Android 異步加載圖片分析總結(jié)

更新時(shí)間：2013年06月03日 16:18:29 作者：

研究了android從網(wǎng)絡(luò)上異步加載圖像，現(xiàn)總結(jié)如下，感興趣的朋友可以了解下哈

研究了android從網(wǎng)絡(luò)上異步加載圖像，現(xiàn)總結(jié)如下：
（1）由于android UI更新支持單一線程原則，所以從網(wǎng)絡(luò)上取數(shù)據(jù)并更新到界面上，為了不阻塞主線程首先可能會(huì)想到以下方法。

在主線程中new 一個(gè)Handler對(duì)象，加載圖像方法如下所示

復(fù)制代碼代碼如下:

 
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); 
} 
}); 
} 

上面這個(gè)方法缺點(diǎn)很顯然，經(jīng)測(cè)試，如果要加載多個(gè)圖片，這并不能實(shí)現(xiàn)異步加載，而是等到所有的圖片都加載完才一起顯示，因?yàn)樗鼈兌歼\(yùn)行在一個(gè)線程中。

然后，我們可以簡(jiǎn)單改進(jìn)下，將Handler+Runnable模式改為Handler+Thread+Message模式不就能實(shí)現(xiàn)同時(shí)開啟多個(gè)線程嗎？
（2）在主線程中new 一個(gè)Handler對(duì)象，代碼如下：

復(fù)制代碼代碼如下:

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

對(duì)應(yīng)加載圖像代碼如下：

復(fù)制代碼代碼如下:

 
//采用handler+Thread模式實(shí)現(xiàn)多線程異步加載 
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) { 
} 
&nbsp; 
Message message= handler2.obtainMessage() ; 
message.arg1 = id; 
message.obj = drawable; 
handler2.sendMessage(message); 
} 
}; 
thread.start(); 
thread = null; 
} 

這樣就簡(jiǎn)單實(shí)現(xiàn)了異步加載了。細(xì)想一下，還可以優(yōu)化的，比如引入線程池、引入緩存等，我們先介紹線程池。
（3）引入ExecutorService接口，于是代碼可以優(yōu)化如下：
在主線程中加入：private ExecutorService executorService = Executors.newFixedThreadPool(5);
對(duì)應(yīng)加載圖像方法更改如下：

復(fù)制代碼代碼如下:

 
// 引入線程池來管理多線程 
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() { 
&nbsp; 
public void run() { 
((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable); 
} 
}); 
} catch (Exception e) { 
throw new RuntimeException(e); 
} 
} 
}); 
} 

4）為了更方便使用我們可以將異步加載圖像方法封裝一個(gè)類，對(duì)外界只暴露一個(gè)方法即可，考慮到效率問題我們可以引入內(nèi)存緩存機(jī)制，做法是建立一個(gè)HashMap，其鍵（key）為加載圖像url，其值（value）是圖像對(duì)象Drawable。先看一下我們封裝的類

復(fù)制代碼代碼如下:

 
public class AsyncImageLoader3 { 
//為了加快速度，在內(nèi)存中開啟緩存（主要應(yīng)用于重復(fù)圖片較多時(shí)，或者同一個(gè)圖片要多次被訪問，比如在ListView時(shí)來回滾動(dòng)） 
public Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>(); 
private ExecutorService executorService = Executors.newFixedThreadPool(5); //固定五個(gè)線程來執(zhí)行任務(wù) 
private final Handler handler=new Handler(); 
&nbsp; 
/** 
* 
* @param imageUrl 圖像url地址 
* @param callback 回調(diào)接口 
* <a href="\"http://www.eoeandroid.com/home.php?mod=space&uid=7300\"" target="\"_blank\"">@return</a> 返回內(nèi)存中緩存的圖像，第一次加載返回null 
*/ 
public Drawable loadDrawable(final String imageUrl, final ImageCallback callback) { 
//如果緩存過就從緩存中取出數(shù)據(jù) 
if (imageCache.containsKey(imageUrl)) { 
SoftReference<Drawable> softReference = imageCache.get(imageUrl); 
if (softReference.get() != null) { 
return softReference.get(); 
} 
} 
//緩存中沒有圖像，則從網(wǎng)絡(luò)上取出數(shù)據(jù)，并將取出的數(shù)據(jù)緩存到內(nèi)存中 
executorService.submit(new Runnable() { 
public void run() { 
try { 
final Drawable drawable = Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png"); 
&nbsp; 
imageCache.put(imageUrl, new SoftReference<Drawable>(drawable)); 
&nbsp; 
handler.post(new Runnable() { 
public void run() { 
callback.imageLoaded(drawable); 
} 
}); 
} catch (Exception e) { 
throw new RuntimeException(e); 
} 
} 
}); 
return null; 
} 
//從網(wǎng)絡(luò)上取數(shù)據(jù)方法 
protected Drawable loadImageFromUrl(String imageUrl) { 
try { 
return Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png"); 
} catch (Exception e) { 
throw new RuntimeException(e); 
} 
} 
//對(duì)外界開放的回調(diào)接口 
public interface ImageCallback { 
//注意 此方法是用來設(shè)置目標(biāo)對(duì)象的圖像資源 
public void imageLoaded(Drawable imageDrawable); 
} 
} 

這樣封裝好后使用起來就方便多了。在主線程中首先要引入AsyncImageLoader3 對(duì)象，然后直接調(diào)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載的圖像設(shè)置到目標(biāo)ImageView或其相關(guān)的組件上。

在主線程調(diào)用代碼：
先實(shí)例化對(duì)象 private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3();
調(diào)用異步加載方法：

復(fù)制代碼代碼如下:

 
//引入線程池，并引入內(nèi)存緩存功能,并對(duì)外部調(diào)用封裝了接口，簡(jiǎn)化調(diào)用過程 
private void loadImage4(final String url, final int id) { 
//如果緩存過就會(huì)從緩存中取出圖像，ImageCallback接口中方法也不會(huì)被執(zhí)行 
Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() { 
//請(qǐng)參見實(shí)現(xiàn)：如果第一次加載url時(shí)下面方法會(huì)執(zhí)行 
public void imageLoaded(Drawable imageDrawable) { 
((ImageView) findViewById(id)).setImageDrawable(imageDrawable); 
} 
}); 
if(cacheImage!=null){ 
((ImageView) findViewById(id)).setImageDrawable(cacheImage); 
} 
} 

5）同理，下面也給出采用Thread+Handler+MessageQueue+內(nèi)存緩存代碼，原則同（4），只是把線程池?fù)Q成了Thread+Handler+MessageQueue模式而已。代碼如下：

復(fù)制代碼代碼如下:

 
public class AsyncImageLoader { 
//為了加快速度，加入了緩存（主要應(yīng)用于重復(fù)圖片較多時(shí)，或者同一個(gè)圖片要多次被訪問，比如在ListView時(shí)來回滾動(dòng)） 
private Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>(); 
&nbsp; 
/** 
* 
* @param imageUrl 圖像url地址 
* @param callback 回調(diào)接口 
* @return 返回內(nèi)存中緩存的圖像，第一次加載返回null 
*/ 
public Drawable loadDrawable(final String imageUrl, final ImageCallback callback) { 
//如果緩存過就從緩存中取出數(shù)據(jù) 
if (imageCache.containsKey(imageUrl)) { 
SoftReference<Drawable> softReference = imageCache.get(imageUrl); 
if (softReference.get() != null) { 
return softReference.get(); 
} 
} 
&nbsp; 
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)); 
&nbsp; 
} 
&nbsp; 
}.start(); 
/* 
下面注釋的這段代碼是Handler的一種代替方法 
*/ 
// 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; 
} 
&nbsp; 
protected Drawable loadImageFromUrl(String imageUrl) { 
try { 
return Drawable.createFromStream(new URL(imageUrl).openStream(), "src"); 
} catch (Exception e) { 
throw new RuntimeException(e); 
} 
} 
//對(duì)外界開放的回調(diào)接口 
public interface ImageCallback { 
public void imageLoaded(Drawable imageDrawable); 
} 
} 

