Android异步加载图像(含线程池,缓存方法)
Android异步加载图像(含线程池,缓存方法)
研究了android从网络上异步加载图像:
(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
[java] view plain copy print ?- privatevoidloadImage(finalStringurl,finalintid){
- handler.post(newRunnable(){
- publicvoidrun(){
- Drawabledrawable=null;
- try{
- drawable=Drawable.createFromStream(newURL(url).openStream(),"image.png");
- }catch(IOExceptione){
- }
- ((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:
- finalHandlerhandler2=newHandler(){
- @Override
- publicvoidhandleMessage(Messagemsg){
- ((ImageView)LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.obj);
- }
- };
对应加载图像代码如下:对应加载图像代码如下:对应加载图像代码如下:
- //引入线程池来管理多线程
- privatevoidloadImage3(finalStringurl,finalintid){
- executorService.submit(newRunnable(){
- publicvoidrun(){
- try{
- finalDrawabledrawable=Drawable.createFromStream(newURL(url).openStream(),"image.png");
- handler.post(newRunnable(){
- publicvoidrun(){
- ((ImageView)LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
- }
- });
- }catch(Exceptione){
- thrownewRuntimeException(e);
- }
- }
- });
- }
(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
[java] view plain copy print ?- publicclassAsyncImageLoader3{
- //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)
- publicMap<String,SoftReference<Drawable>>imageCache=newHashMap<String,SoftReference<Drawable>>();
- privateExecutorServiceexecutorService=Executors.newFixedThreadPool(5);//固定五个线程来执行任务
- privatefinalHandlerhandler=newHandler();
- /**
- *
- *@paramimageUrl图像url地址
- *@paramcallback回调接口
- *@return返回内存中缓存的图像,第一次加载返回null
- */
- publicDrawableloadDrawable(finalStringimageUrl,finalImageCallbackcallback){
- //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){
- returnsoftReference.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(newRunnable(){
- publicvoidrun(){
- try{
- finalDrawabledrawable=Drawable.createFromStream(newURL(imageUrl).openStream(),"image.png");
- imageCache.put(imageUrl,newSoftReference<Drawable>(drawable));
- handler.post(newRunnable(){
- publicvoidrun(){
- callback.imageLoaded(drawable);
- }
- });
- }catch(Exceptione){
- thrownewRuntimeException(e);
- }
- }
- });
- returnnull;
- }
- //从网络上取数据方法
- protectedDrawableloadImageFromUrl(StringimageUrl){
- try{
- returnDrawable.createFromStream(newURL(imageUrl).openStream(),"image.png");
- }catch(Exceptione){
- thrownewRuntimeException(e);
- }
- }
- //对外界开放的回调接口
- publicinterfaceImageCallback{
- //注意此方法是用来设置目标对象的图像资源
- publicvoidimageLoaded(DrawableimageDrawable);
- }
- }
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:
[java] view plain copy print ?- //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
- privatevoidloadImage4(finalStringurl,finalintid){
- //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
- DrawablecacheImage=asyncImageLoader.loadDrawable(url,newAsyncImageLoader.ImageCallback(){
- //请参见实现:如果第一次加载url时下面方法会执行
- publicvoidimageLoaded(DrawableimageDrawable){
- ((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: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:
- publicclassAsyncImageLoader{
- //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)
- privateMap<String,SoftReference<Drawable>>imageCache=newHashMap<String,SoftReference<Drawable>>();
- /**
- *
- *@paramimageUrl图像url地址
- *@paramcallback回调接口
- *@return返回内存中缓存的图像,第一次加载返回null
- */
- publicDrawableloadDrawable(finalStringimageUrl,finalImageCallbackcallback){
- //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){
- returnsoftReference.get();
- }
- }
- finalHandlerhandler=newHandler(){
- @Override
- publicvoidhandleMessage(Messagemsg){
- callback.imageLoaded((Drawable)msg.obj);
- }
- };
- newThread(){
- publicvoidrun(){
- Drawabledrawable=loadImageFromUrl(imageUrl);
- imageCache.put(imageUrl,newSoftReference<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
- */
- //newAsyncTask(){
- //@Override
- //protectedDrawabledoInBackground(Object...objects){
- //Drawabledrawable=loadImageFromUrl(imageUrl);
- //imageCache.put(imageUrl,newSoftReference<Drawable>(drawable));
- //returndrawable;
- //}
- //
- //@Override
- //protectedvoidonPostExecute(Objecto){
- //callback.imageLoaded((Drawable)o);
- //}
- //}.execute();
- returnnull;
- }
- protectedDrawableloadImageFromUrl(StringimageUrl){
- try{
- returnDrawable.createFromStream(newURL(imageUrl).openStream(),"src");
- }catch(Exceptione){
