Android 图片加载框架的简单设计
ljqlovelxy
8年前
<p>目前Android 发展至今优秀的图片加载框架太多，例如： Volley ,Picasso,Imageloader，Glide等等。但是作为程序猿，懂得其中的实现原理还是相当重要的，只有懂得才能更好地使用。于是乎，今天我就简单设计一个网络加载图片框架。主要就是熟悉图片的网络加载机制。</p> <p>一般来说，一个优秀的 图片加载框架(ImageLoader) 应该具备如下功能：</p> <ul> <li> <p>图片压缩</p> </li> <li> <p>内存缓存</p> </li> <li> <p>磁盘缓存</p> </li> <li> <p>图片的同步加载</p> </li> <li> <p>图片的异步加载</p> </li> <li> <p>网络拉取</p> </li> </ul> <p>那我们就从以上几个方面进行介绍：</p> <p><strong>1.图片压缩（有效的降低OOM的发生概率）</strong></p> <p>图片压缩功能我在Bitmap 的高效加载中已经做了介绍这里不多说直接上代码。这里直接抽象一个类用于完成图片压缩功能。</p> <pre> <code class="language-java">public class ImageResizer { private static final String TAG = "ImageResizer"; public ImageResizer() { super(); // TODO Auto-generated constructor stub } public Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight) { final BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeResource(res, resId, options); options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight); options.inJustDecodeBounds = false; return BitmapFactory.decodeResource(res, resId, options); } public Bitmap decodeSampledBitmapFromBitmapFileDescriptor(FileDescriptor fd, int reqWidth,int reqHeight){ final BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeFileDescriptor(fd, null, options); options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight); options.inJustDecodeBounds = false; return BitmapFactory.decodeFileDescriptor(fd, null, options); } public int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) { final int width = options.outWidth; final int height = options.outHeight; int inSampleSize = 1; if (height > reqHeight || width > reqWidth) { final int halfHeight = height / 2; final int halfWidth = width / 2; while ((halfHeight / inSampleSize) > reqHeight && (halfWidth / inSampleSize) > halfWidth) { inSampleSize *= 2; } } return inSampleSize; } }</code></pre> <p><strong>2.内存缓存和磁盘缓存</strong></p> <p>缓存直接选择 LruCache 和 DiskLruCache 来完成内存缓存和磁盘缓存工作。</p> <p>首先对其初始化：</p> <pre> <code class="language-java">private LruCache<String, Bitmap> mMemoryCache; private DiskLruCache mDiskLruCache; public ImageLoader(Context context) { mContext = context.getApplicationContext(); //分配内存缓存为当前进程的1/8,磁盘缓存容量为50M int maxMemory = (int) (Runtime.getRuntime().maxMemory() * 1024); int cacheSize = maxMemory / 8; mMemoryCache = new LruCache<String, Bitmap>(cacheSize) { @Override protected int sizeOf(String key, Bitmap value) { return value.getRowBytes() * value.getHeight() / 1024; } }; File diskCacheDir = getDiskChaheDir(mContext, "bitmap"); if (!diskCacheDir.exists()) { diskCacheDir.mkdirs(); } if (getUsableSpace(diskCacheDir) > DISK_CACHE_SIZE) { try { mDiskLruCache = DiskLruCache.open(diskCacheDir, 1, 1, DISK_CACHE_SIZE); mIsDiskLruCacheCreated = true; } catch (IOException e) { e.printStackTrace(); } } }</code></pre> <p>创建完毕后，接下来则需要提供方法来视线添加以及获取的功能。首先来看内存缓存。</p> <pre> <code class="language-java">private void addBitmapToMemoryCache(String key, Bitmap bitmap) { if (getBitmapFromMemCache(key) == null) { mMemoryCache.put(key, bitmap); } } private Bitmap getBitmapFromMemCache(String key) { return mMemoryCache.get(key); }</code></pre> <p>相对来说内存缓存比较简单，而磁盘缓存则复杂的多。磁盘缓存（LruDiskCache）并没有直接提供方法来实现，而是要通过Editor以及Snapshot 来实现对于文件系统的添加以及读取的操作。</p> <p>首先看一下，Editor，它提供了commit 和 abort 方法来提交和撤销对文件系统的写操作。</p> <pre> <code class="language-java">//将下载的图片写入文件系统，实现磁盘缓存 private Bitmap loadBitmapFromHttp(String url, int reqWidth, int reqHeight) throws IOException { if (Looper.myLooper() == Looper.getMainLooper()) { throw new RuntimeException("can not visit network from UI Thread."); } if (mDiskLruCache == null) return null; String key = hashKeyFormUrl(url); DiskLruCache.Editor editor = mDiskLruCache.edit(key); if (editor != null) { OutputStream outputStream = editor .newOutputStream(DISK_CACHE_INDEX); if (downloadUrlToStream(url, outputStream)) { editor.commit(); } else { editor.abort(); } } mDiskLruCache.flush(); return loadBitmapForDiskCache(url, reqWidth, reqHeight); }</code></pre> <p>Snapshot, 通过它可以获取磁盘缓存对象对应的 FileInputStream，但是FileInputStream 无法便捷的进行压缩，所以通过FileDescriptor 来加载压缩后的图片，最后将加载后的bitmap添加到内存缓存中。