Android IPC分析

说到Android的IPC（Inter-Process Conmmunication）首先想到的就是Handler和Looper，Handler用于多进程之间的通信和数据交换，将各进程之间通信的数据Message放置到Message Queue里，而Looper用于创建各进程自身的message queue，然后在适当的时候分发给相应的进程。

我们知道在Android中，每一个UI线程是一个主线程（Main Thread），Android为每一个主线程维护一个Message Queue，当用户需要长时间的背景线程操作的时候，需要create自己的new thread，这样的new thread是没有自己的message queue的，只能共享主线程的message queue并且将所做的运算结果和数据通过Handler发送到主线程的message queue里，被主线程共享。

    <?xml version="1.0" encoding="utf-8"?>  
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  
        android:orientation="vertical"  
        android:layout_width="fill_parent"  
        android:layout_height="fill_parent"  
        >  
      
        <ProgressBar   
            android:id="@+id/ProgressBar01"   
            android:layout_width="150dip"  
            android:layout_height="wrap_content"  
            style="?android:attr/progressBarStyleHorizontal">  
        </ProgressBar>  
          
    </LinearLayout>  

 这个xml文件创建了一个progressbar，并且将style设置成水平，

style="?android:attr/progressBarStyleHorizontal

    <?xml version="1.0" encoding="utf-8"?>  
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"  
          package="waterlife.ipc.demo"  
          android:versionCode="1"  
          android:versionname="1.0">  
        <application android:icon="@drawable/icon" android:label="@string/app_name">  
            <activity android:name=".IPCConmunication"  
                      android:label="@string/app_name">  
                <intent-filter>  
                    <action android:name="android.intent.action.MAIN" />  
                    <category android:name="android.intent.category.LAUNCHER" />  
                </intent-filter>  
            </activity>  
      
        </application>  
        <uses-permission android:name="android.permission.INTERNET" />  
      
    </manifest>   

 为了访问网络，需要在manifest file里设置access internet的permission，

    <uses-permission android:name="android.permission.INTERNET" />

    package waterlife.ipc.demo;  
      
    import java.io.InputStream;  
    import java.net.MalformedURLException;  
    import java.net.URL;  
    import java.net.URLConnection;  
    import android.app.Activity;  
    import android.os.Bundle;  
    import android.os.Handler;  
    import android.os.HandlerThread;  
    import android.os.Looper;  
    import android.os.Message;  
    import android.widget.ProgressBar;  
          
    public class IPCConmunication extends Activity {  
        static ProgressBar pb;  
        final int UPDATE_PROGRESS_BAR = 1000;  
        /** Called when the activity is first created. */  
        @Override  
        public void onCreate(Bundle savedInstanceState) {  
            super.onCreate(savedInstanceState);  
            setContentView(R.layout.main);  
            pb = (ProgressBar)findViewById(R.id.ProgressBar01);  
            Download dl = new Download();  
            new Thread(dl).start();  
              
        }  
          
        Handler mHandle = new Handler()  
        {  
           public void handleMessage(Message msg)  
            {  
                switch(msg.what)  
                {  
                    case UPDATE_PROGRESS_BAR:  
                        pb.setProgress(msg.arg1);  
                        break;  
                    default:  
                        break;  
                }  
            }  
        };  
      
        class Download implements Runnable  
        {  
      
            @Override  
            public void run() {  
                int totalSize = 0;  
                InputStream recevier = null;  
      
                 try {  
                    URL myUrl = new URL("http://bbs.nju.edu.cn");  
      
                    URLConnection urlConn = myUrl.openConnection();  
                    totalSize = urlConn.getContentLength();  
                    recevier = urlConn.getInputStream();  
                    byte[] b =new byte[256];  
                    int length = 0;  
                    length += recevier.read(b);  
                    while(length < totalSize)  
                    {  
                        Message msg = mHandle.obtainMessage(UPDATE_PROGRESS_BAR);  
                        msg.arg1 = (int)(length*100/totalSize);  
                        if(mHandle.hasMessages(UPDATE_PROGRESS_BAR))  
                        {  
                            mHandle.removeMessages(UPDATE_PROGRESS_BAR);  
                        }  
                        mHandle.sendMessage(msg);  
                        length += recevier.read(b);  
                        Thread.sleep(1000);  //睡眠1S，这个方法是不值得推荐的，因为它会使线程独占CPU，在以后的例子会使用更加有效的方法  
                    }  
                    recevier.close();  
                } catch (MalformedURLException e) {  
                    // TODO Auto-generated catch block  
                    e.printStackTrace();  
                }catch (Exception ex)  
                {  
                    ex.printStackTrace();  
                }      
            }  
        }  
    }