Android 实现异步加载图片

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麦洛开通博客以来,有一段时间没有更新博文了.主要是麦洛这段时间因项目开发实在太忙了.今天周六还在公司加班,苦逼程序猿都是这样生活的.

今天在做项目的时候,有一个实现异步加载图片的功能,虽然比较简单但还是记录一下吧.因为麦洛之前实现异步加载图片都是使用了AsynTask这个API,继续这个类,实现起来非常简单也很方便.在doInBackground()方法里实现下载逻辑.具体实现如下

实现逻辑是:先从内存中读取,如果内存中有这张图片,则直接使用;如果内存没有再到sdcard上读取,如果有则显示;如果sdcard上还没有则到网络上读取.内存中开启缓存是参考了网上的实现.麦洛在这里非常感谢喜欢分享的程序猿们.

public class ImageDownloader extends AsyncTask<String, Integer, Object> { private static final String TAG = "ImageDownloader";
// 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）
private Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();
/**
* 显示图片的控件
*/
private ImageView mImageView; public ImageDownloader(ImageView image) {
mImageView = image;
} @Override
protected void onPreExecute() {
super.onPreExecute();
} @Override
protected Object doInBackground(String... params) {
// Log.i("ImageDownloader", "loading image...");
String url = params[0];
Drawable drawable = null;
try {
if (!"".equals(url) && url != null) {
String fileName = url.hashCode()+".jpg";
// 如果缓存过就从缓存中取出数据
if (imageCache.containsKey(fileName)) {
SoftReference<Drawable> softReference = imageCache.get(fileName);
drawable = softReference.get();
if (drawable != null) {
return drawable;
}
}
File dir = new File(FileConstant.IMAGE_FILE_PATH);
if (!dir.exists()) {
boolean m = dir.mkdirs();
}
File file = new File(dir, fileName);
if (file.exists() && file.length() > 0) {
Log.i(TAG, "load image from sd card");
// 如果文件存在则直接读取sdcard
drawable = readFromSdcard(file);
} else {
//file.createNewFile();
Log.i(TAG, "load image from network");
URL imageUrl = new URL(url);
// 写入sdcard
if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
saveImageFile(imageUrl, file);
drawable = Drawable.createFromStream(new FileInputStream(file), fileName);
}else{
//直接从流读取
drawable = Drawable.createFromStream(imageUrl.openStream(), fileName);
}
}
if(drawable!=null){
//保存在缓存中
imageCache.put(fileName, new SoftReference<Drawable>(drawable));
}
}
} catch (Exception e) {
e.printStackTrace();
}
return drawable;
}
/**
* save image
*/
private void saveImageFile(URL url, File file) {
FileOutputStream out = null;
InputStream in = null;
try {
file.deleteOnExit();
out = new FileOutputStream(file);
in = url.openStream();
byte[] buf = new byte[1024];
int len = -1;
while((len = in.read(buf))!=-1){
out.write(buf, 0, len);
out.flush();
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if(out!=null){
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if(in!=null){
try {
in.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
} /**
* 从sdcard中获取图片
*/
private Drawable readFromSdcard(File file) throws Exception {
FileInputStream in = new FileInputStream(file);
return Drawable.createFromStream(in, file.getName());
} @Override
protected void onPostExecute(Object result) {
super.onPostExecute(result);
Drawable drawable = (Drawable) result;
if (mImageView != null && drawable != null) {
mImageView.setBackgroundDrawable(drawable);
}
} @Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
} @Override
protected void onCancelled() {
super.onCancelled();
}}

使用时:

ImageDownloader loader = new ImageDownloader(imageView);
loader.execute(url);

其实这样的话,还有一些隐患的,就是说这个类实现还是有些问题的.比如每次都在imageView中设置网络上的图片时,其实是没有使用到这个类里面的内存缓存的,就是imageCache

Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();

因为每次设置imageView的时候,都是new了一个ImageDownloader的对象.所以每个ImageDownloader对象里面都是独立的一个imageCache.

另外,AsynTask也是一个线程.而每次使用都开一个线程来load 图片,对线程个数没有进行显示,毕竟线程数目还是有限制的.

所以麦洛今天发现了这个问题,于是参考了别人的实现,使用了线程池,实现逻辑也上面的代码一样,先从内存读取,如果没有到sdcard读取,如果还是没有,则是网络读取;实现没有使用AsynTask,具体代码如下:

/**
* 异步加载图片，并将图片设置到ImageView控件中
*/
public class ImageDownloader extends AsyncTask<String, Integer, Object> { private static final String TAG = "ImageDownloader";
// 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）
private Map<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();
/**
* 显示图片的控件
*/
private ImageView mImageView; public ImageDownloader(ImageView image) {
mImageView = image;
} @Override
protected void onPreExecute() {
super.onPreExecute();
} @Override
protected Object doInBackground(String... params) {
// Log.i("ImageDownloader", "loading image...");
String url = params[0];
Drawable drawable = null;
try {
if (!"".equals(url) && url != null) {
String fileName = url.hashCode()+".jpg";
// 如果缓存过就从缓存中取出数据
if (imageCache.containsKey(fileName)) {
SoftReference<Drawable> softReference = imageCache.get(fileName);
drawable = softReference.get();
if (drawable != null) {
return drawable;
}
}
File dir = new File(FileConstant.IMAGE_FILE_PATH);
if (!dir.exists()) {
boolean m = dir.mkdirs();
}
File file = new File(dir, fileName);
if (file.exists() && file.length() > 0) {
Log.i(TAG, "load image from sd card");
// 如果文件存在则直接读取sdcard
drawable = readFromSdcard(file);
} else {
//file.createNewFile();
Log.i(TAG, "load image from network");
URL imageUrl = new URL(url);
// 写入sdcard
if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
saveImageFile(imageUrl, file);
drawable = Drawable.createFromStream(new FileInputStream(file), fileName);
}else{
//直接从流读取
drawable = Drawable.createFromStream(imageUrl.openStream(), fileName);
}
}
if(drawable!=null){
//保存在缓存中
imageCache.put(fileName, new SoftReference<Drawable>(drawable));
}
}
} catch (Exception e) {
e.printStackTrace();
}
return drawable;
}
/**
* save image
*/
private void saveImageFile(URL url, File file) {
FileOutputStream out = null;
InputStream in = null;
try {
file.deleteOnExit();
out = new FileOutputStream(file);
in = url.openStream();
byte[] buf = new byte[1024];
int len = -1;
while((len = in.read(buf))!=-1){
out.write(buf, 0, len);
out.flush();
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if(out!=null){
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if(in!=null){
try {
in.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
} /**
* 从sdcard中获取图片
*/
private Drawable readFromSdcard(File file) throws Exception {
FileInputStream in = new FileInputStream(file);
return Drawable.createFromStream(in, file.getName());
} @Override
protected void onPostExecute(Object result) {
super.onPostExecute(result);
Drawable drawable = (Drawable) result;
if (mImageView != null && drawable != null) {
mImageView.setBackgroundDrawable(drawable);
}
} @Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
} @Override
protected void onCancelled() {
super.onCancelled();
}}

这个ImageDownloader2的使用也很简单

public class ImageUtil {
/**
* image loader
*/
static ImageDownloader2 loader = null; /**
* load image
*/
public static void loadImage(String url,final ImageView imageView){
if(loader == null){
loader = new ImageDownloader2();
}
loader.loadDrawable(url, new ImageCallback() { @Override
public void imageLoaded(Drawable imageDrawable) {
if(imageDrawable!=null){
imageView.setBackgroundDrawable(imageDrawable);
}
}
});
}}