【Android】《第一行代码—Android》第九章总结

前面几章介绍了Activity、BroadcastReceiver、ContentProvider等内容，本章对Android四大组件的最后一个组件Service进行了较为全面的讲解，涉及到Android多线程、Service的基本用法、Service生命周期的介绍等等东西，内容比较多，但都很好理解，我也是在一天之内就把这一章的内容全部看完的，所以趁现在夜深人静的时候，把总结尽可能详细的写下来，说做就做现在动手！

服务—Service—Android实现应用程序后台运行的解决方案，它非常适合去执行那些不需要和用户交互而且还要长期运行的任务。需要注意的是，服务并不是运行在一个独立的进程之中的，而是依赖于创建服务时所在的应用程序进程。换句话说，当某个应用程序的进程被杀掉时，所有依赖于该进程的服务也会停止运行。

Android多线程编程

Android多线程编程无论从语法上，还是从使用技巧上都与Java多线程编程类似，有三种方式开启子线程。下面会分别介绍。方式一很常规，直接自定义Thread抽象类的实现类，重写父类的run()方法并在方法体内添加耗时处理逻辑，如下

class MyThread extends Thread {    @Override    public void run() {        //耗时处理逻辑    }}

这样我们就只需要创建线程实现类的对象，通过对象调用器start()方法，这样run()方法体中的逻辑就会在子线程中运行了。如下

new MyThread().start();

鉴于方式一使用继承的方式，耦合性有点高。方式二主要使用实现Runnable()接口的方式来定义一个线程。如下

class MyThread implements Runnable {    @Override    public void run() {        //处理具体逻辑    }}

Thread构造函数接收一个Runnable参数，我们正好可以传入new MyThread() 匿名对象作为参数，接着调用start()方法启动线程，如下

new Thread(new MyThread()).start();

方式三的写法更为常见，使用匿名类的方式，如下

new Thread(new Runnable() {    @Override    public void run() {        //具体处理逻辑    }}).start();

接下来我们来尝试“通过子线程进行UI操作”，如果你真的以为子线程可以直接操作UI，程序会直接崩溃掉。事实上也确实如此，Android确实是不允许在子线程中进行UI操作的。但我们可以借助Android异步消息处理机制，完美解决面临的问题。先看代码如下

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical" >    <Button         android:id="@+id/change_text"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="@string/change_text"        />    <TextView        android:id="@+id/text"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_centerInParent="true"        android:text="@string/hello_world"        android:textSize="20sp"        /></RelativeLayout>

package com.example.androidthreadtest;import android.app.Activity;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import android.widget.TextView;public class MainActivity extends Activity implements OnClickListener {    private static final int UPDATE_TEXT = 1;    private TextView text;    private Button changeText;    private Handler handler = new Handler() {        public void handleMessage(Message msg) {            switch (msg.what) {            case UPDATE_TEXT:                // 在这里可以进行UI操作                text.setText("nice to meet you");                break;            default:                break;            }        }    };    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        text = (TextView) findViewById(R.id.text);        changeText = (Button) findViewById(R.id.change_text);        changeText.setOnClickListener(this);    }    @Override    public void onClick(View v) {        switch (v.getId()) {        case R.id.change_text:            new Thread(new Runnable() {                @Override                public void run() {                    Message message = new Message();                    message.what = UPDATE_TEXT;                    handler.sendMessage(message);   // 将Message对象发送出去                }            }).start();            break;        default:            break;        }    }}

上面分别给出了activity_main.xml布局文件和MainActivity.java文件的代码，我们可以通过点击按钮把hello world变成nice to meet you，那么处理的思路是什么呢？

首先Android的异步消息处理机制提供了一整套的消息处理流程，Handler既可以发送消息，又包含消息的处理逻辑。我们可以利用Handler对象调用其sendMessage()方法在子线程中发送出一条消息，这条消息就包含更新UI的指令，注意！这条消息是在子线程中发送出去的。很快主线程Handler对象就会接收到这条带有更新UI指令的消息，接着就会在主线程中完成对UI的更新操作。哈哈，到这里问题是不是解决很完美呢？其实上面的代码就是按照这个逻辑去写的。

Service基本用法

我们先要明确，服务的处理逻辑是独立于Activity之外的，换句话说，服务并不需要依赖于Activity而存在，所以自然的，定义Service的代码我们就用MyService类去实现，MyService类作为Service的实现类，不仅要继承自Service，还要实现Service的抽象方法。下面我们来看代码如下

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical" >    <Button         android:id="@+id/start_service"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="@string/start_service"        />    <Button         android:id="@+id/stop_service"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="@string/stop_service"        />    <Button         android:id="@+id/bind_service"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="@string/bind_service"        />    <Button         android:id="@+id/unbind_service"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="@string/unbind_service"        />    <Button         android:id="@+id/start_intent_service"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="@string/start_intentservice"        /></LinearLayout>

