Android图片异步加载的兑现_Android

Android图片异步加载的实现

转帖自http://blog.csdn.net/sgl870927/article/details/6285535，谢谢该童鞋为Android开发做出的贡献。

研究了android从网络上异步加载图像，现总结如下：

（1）由于android UI更新支持单一线程原则，所以从网络上取数据并更新到界面上，为了不阻塞主线程首先可能会想到以下方法。

???? 在主线程中new 一个Handler对象，加载图像方法如下所示

??? private void loadImage(final String url, final int id) {
??????? ?handler.post(new Runnable() {
??????????????? public void run() {
??????????????????? Drawable drawable = null;
??????????????????? try {
??????????????????????? drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
??????????????????? } catch (IOException e) {
??????????????????? }
??????????????????? ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
??????????????? }
??????????? });
??? }

上面这个方法缺点很显然，经测试，如果要加载多个图片，这并不能实现异步加载，而是等到所有的图片都加载完才一起显示，因为它们都运行在一个线程中。

然后，我们可以简单改进下，将Handler+Runnable模式改为Handler+Thread+Message模式不就能实现同时开启多个线程吗？

（2）在主线程中new 一个Handler对象，代码如下：

??? final Handler handler2=new Handler(){
????????? @Override
????????? public void handleMessage(Message msg) {
???????????? ((ImageView) LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.obj);
????????? }
????? };

??? 对应加载图像代码如下：

? //采用handler+Thread模式实现多线程异步加载
???? private void loadImage2(final String url, final int id) {
???????? Thread thread = new Thread(){
???????????? @Override
???????????? public void run() {
?????????????? Drawable drawable = null;
??????????????????? try {
??????????????????????? drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
??????????????????? } catch (IOException e) {
??????????????????? }

这样就简单实现了异步加载了。细想一下，还可以优化的，比如引入线程池、引入缓存等，我们先介绍线程池。

（3）引入ExecutorService接口，于是代码可以优化如下：

?????? 在主线程中加入：private ExecutorService executorService = Executors.newFixedThreadPool(5);???

?????? 对应加载图像方法更改如下：

????? // 引入线程池来管理多线程
??? private void loadImage3(final String url, final int id) {
??????? executorService.submit(new Runnable() {
??????????? public void run() {
??????????????? try {
??????????????????? final Drawable drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
??????????????????? handler.post(new Runnable() {

（4）为了更方便使用我们可以将异步加载图像方法封装一个类，对外界只暴露一个方法即可，考虑到效率问题我们可以引入内存缓存机制，做法是

建立一个HashMap，其键（key）为加载图像url，其值（value）是图像对象Drawable。先看一下我们封装的类

public class AsyncImageLoader3 {
?? //为了加快速度，在内存中开启缓存（主要应用于重复图片较多时，或者同一个图片要多次被访问，比如在ListView时来回滚动）
??? public Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();
??? private ExecutorService executorService = Executors.newFixedThreadPool(5);??? //固定五个线程来执行任务
??? private final Handler handler=new Handler();

???? /**
???? *
???? * @param imageUrl???? 图像url地址
???? * @param callback???? 回调接口
???? * @return???? 返回内存中缓存的图像，第一次加载返回null
???? */
??? public Drawable loadDrawable(final String imageUrl, final ImageCallback callback) {
??????? //如果缓存过就从缓存中取出数据
??????? if (imageCache.containsKey(imageUrl)) {
??????????? SoftReference<Drawable> softReference = imageCache.get(imageUrl);
??????????? if (softReference.get() != null) {
??????????????? return softReference.get();
??????????? }
??????? }
??????? //缓存中没有图像，则从网络上取出数据，并将取出的数据缓存到内存中
???????? executorService.submit(new Runnable() {
??????????? public void run() {
??????????????? try {
??????????????????? final Drawable drawable = Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png");

??????????????????? imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));

