Android实现异步加载图片
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); } } }); } }
每次在使用是需要调用ImageUtil.loadImage(url,imageView)将图片url已经需要显示图片的控件ImageView的引用传入就可以了.
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