package com.example.servicetest;import android.app.Notification;import android.app.PendingIntent;import android.app.Service;import android.content.Intent;import android.os.Binder;import android.os.IBinder;import android.util.Log;public class MyService extends Service {    private DownloadBinder mBinder = new DownloadBinder();    class DownloadBinder extends Binder {        public void startDownload() {            Log.d("MyService", "startDownload executed");        }        public int getProgress() {            Log.d("MyService", "getProgress executed");            return 0;        }    }    @Override    public IBinder onBind(Intent intent) {        return mBinder;    }    @Override    public void onCreate() {        super.onCreate();        /*         * 下面的notification部分写法Android官方已经不推荐这样用         * 由于《第一行代码——Android》实在Android 4.0的基础上开发的         * 所以会这样。         *          * */        Notification notification = new Notification(R.drawable.ic_launcher, "notification comes", System.currentTimeMillis());        Intent notificationIntent = new Intent(this, MainActivity.class);        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);        notification.setLatestEventInfo(this, "this is title", "this is content", pendingIntent);        startForeground(1, notification);        Log.d("MyService", "onCreate executed");    }    @Override    public int onStartCommand(Intent intent, int flags, int startId) {        Log.d("MyService", "onStartCommand executed");        return super.onStartCommand(intent, flags, startId);    }    @Override    public void onDestroy() {        super.onDestroy();        Log.d("MyService", "onDestroy executed");    }}

package com.example.servicetest;import android.app.IntentService;import android.content.Intent;import android.util.Log;public class MyIntentService extends IntentService {    public MyIntentService() {        super("MyIntentService");   // 提供一个无参构造，在其内部调用其父类的有参构造函数    }    @Override    public void onDestroy() {        super.onDestroy();        Log.d("MyIntnetService", "onDestroy executed");    }    @Override    protected void onHandleIntent(Intent intent) {        // 打印当前线程的id        Log.d("MyIntnetService", "thread id is " + Thread.currentThread().getId());    }}

package com.example.servicetest;import android.app.Activity;import android.content.ComponentName;import android.content.Intent;import android.content.ServiceConnection;import android.os.Bundle;import android.os.IBinder;import android.util.Log;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;public class MainActivity extends Activity implements OnClickListener {    private Button startService;    private Button stopService;    private Button bindService;    private Button unbindService;    private MyService.DownloadBinder downloadBinder;    private ServiceConnection connection = new ServiceConnection() {        @Override        public void onServiceConnected(ComponentName name, IBinder service) {            downloadBinder = (MyService.DownloadBinder) service;            downloadBinder.startDownload();            downloadBinder.getProgress();        }        @Override        public void onServiceDisconnected(ComponentName name) {        }    };    private Button startIntentService;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        startService = (Button) findViewById(R.id.start_service);        stopService = (Button) findViewById(R.id.stop_service);        startService.setOnClickListener(this);        stopService.setOnClickListener(this);        bindService = (Button) findViewById(R.id.bind_service);        unbindService = (Button) findViewById(R.id.unbind_service);        bindService.setOnClickListener(this);        unbindService.setOnClickListener(this);        startIntentService = (Button) findViewById(R.id.start_intent_service);        startIntentService.setOnClickListener(this);    }    @Override    public void onClick(View v) {        switch (v.getId()) {        case R.id.start_service:            Intent startIntent = new Intent(this, MyService.class);            startService(startIntent);            break;        case R.id.stop_service:            Intent stopIntent = new Intent(this, MyService.class);            stopService(stopIntent);            break;        case R.id.bind_service:            Intent bindIntent = new Intent(this, MyService.class);            bindService(bindIntent, connection, BIND_AUTO_CREATE);            break;        case R.id.unbind_service:            unbindService(connection);  // 解绑服务            break;        case R.id.start_intent_service:            // 打印主线程id            Log.d("MainActivity", "thread id is " + Thread.currentThread().getId());            Intent intentService = new Intent(this, MyIntentService.class);            startService(intentService);            break;        default:            break;        }    }}

主要介绍了服务的创建、启动、销毁，以及可以自动销毁的服务—IntentService。

Service实践：后台执行定时服务

实践这部分我只贴出代码如下

<RelativeLayout 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"    tools:context="${relativePackage}.${activityClass}" >    <TextView        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="@string/hello_world" /></RelativeLayout>

package com.example.servicebestpractice;import java.util.Date;import android.app.AlarmManager;import android.app.PendingIntent;import android.app.Service;import android.content.Intent;import android.os.IBinder;import android.os.SystemClock;import android.util.Log;public class LongRunningService extends Service {    @Override    public IBinder onBind(Intent intent) {        return null;    }    @Override    public int onStartCommand(Intent intent, int flags, int startId) {        new Thread(new Runnable() {            @Override            public void run() {                Log.d("LongRunningService", "excuted at " + new Date().toString());            }        }).start();        AlarmManager manager = (AlarmManager) getSystemService(ALARM_SERVICE);        int anHour = 1 * 1000;        long triggerAtTime = SystemClock.elapsedRealtime() + anHour;        Intent i = new Intent(this, AlarmReceiver.class);        PendingIntent pi = PendingIntent.getBroadcast(this, 0, i, 0);        manager.setExact(AlarmManager.ELAPSED_REALTIME_WAKEUP, triggerAtTime, pi);        return super.onStartCommand(intent, flags, startId);    }}

package com.example.servicebestpractice;import android.content.BroadcastReceiver;import android.content.Context;import android.content.Intent;public class AlarmReceiver extends BroadcastReceiver {    @Override    public void onReceive(Context context, Intent intent) {        Intent i = new Intent(context, LongRunningService.class);        context.startService(i);    }}

package com.example.servicebestpractice;import android.app.Activity;import android.content.Intent;import android.os.Bundle;public class MainActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Intent intent = new Intent(this, LongRunningService.class);        startService(intent);    }}