- thrownewRuntimeException(e);
- }
- }
- //对外界开放的回调接口
- publicinterfaceImageCallback{
- publicvoidimageLoaded(DrawableimageDrawable);
- }
- }
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:
- packagecom.bshark.supertelphone.activity;
- importandroid.app.Activity;
- importandroid.graphics.drawable.Drawable;
- importandroid.os.Bundle;
- importandroid.os.Handler;
- importandroid.os.Message;
- importandroid.widget.ImageView;
- importcom.bshark.supertelphone.R;
- importcom.bshark.supertelphone.ui.adapter.util.AsyncImageLoader;
- importcom.bshark.supertelphone.ui.adapter.util.AsyncImageLoader3;
- importjava.io.IOException;
- importjava.net.URL;
- importjava.util.concurrent.ExecutorService;
- importjava.util.concurrent.Executors;
- publicclassLazyLoadImageActivityextendsActivity{
- finalHandlerhandler=newHandler();
- finalHandlerhandler2=newHandler(){
- @Override
- publicvoidhandleMessage(Messagemsg){
- ((ImageView)LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.obj);
- }
- };
- privateExecutorServiceexecutorService=Executors.newFixedThreadPool(5);//固定五个线程来执行任务
- privateAsyncImageLoaderasyncImageLoader=newAsyncImageLoader();
- privateAsyncImageLoader3asyncImageLoader3=newAsyncImageLoader3();
- @Override
- publicvoidonCreate(BundlesavedInstanceState){
- 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);
- }
- @Override
- protectedvoidonDestroy(){
- executorService.shutdown();
- super.onDestroy();
- }
- //线程加载图像基本原理
- privatevoidloadImage(finalStringurl,finalintid){
- handler.post(newRunnable(){
- publicvoidrun(){
- Drawabledrawable=null;
- try{
- drawable=Drawable.createFromStream(newURL(url).openStream(),"image.png");
- }catch(IOExceptione){
- }
- ((ImageView)LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
- }
- });
- }
- //采用handler+Thread模式实现多线程异步加载
- privatevoidloadImage2(finalStringurl,finalintid){
- Threadthread=newThread(){
- @Override
- publicvoidrun(){
- Drawabledrawable=null;
- try{
- drawable=Drawable.createFromStream(newURL(url).openStream(),"image.png");
- }catch(IOExceptione){
- }
- Messagemessage=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
- privatevoidloadImage3(finalStringurl,finalintid){
- executorService.submit(newRunnable(){
- publicvoidrun(){
- try{
- finalDrawabledrawable=Drawable.createFromStream(newURL(url).openStream(),"image.png");
- handler.post(newRunnable(){
- publicvoidrun(){
- ((ImageView)LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable);
- }
- });
- }catch(Exceptione){
- thrownewRuntimeException(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
- privatevoidloadImage4(finalStringurl,finalintid){
- //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
- DrawablecacheImage=asyncImageLoader.loadDrawable(url,newAsyncImageLoader.ImageCallback(){
- //请参见实现:如果第一次加载url时下面方法会执行
- publicvoidimageLoaded(DrawableimageDrawable){
- ((ImageView)findViewById(id)).setImageDrawable(imageDrawable);
- }
- });
- if(cacheImage!=null){
- ((ImageView)findViewById(id)).setImageDrawable(cacheImage);
- }
- }
- //采用Handler+Thread+封装外部接口
- privatevoidloadImage5(finalStringurl,finalintid){
- //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
- DrawablecacheImage=asyncImageLoader3.loadDrawable(url,newAsyncImageLoader3.ImageCallback(){
- //请参见实现:如果第一次加载url时下面方法会执行
- publicvoidimageLoaded(DrawableimageDrawable){
- ((ImageView)findViewById(id)).setImageDrawable(imageDrawable);
- }
- });
- if(cacheImage!=null){
- ((ImageView)findViewById(id)).setImageDrawable(cacheImage);
- }
- }
- }
xml文件大致如下:
- <?xmlversion="1.0"encoding="utf-8"?>
- <LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"
- android:layout_width="fill_parent"
- android:orientation="vertical"
- android:layout_height="fill_parent">
- <ImageViewandroid:id="@+id/image1"android:layout_height="wrap_content"android:layout_width="fill_parent"></ImageView>
- <ImageViewandroid:id="@+id/image2"android:layout_height="wrap_content"android:layout_width="fill_parent"></ImageView>
- <ImageViewandroid:id="@+id/image3"android:layout_height="wrap_content"android:layout_width="fill_parent"></ImageView>
- <ImageViewandroid:id="@+id/image5"android:layout_height="wrap_content"android:layout_width="fill_parent"></ImageView>
- <ImageViewandroid:id="@+id/image4"android:layout_height="wrap_content"android:layout_width="fill_parent"></ImageView>
- </LinearLayout>
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