至此，異步加載就介紹完了，下面給出的代碼為測(cè)試用的完整代碼：

復(fù)制代碼代碼如下:

 
package com.bshark.supertelphone.activity; 
&nbsp; 
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; 
&nbsp; 
import java.io.IOException; 
import java.net.URL; 
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 
&nbsp; 
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); //固定五個(gè)線程來執(zhí)行任務(wù) 
private AsyncImageLoader asyncImageLoader = new AsyncImageLoader(); 
private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3(); 
&nbsp; 
@Override 
public void onCreate(Bundle savedInstanceState) { 
super.onCreate(savedInstanceState); 
setContentView(R.layout.main); 
&nbsp; 
// 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); 
&nbsp; 
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); 
&nbsp; 
// 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); 
&nbsp; 
// loadImage5("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1); 
// //為了測(cè)試緩存而模擬的網(wǎng)絡(luò)延時(shí) 
// 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); 
} 
&nbsp; 
@Override 
protected void onDestroy() { 
executorService.shutdown(); 
super.onDestroy(); 
} 
//線程加載圖像基本原理 
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模式實(shí)現(xiàn)多線程異步加載 
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) { 
} 
&nbsp; 
Message message= handler2.obtainMessage() ; 
message.arg1 = id; 
message.obj = drawable; 
handler2.sendMessage(message); 
} 
}; 
thread.start(); 
thread = null; 
} 
// 引入線程池來管理多線程 
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() { 
&nbsp; 
public void run() { 
((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable); 
} 
}); 
} catch (Exception e) { 
throw new RuntimeException(e); 
} 
} 
}); 
} 
//引入線程池，并引入內(nèi)存緩存功能,并對(duì)外部調(diào)用封裝了接口，簡(jiǎn)化調(diào)用過程 
private void loadImage4(final String url, final int id) { 
//如果緩存過就會(huì)從緩存中取出圖像，ImageCallback接口中方法也不會(huì)被執(zhí)行 
Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() { 
//請(qǐng)參見實(shí)現(xiàn)：如果第一次加載url時(shí)下面方法會(huì)執(zhí)行 
public void imageLoaded(Drawable imageDrawable) { 
((ImageView) findViewById(id)).setImageDrawable(imageDrawable); 
} 
}); 
if(cacheImage!=null){ 
((ImageView) findViewById(id)).setImageDrawable(cacheImage); 
} 
} 
&nbsp; 
//采用Handler+Thread+封裝外部接口 
private void loadImage5(final String url, final int id) { 
//如果緩存過就會(huì)從緩存中取出圖像，ImageCallback接口中方法也不會(huì)被執(zhí)行 
Drawable cacheImage = asyncImageLoader3.loadDrawable(url,new AsyncImageLoader3.ImageCallback() { 
//請(qǐng)參見實(shí)現(xiàn)：如果第一次加載url時(shí)下面方法會(huì)執(zhí)行 
public void imageLoaded(Drawable imageDrawable) { 
((ImageView) findViewById(id)).setImageDrawable(imageDrawable); 
} 
}); 
if(cacheImage!=null){ 
((ImageView) findViewById(id)).setImageDrawable(cacheImage); 
} 
} 
&nbsp; 
&nbsp; 
} 

xml文件大致如下：

復(fù)制代碼代碼如下:

 
<SPAN style="FONT-SIZE: 18px"><STRONG>< ?xml version="1.0" encoding="utf-8"?> 
&nbsp; 
< LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="fill_parent" 
android:orientation="vertical" 
android:layout_height="fill_parent" > 
<ImageView android:id="@+id/image1" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView> 
<ImageView android:id="@+id/image2" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView> 
<ImageView android:id="@+id/image3" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView> 
<ImageView android:id="@+id/image5" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView> 
<ImageView android:id="@+id/image4" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView> 
< /LinearLayout></STRONG></SPAN> 

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