??????????????????? handler.post(new Runnable() {
??????????????????????? public void run() {
?????????????????????????? callback.imageLoaded(drawable);
??????????????????????? }
??????????????????? });
??????????????? } catch (Exception e) {
??????????????????? throw new RuntimeException(e);
??????????????? }
??????????? }
??????? });
??????? return null;
??? }
???? //从网络上取数据方法
??? protected Drawable loadImageFromUrl(String imageUrl) {
??????? try {
??????????? return Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png");
??????? } catch (Exception e) {
??????????? throw new RuntimeException(e);
??????? }
??? }
??? //对外界开放的回调接口
??? public interface ImageCallback {
??????? //注意此方法是用来设置目标对象的图像资源
??????? public void imageLoaded(Drawable imageDrawable);
??? }
}这样封装好后使用起来就方便多了。在主线程中首先要引入AsyncImageLoader3 对象，然后直接调用其loadDrawable方法即可，需要注意的是ImageCallback接口的imageLoaded方法是唯一可以把加载的图像设置到目标ImageView或其相关的组件上。

在主线程调用代码：

? 先实例化对象?private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3();

? 调用异步加载方法：

//引入线程池，并引入内存缓存功能,并对外部调用封装了接口，简化调用过程
??? private void loadImage4(final String url, final int id) {
????????? //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行
???????? Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() {
???????????? //请参见实现：如果第一次加载url时下面方法会执行
???????????? public void imageLoaded(Drawable imageDrawable) {
?????????????? ((ImageView) findViewById(id)).setImageDrawable(imageDrawable);
???????????? }
???????? });
??????? if(cacheImage!=null){
????????? ((ImageView) findViewById(id)).setImageDrawable(cacheImage);
??????? }
??? }

（5）同理，下面也给出采用Thread+Handler+MessageQueue+内存缓存代码，原则同（4），只是把线程池换成了Thread+Handler+MessageQueue模式而已。代码如下：

public class AsyncImageLoader {
?? //为了加快速度，加入了缓存（主要应用于重复图片较多时，或者同一个图片要多次被访问，比如在ListView时来回滚动）
??? private Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();

??????? final Handler handler = new Handler() {
??????????? @Override
??????????? public void handleMessage(Message msg) {
??????????????? callback.imageLoaded((Drawable) msg.obj);
??????????? }
??????? };
??????? new Thread() {
??????????? public void run() {
??????????????? Drawable drawable = loadImageFromUrl(imageUrl);
??????????????? imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
??????????????? handler.sendMessage(handler.obtainMessage(0, drawable));

??????????? }

??????? }.start();
??????? /*
??????? 下面注释的这段代码是Handler的一种代替方法
???????? */
//??????? new AsyncTask() {
//??????????? @Override
//??????????? protected Drawable doInBackground(Object... objects) {
//????????????????? Drawable drawable = loadImageFromUrl(imageUrl);
//??????????????? imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
//??????????????? return? drawable;
//??????????? }
//
//??????????? @Override
//??????????? protected void onPostExecute(Object o) {
//????????????????? callback.imageLoaded((Drawable) o);
//??????????? }
//??????? }.execute();
??????? return null;
??? }

??? protected Drawable loadImageFromUrl(String imageUrl) {
??????? try {
??????????? return Drawable.createFromStream(new URL(imageUrl).openStream(), "src");
??????? } catch (Exception e) {
??????????? throw new RuntimeException(e);
??????? }
??? }
??? //对外界开放的回调接口
??? public interface ImageCallback {
??????? public void imageLoaded(Drawable imageDrawable);
??? }
}

至此，异步加载就介绍完了，下面给出的代码为测试用的完整代码：

package com.bshark.supertelphone.activity;

import android.app.Activity;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.ImageView;
import com.bshark.supertelphone.R;
import com.bshark.supertelphone.ui.adapter.util.AsyncImageLoader;
import com.bshark.supertelphone.ui.adapter.util.AsyncImageLoader3;

import java.io.IOException;
import java.net.URL;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class LazyLoadImageActivity extends Activity {
?????? final Handler handler=new Handler();
????? final Handler handler2=new Handler(){
????????? @Override
????????? public void handleMessage(Message msg) {
???????????? ((ImageView) LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.obj);
????????? }
????? };
?private ExecutorService executorService = Executors.newFixedThreadPool(5);??? //固定五个线程来执行任务
??? private AsyncImageLoader asyncImageLoader = new AsyncImageLoader();
??? private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3();