</p> <pre> <code class="language-java">public Bitmap loadBitmapForDiskCache(String url, int reqWidth, int reqHeight) throws IOException { if (Looper.myLooper() == Looper.getMainLooper()) { Log.w(TAG, "load bitmap from UI Thread , it's not recommended"); } if (mDiskLruCache == null) return null; Bitmap bitmap = null; String key = hashKeyFormUrl(url); DiskLruCache.Snapshot snapshot = mDiskLruCache.get(key); if (snapshot != null) { FileInputStream fileInputStream = (FileInputStream) snapshot .getInputStream(DISK_CACHE_INDEX); FileDescriptor fileDescriptor = fileInputStream.getFD(); bitmap = mImageResizer.decodeSampledBitmapFromBitmapFileDescriptor( fileDescriptor, reqWidth, reqHeight); if (bitmap != null) { addBitmapToMemoryCache(key, bitmap); } } return bitmap; }</code></pre> <p><strong>3.同步加载</strong></p> <p>同步加载的方法需要外部在子线程中调用。</p> <pre> <code class="language-java">//同步加载 public Bitmap loadBitmap(String uri, int reqWidth, int reqHeight) { Bitmap bitmap = loadBitmpaFromMemCache(uri); if (bitmap != null) { return bitmap; } try { bitmap = loadBitmapForDiskCache(uri, reqWidth, reqHeight); if (bitmap != null) { return bitmap; } bitmap = loadBitmapFromHttp(uri, reqWidth, reqHeight); } catch (IOException e) { e.printStackTrace(); } if (bitmap == null && !mIsDiskLruCacheCreated) { bitmap = downloadBitmapFromUrl(uri); } return bitmap; }</code></pre> <p>从方法中可以看出工作过程遵循如下几步：</p> <p>首先尝试从内存缓存中读取图片，接着尝试从磁盘缓存中读取图片，最后才会从网络中拉取。此方法不能再主线程中执行，执行环境的检测是在loadBitmapFromHttp中实现的。</p> <pre> <code class="language-java">if (Looper.myLooper() == Looper.getMainLooper()) { throw new RuntimeException("can not visit network from UI Thread."); }</code></pre> <p><strong>4.异步加载</strong></p> <pre> <code class="language-java">//异步加载 public void bindBitmap(final String uri, final ImageView imageView, final int reqWidth, final int reqHeight) { imageView.setTag(TAG_KEY_URI, uri); Bitmap bitmap = loadBitmpaFromMemCache(uri); if (bitmap != null) { imageView.setImageBitmap(bitmap); return; } Runnable loadBitmapTask = new Runnable() { @Override public void run() { Bitmap bitmap = loadBitmap(uri, reqWidth, reqHeight); if (bitmap != null) { LoaderResult result = new LoaderResult(imageView, uri, bitmap); mMainHandler.obtainMessage(MESSAGE_POST_RESULT, result) .sendToTarget(); } } }; THREAD_POOL_EXECUTOR.execute(loadBitmapTask); }</code></pre> <p>从bindBitmap的实现来看，bindBitmap 方法会尝试从内存缓存中读取图片，如果读取成功就直接返回结果，否则会在线程池中去调用loadBitmap方法，当图片加载成功后再将图片、图片的地址以及需要绑定的imageView封装成一个LoaderResult对象，然后再通过mMainHandler向主线程发送一个消息，这样就可以在主线程中给imageView设置图片了。</p> <p>下面来看一下，bindBitmap这个方法中用到的线程池和Handler，首先看一下线程池 THREAD_POOL_EXECUTOR 的实现。</p> <pre> <code class="language-java">private static final int CPU_COUNT = Runtime.getRuntime() .availableProcessors(); private static final int CORE_POOL_SIZE = CPU_COUNT + 1; private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1; private static final long KEEP_ALIVE = 10L; private static final ThreadFactory sThreadFactory = new ThreadFactory() { private final AtomicInteger mCount = new AtomicInteger(); @Override public Thread newThread(Runnable r) { // TODO Auto-generated method stub return new Thread(r, "ImageLoader#" + mCount.getAndIncrement()); } }; public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor( CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, new LinkedBlockingDeque<Runnable>(), sThreadFactory);</code></pre> <p>1.使用线程池和handler的原因。</p> <p>首先不能用普通线程去实现，如果采用普通线程去加载图片，随着列表的滑动可能会产生大量的线程，这样不利于效率的提升。 Handler 的实现 ，直接采用了 主线程的Looper来构造Handler 对象，这就使得 ImageLoader 可以在非主线程构造。另外为了解决由于View复用所导致的列表错位这一问题再给ImageView 设置图片之前会检查他的url有没有发生改变，如果发生改变就不再给它设置图片，这样就解决了列表错位问题。</p> <pre> <code class="language-java">private Handler mMainHandler = new Handler(Looper.getMainLooper()) { @Override public void handleMessage(Message msg) { LoaderResult result = (LoaderResult) msg.obj; ImageView imageView = result.imageView; imageView.setImageBitmap(result.bitmap); String uri = (String) imageView.getTag(TAG_KEY_URI); if (uri.equals(result.uri)) { imageView.setImageBitmap(result.bitmap); } else { Log.w(TAG, "set image bitmap,but url has changed , ignored!"); } } };</code></pre> <p><strong>总结：</strong></p> <p>图片加载的问题 ，尤其是大量图片的加载，对于android 开发者来说一直是比较困扰的问题。本文只是提到了最基础的一种解决方法，用于学习还是不错的。</p> <p> </p> <p> </p> <p>来自：http://www.jianshu.com/p/2319b16d269f</p> <p> </p>