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(xiàn)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(xiàn)程首先可能會(huì)想到以下方法。
在主線(xiàn)程中new 一個(gè)Handler對(duì)象，加載圖像方法如下所示
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(xiàn)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(kāi)啟多個(gè)線(xiàn)程嗎？
（2）在主線(xiàn)程中new 一個(gè)Handler對(duì)象，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：
//采用handler+Thread模式實(shí)現(xiàn)多線(xiàn)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) {
}
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?xún)?yōu)化的，比如引入線(xiàn)程池、引入緩存等，我們先介紹線(xiàn)程池。
（3）引入ExecutorService接口，于是代碼可以?xún)?yōu)化如下：
在主線(xiàn)程中加入：private ExecutorService executorService = Executors.newFixedThreadPool(5);
對(duì)應(yīng)加載圖像方法更改如下：
// 引入線(xiàn)程池來(lái)管理多線(xiàn)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);
}
}
});
}
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(lèi)，對(duì)外界只暴露一個(gè)方法即可，考慮到效率問(wèn)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(lèi)
public class AsyncImageLoader3 {
//為了加快速度，在內(nèi)存中開(kāi)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(fǎng)問(wèn)，比如在ListView時(shí)來(lái)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(xiàn)程來(lái)執(zhí)行任務(wù)
private final Handler handler=new Handler();
/**
*
* @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(guò)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(méi)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");
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;
}
//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(kāi)放的回調(diào)接口
public interface ImageCallback {
//注意 此方法是用來(lái)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(lái)就方便多了。在主線(xiàn)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(xiàn)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：
//引入線(xiàn)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(guò)程
private void loadImage4(final String url, final int id) {
//如果緩存過(guò)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(jiàn)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(xiàn)程池?fù)Q成了Thread+Handler+MessageQueue模式而已。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(fǎng)問(wèn)，比如在ListView時(shí)來(lái)回滾動(dòng)）
private Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();
/**
*
* @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(guò)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();
}
}
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;
}
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(kāi)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：
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); //固定五個(gè)線(xiàn)程來(lái)執(zhí)行任務(wù)
private AsyncImageLoader asyncImageLoader = new AsyncImageLoader();
private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3();
@Override
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);
// //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);
}
@Override
protected void onDestroy() {
executorService.shutdown();
super.onDestroy();
}
//線(xiàn)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(xiàn)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) {
}
Message message= handler2.obtainMessage() ;
message.arg1 = id;
message.obj = drawable;
handler2.sendMessage(message);
}
};
thread.start();
thread = null;
}
// 引入線(xiàn)程池來(lái)管理多線(xiàn)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);
}
}
});
}
//引入線(xiàn)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(guò)程
private void loadImage4(final String url, final int id) {
//如果緩存過(guò)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(jiàn)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);
}
}
//采用Handler+Thread+封裝外部接口
private void loadImage5(final String url, final int id) {
//如果緩存過(guò)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(jiàn)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);
}
}
}
xml文件大致如下：
<SPAN style="FONT-SIZE: 18px"><STRONG>< ?xml version="1.0" encoding="utf-8"?>
< 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>