[email protected]
?public void onCreate(Bundle savedInstanceState) {
??super.onCreate(savedInstanceState);
??setContentView(R.layout.main);
??
//??loadImage("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//??loadImage("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//??loadImage("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//??????? loadImage("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//??loadImage("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);

??????? loadImage2("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
??loadImage2("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
??loadImage2("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
??????? loadImage2("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
??loadImage2("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);
//??????? loadImage3("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//??loadImage3("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//??loadImage3("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//??????? loadImage3("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//??loadImage3("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);

//??????? loadImage4("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//??loadImage4("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//??loadImage4("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//??????? loadImage4("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//??loadImage4("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);

//??????? loadImage5("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);
//??????? //为了测试缓存而模拟的网络延时
//??????? SystemClock.sleep(2000);
//??loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);
//??????? SystemClock.sleep(2000);
//??loadImage5("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);
//??????? SystemClock.sleep(2000);
//??????? loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
//??????? SystemClock.sleep(2000);
//??loadImage5("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);
//??????? SystemClock.sleep(2000);
//???????? loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);
?}

[email protected]
?protected void onDestroy() {
??executorService.shutdown();
??super.onDestroy();
?}
??? //线程加载图像基本原理
??? private void loadImage(final String url, final int id) {
??????? ?handler.post(new Runnable() {
??????????????? public void run() {
??????????????????? Drawable drawable = null;
??????????????????? try {
??????????????????????? drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
??????????????????? } catch (IOException e) {
??????????????????? }
??????????????????? ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
??????????????? }
??????????? });
??? }
???? //采用handler+Thread模式实现多线程异步加载
???? private void loadImage2(final String url, final int id) {
???????? Thread thread = new Thread(){
???????????? @Override
???????????? public void run() {
?????????????? Drawable drawable = null;
??????????????????? try {
??????????????????????? drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
??????????????????? } catch (IOException e) {
??????????????????? }

??????????????? Message message= handler2.obtainMessage() ;
???????????????? message.arg1 = id;
???????????????? message.obj = drawable;
???????????????? handler2.sendMessage(message);
???????????? }
???????? };
???????? thread.start();
???????? thread = null;
??? }
??? // 引入线程池来管理多线程
??? private void loadImage3(final String url, final int id) {
??????? executorService.submit(new Runnable() {
??????????? public void run() {
??????????????? try {
??????????????????? final Drawable drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png");
??????????????????? handler.post(new Runnable() {

??????????????????????? public void run() {
??????????????????????????? ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
??????????????????????? }
??????????????????? });
??????????????? } catch (Exception e) {
??????????????????? throw new RuntimeException(e);
??????????????? }
??????????? }
??????? });
??? }
??? //引入线程池，并引入内存缓存功能,并对外部调用封装了接口，简化调用过程
??? private void loadImage4(final String url, final int id) {
????????? //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行
???????? Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() {
???????????? //请参见实现：如果第一次加载url时下面方法会执行
???????????? public void imageLoaded(Drawable imageDrawable) {
?????????????? ((ImageView) findViewById(id)).setImageDrawable(imageDrawable);
???????????? }
???????? });
??????? if(cacheImage!=null){
????????? ((ImageView) findViewById(id)).setImageDrawable(cacheImage);
??????? }
??? }

??? //采用Handler+Thread+封装外部接口
??? private void loadImage5(final String url, final int id) {
????????? //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行
???????? Drawable cacheImage = asyncImageLoader3.loadDrawable(url,new AsyncImageLoader3.ImageCallback() {
???????????? //请参见实现：如果第一次加载url时下面方法会执行
???????????? public void imageLoaded(Drawable imageDrawable) {
?????????????? ((ImageView) findViewById(id)).setImageDrawable(imageDrawable);
???????????? }
???????? });
??????? if(cacheImage!=null){
??????????????????? ((ImageView) findViewById(id)).setImageDrawable(cacheImage);
??????? }
??? }

}

xml文件大致如下：

<?xml version="1.0" encoding="utf-8"?>