安卓开发--AsyncTask

package com.cnn.asynctask;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;


public class MainActivity extends Activity {
	private Button btnButton;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        btnButton=(Button) findViewById(R.id.button1);
        btnButton.setOnClickListener(new OnClickListener() {
			
			@Override
			public void onClick(View v) {
				// TODO 自动生成的方法存根
				Intent intent = new Intent(MainActivity.this, ImageTest.class);
				startActivity(intent);
			}
		});
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

　　

package com.cnn.asynctask;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.opengl.Visibility;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.ImageView;
import android.widget.ProgressBar;

public class ImageTest extends Activity {
	private ImageView imageView;
	private ProgressBar bar;
	private String urlString="http://pic2.ooopic.com/01/03/51/25b1OOOPIC19.jpg";
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		// TODO 自动生成的方法存根
		super.onCreate(savedInstanceState);
		setContentView(R.layout.image);
		
		imageView=(ImageView) findViewById(R.id.image);
		bar=(ProgressBar) findViewById(R.id.bar);
		
		new ImageAsync().execute(urlString);
	}
	
	class ImageAsync extends AsyncTask<String, Void, Bitmap> {

		@Override
		protected Bitmap doInBackground(String... params) {
			// TODO 自动生成的方法存根
			//获取传递进来的参数
			String urlString=params[0];
			Bitmap bitmap=null;
			URLConnection connection;
			InputStream inputStream;
			try {
				connection=new URL(urlString).openConnection();
				inputStream=connection.getInputStream();
				BufferedInputStream bisBufferedInputStream= new BufferedInputStream(inputStream);
				//通过decodeStream方法解析输入流 转化为bitmap 
				bitmap=BitmapFactory.decodeStream(bisBufferedInputStream);
				inputStream.close();
				bisBufferedInputStream.close();
			} catch (MalformedURLException e) {
				// TODO 自动生成的 catch 块
				e.printStackTrace();
			} catch (IOException e) {
				// TODO 自动生成的 catch 块
				e.printStackTrace();
			}
			//bitmap作为返回值返回给后面调用的方法
			return bitmap;
		}
		
		@Override
		protected void onPreExecute() {
			// TODO 自动生成的方法存根
			super.onPreExecute();
			bar.setVisibility(View.VISIBLE);
		}
		
		@Override
		protected void onPostExecute(Bitmap result) {
			// TODO 自动生成的方法存根
			super.onPostExecute(result);
			bar.setVisibility(View.GONE);
			imageView.setImageBitmap(result);
		}
	}
}

　　

<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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.cnn.asynctask.MainActivity" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:text="Button" />

</RelativeLayout>

　　

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:padding="16dp"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" >
	<ImageView 
	    android:id="@+id/image"
	    android:layout_width="match_parent"
	    android:layout_height="match_parent"
	    />
	<ProgressBar 
	    android:id="@+id/bar"
	    android:visibility="gone"
	    android:layout_width="wrap_content"
	    android:layout_height="wrap_content"
	    android:layout_centerInParent="true"
	    />
</RelativeLayout>

　　

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.cnn.asynctask"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="21" />
    <uses-permission android:name="android.permission.INTERNET"/>

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        
         <activity
            android:name=".ImageTest"
            android:label="@string/app_name" >
        </activity>
    </application>

</manifest>