text="启动Service"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

    <Button
        android:id="@+id/stopService"
        android:text="停止Service"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

    <Button
        android:id="@+id/startDownloadService"
        android:text="绑定Service Download"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

    <Button
        android:id="@+id/stopDownloadService"
        android:text="解绑Service Download"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

    <Button
        android:id="@+id/startGetProgressService"
        android:text="绑定Service GetProgress"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

    <Button
        android:id="@+id/stopGetProgressService"
        android:text="解绑Service GetProgress"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAllCaps="false"/>

</LinearLayout>

    这里添加了启动下载的服务和获取当前下载状态的服务。Service中添加Binder，代码如下：

package com.example.jared.servicetest;

import android.app.Service;
import android.content.Intent;
import android.os.Binder;
import android.os.IBinder;
import android.support.annotation.Nullable;
import android.util.Log;
import android.widget.Toast;

/**
 * Created by jared on 16/2/18.
 */
public class MyService extends Service{

    private static final String TAG = "MyService";

    private DownloadBinder mDownloadBinder = new DownloadBinder();

    class DownloadBinder extends Binder {
        private int progress = 0;
        public void startDownload() {
            Log.d(TAG, "startDownload is called");
            Toast.makeText(MyService.this, "startDownload is called",Toast.LENGTH_SHORT).show();
            progress = 0;
            new Thread(new Runnable(){
                @Override
                public void run() {
                    for (int i=0; i<=100; i++) {
                        try {
                            Thread.sleep(1000);
                            progress++;
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }).start();
        }
        public int getProgress() {
            Log.d(TAG, "getProgress is called");
            Toast.makeText(MyService.this, "getProgress is called",Toast.LENGTH_SHORT).show();
            return progress;
        }
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return mDownloadBinder;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        Log.d(TAG, "onCreate is called");
        Toast.makeText(MyService.this, "onCreate is called",Toast.LENGTH_SHORT).show();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.d(TAG, "onStartCommand is called");
        Toast.makeText(MyService.this, "onStartCommand is called",Toast.LENGTH_SHORT).show();
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy is called");
        Toast.makeText(MyService.this, "onDestroy is called",Toast.LENGTH_SHORT).show();
    }
}

    这里模拟了一个下载的过程，启动下载后，线程每秒更新个数值，然后可以通过getProgress去获取数值。接着修改MainActivity代码如下：

package com.example.jared.servicetest;

import android.content.ComponentName;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    private Button mStartServiceBtn;
    private Button mStopServiceBtn;
    private Button mBindService1Btn;
    private Button mUnbindService1Btn;
    private Button mBindService2Btn;
    private Button mUnbindService2Btn;

    private MyService.DownloadBinder downloadBinder;

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

        mStartServiceBtn = (Button)findViewById(R.id.startService);
        mStopServiceBtn = (Button)findViewById(R.id.stopService);
        mBindService1Btn = (Button)findViewById(R.id.startDownloadService);
        mUnbindService1Btn = (Button)findViewById(R.id.stopDownloadService);
        mBindService2Btn = (Button)findViewById(R.id.startGetProgressService);
        mUnbindService2Btn = (Button)findViewById(R.id.stopGetProgressService);

        mStartServiceBtn.setOnClickListener(new myOnClickListener());
        mStopServiceBtn.setOnClickListener(new myOnClickListener());
        mBindService1Btn.setOnClickListener(new myOnClickListener());
        mUnbindService1Btn.setOnClickListener(new myOnClickListener());
        mBindService2Btn.setOnClickListener(new myOnClickListener());
        mUnbindService2Btn.setOnClickListener(new myOnClickListener());
    }

    private class myOnClickListener implements View.OnClickListener {
        @Override
        public void onClick(View view) {
            switch (view.getId()) {
                case R.id.startService:
                    Intent mStartIntent = new Intent(getApplicationContext(), MyService.class);
                    startService(mStartIntent);
                    break;
                case R.id.stopService:
                    Intent mSopIntent = new Intent(getApplicationContext(), MyService.class);
                    stopService(mSopIntent);
                    break;
                case R.id.startDownloadService:
                    Intent mBindIntent = new Intent(getApplicationContext(), MyService.class);
                    bindService(mBindIntent, startDownloadInService, BIND_AUTO_CREATE);
                    break;
                case R.id.stopDownloadService:
                    unbindService(startDownloadInService);
                    break;
                case R.id.startGetProgressService:
                    Intent mGetProgress = new Intent(getApplicationContext(), MyService.class);
                    bindService(mGetProgress, getProgressInService, BIND_AUTO_CREATE);
                    break;
                case R.id.stopGetProgressService:
                    unbindService(getProgressInService);
                    break;
                default:
                    break;
            }
        }
    }

    private ServiceConnection startDownloadInService = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            downloadBinder = (MyService.DownloadBinder)iBinder;
            downloadBinder.startDownload();
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {

        }
    };

    private ServiceConnection getProgressInService = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            downloadBinder = (MyService.DownloadBinder)iBinder;
            String progressValue = String.valueOf(downloadBinder.getProgress());
            Toast.makeText(getApplicationContext(), "ProgressNow is:"+progressValue, Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {

        }
    };
}

    这里实例化了ServiceConnection，当通过bindService方法的时候，会通过ServiceConnection的onServiceConnected方法来调用Service中刚创建的下载类的方法。运行看下效果，先启动下载，再获取progress，运行效果如下：

    

     这里要注意的是，服务是单例的，所谓单例就是只有一个实例，只能被new一次，不同进程也只能调用一次。当我们调用bind download的时候会直接调用Service的onCreate，当我们解绑的时候，也会调用Service的onDestroy方法。不过当我们调用了启动Service，又调用了binder的话，那就得ubind和stop方法都得调用。

    上述当我们获取progress的时候，如果不解绑，那么之后的都是无效的。只有获取了一次，解绑了，再获取才有效。

    android系统当内存不够的时候会自动回收优先级比较低的服务，所以为了让服务可以一直保存运行状态，而不会由于内存不足的原因导致被回收，就需要使用前台服务。前台服务就是一直运行并在状态栏显示，类似音乐播放器就会在状态栏一直保存着。简单修改Service代码如下：

package com.example.jared.servicetest;

import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.os.Binder;
import android.os.IBinder;
import android.support.annotation.Nullable;
import android.support.v4.app.NotificationCompat;
import android.util.Log;
import android.widget.Toast;

/**
 * Created by jared on 16/2/18.
 */
public class MyService extends Service{

    private static final String TAG = "MyService";
    private static final int NOTIFICATION_ID = 1;

    private DownloadBinder mDownloadBinder = new DownloadBinder();

    class DownloadBinder extends Binder {
        private int progress = 0;
        public void startDownload() {
            Log.d(TAG, "startDownload is called");
            Toast.makeText(MyService.this, "startDownload is called",Toast.LENGTH_SHORT).show();
            progress = 0;
            new Thread(new Runnable(){
                @Override
                public void run() {
                    for (int i=0; i<=100; i++) {
                        try {
                            Thread.sleep(1000);
                            progress++;
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }).start();
        }
        public int getProgress() {
            Log.d(TAG, "getProgress is called");
            Toast.makeText(MyService.this, "getProgress is called",Toast.LENGTH_SHORT).show();
            return progress;
        }
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return mDownloadBinder;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        Log.d(TAG, "onCreate is called");
        Toast.makeText(MyService.this, "onCreate is called",Toast.LENGTH_SHORT).show();
        NotificationCompat.Builder notification = new NotificationCompat.Builder(this);
        notification.setSmallIcon(R.mipmap.ic_launcher);
        notification.setContentTitle("前台服务");
        notification.setContentText("这是个前台服务");
        Intent intent = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);
        notification.setContentIntent(pendingIntent);
        startForeground(NOTIFICATION_ID, notification.build());
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.d(TAG, "onStartCommand is called");
        Toast.makeText(MyService.this, "onStartCommand is called",Toast.LENGTH_SHORT).show();
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy is called");
        Toast.makeText(MyService.this, "onDestroy is called",Toast.LENGTH_SHORT).show();
        stopForeground(true);
    }
}

    这里用到了notification，然后通过startForegroid方法来启动一个前台服务，运行看下效果：

      

     从上图可知，当启动了前台服务的时候，在状态栏有个图标显示，当activity退出的时候，还是存在，只有当停止服务的时候才消失，那么就可以理解了为什么qq音乐退出了还在播放歌曲，还可以通过前台服务点击进入app。其实就是这个道理了。

    关于Service就先学习到这里了。总算把Service学习好了。接下去继续学习别的知识点了。