Android后台进程与UI进程的交互

当一个程序第一次启动时，Android会同时启动一个对应的主线程(MainThread)，主线程主要负责处理与UI相关的事件，如：用户的按键事件，用户接触屏幕的事件以及屏幕绘图事件，并把相关的事件分发到对应的组件进行处理。所以主线程通常又被叫做UI线程。 

比如说从网上获取一个图片，在一个ImageView中将其显示出来，这种涉及到网络操作的程序一般都是需要开一个线程完成网络访问，但是在获得图片后，是不能直接在网络操作线程中调用ImageView的相关方法的，因为其他线程中是不能直接访问主UI线程成员。

完整代码：

注意的是：Manifest.xml文件中，要声明网络权限


001 package com.xsjayz.thread;

002 

003importorg.apache.http.HttpResponse;

004importorg.apache.http.client.HttpClient;

005importorg.apache.http.client.methods.HttpGet;

006importorg.apache.http.impl.client.DefaultHttpClient;

007 

008importandroid.app.Activity;

009importandroid.graphics.Bitmap;

010importandroid.graphics.BitmapFactory;

011importandroid.os.Bundle;

012importandroid.os.Handler;

013importandroid.os.Message;

014importandroid.view.View;

015importandroid.view.View.OnClickListener;

016importandroid.widget.Button;

017importandroid.widget.ImageView;

018importandroid.widget.Toast;

019 

020

026publicclass ThreadHandlerActivity extends Activity {

027 

028    privatestatic final intMSG_SUCCESS = 0;// 获取图片成功的标识

029    privatestatic final intMSG_FAILURE = 1;// 获取图片失败的标识

030 

031    privateImageView mImageView;

032    privateButton mButton;

033    privateThread mThread;

034 

035    @Override

036    publicvoid onCreate(Bundle savedInstanceState) {

037        super.onCreate(savedInstanceState);

038        setContentView(R.layout.main);

039 

040        mImageView= (ImageView) findViewById(R.id.imageView);

041        mButton= (Button) findViewById(R.id.button);

042 

043        mButton.setOnClickListener(newOnClickListener() {

044            @Override

045            publicvoid onClick(Viewv) {

046                //如果线程还没启动，则启动新的线程

047                if(mThread == null) {

048                    mThread= new Thread(runnable);

049                    mThread.start();

050 

051                    //否则提示："线程已经启动"

052                }else {

053                    Toast.makeText(

054                            getApplication(),

055                            getApplication().getString(R.string.thread_started),

056                            Toast.LENGTH_LONG).show();

057                }

058            }

059        });

060    }

061 

062    privateHandler mHandler = new Handler() {

063        //重写handleMessage()方法，此方法在UI线程运行

064        @Override

065        publicvoid handleMessage(Message msg) {

066            switch(msg.what) {

067            //如果成功，则显示从网络获取到的图片

068            caseMSG_SUCCESS:

069                mImageView.setImageBitmap((Bitmap)msg.obj);

070                Toast.makeText(getApplication(),

071                        getApplication().getString(R.string.get_pic_success),

072                        Toast.LENGTH_LONG).show();

073                break;

074            //否则提示失败

075            caseMSG_FAILURE:

076                Toast.makeText(getApplication(),

077                        getApplication().getString(R.string.get_pic_failure),

078                        Toast.LENGTH_LONG).show();

079                break;

080            }

081        }

082    };

083 

084    Runnablerunnable = new Runnable() {

085        //重写run()方法，此方法在新的线程中运行

086        @Override

087        publicvoid run(){

088            HttpClienthttpClient = new DefaultHttpClient();

089            //从网络上获取图片

090            HttpGethttpGet = new HttpGet(

091                    "http://www.oschina.net/img/logo.gif");

092            finalBitmap bitmap;

093            try{

094                HttpResponsehttpResponse = httpClient.execute(httpGet);

095                //解析为图片

096                bitmap= BitmapFactory.decodeStream(httpResponse.getEntity()

097                        .getContent());

098            }catch (Exceptione) {

099                mHandler.obtainMessage(MSG_FAILURE).sendToTarget();//获取图片失败

100                return;

101            }

102 

103            //获取图片成功，向UI线程发送MSG_SUCCESS标识和bitmap对象

104            mHandler.obtainMessage(MSG_SUCCESS,bitmap).sendToTarget();

105        }

106    };

107 

108}