Volley图片加载功能

加载功能"/>

Volley图片加载功能

Gituhb项目

Volley源码中文注释项目我已经上传到github,欢迎大家fork和start.

为什么写这篇博客

本来文章是维护在github上的,但是我在分析ImageLoader源码过程中与到了一个问题,希望大家能帮助解答.

Volley获取网络图片

本来想分析Universal Image Loader的源码,但是发现Volley已经实现了网络图片的加载功能.其实,网络图片的加载也是分几个步骤:
1. 获取网络图片的url.
2. 判断该url对应的图片是否有本地缓存.
3. 有本地缓存,直接使用本地缓存图片,通过异步回调给ImageView进行设置.
4. 无本地缓存,就先从网络拉取,保存在本地后,再通过异步回调给ImageView进行设置.

我们通过Volley源码,看一下Volley是否是按照这个步骤实现网络图片加载的.

ImageRequest.java

按照Volley的架构,我们首先需要构造一个网络图片请求,Volley帮我们封装了ImageRequest类,我们来看一下它的具体实现:

/** 网络图片请求类. */
@SuppressWarnings("unused")
public class ImageRequest extends Request<Bitmap> {/** 默认图片获取的超时时间(单位:毫秒) */public static final int DEFAULT_IMAGE_REQUEST_MS = 1000;/** 默认图片获取的重试次数. */public static final int DEFAULT_IMAGE_MAX_RETRIES = 2;private final Response.Listener<Bitmap> mListener;private final Bitmap.Config mDecodeConfig;private final int mMaxWidth;private final int mMaxHeight;private ImageView.ScaleType mScaleType;/** Bitmap解析同步锁,保证同一时间只有一个Bitmap被load到内存进行解析,防止OOM. */private static final Object sDecodeLock = new Object();/*** 构造一个网络图片请求.* @param url 图片的url地址.* @param listener 请求成功用户设置的回调接口.* @param maxWidth 图片的最大宽度.* @param maxHeight 图片的最大高度.* @param scaleType 图片缩放类型.* @param decodeConfig 解析bitmap的配置.* @param errorListener 请求失败用户设置的回调接口.*/public ImageRequest(String url, Response.Listener<Bitmap> listener, int maxWidth, int maxHeight,ImageView.ScaleType scaleType, Bitmap.Config decodeConfig,Response.ErrorListener errorListener) {super(Method.GET, url, errorListener);mListener = listener;mDecodeConfig = decodeConfig;mMaxWidth = maxWidth;mMaxHeight = maxHeight;mScaleType = scaleType;}/** 设置网络图片请求的优先级. */@Overridepublic Priority getPriority() {return Priority.LOW;}@Overrideprotected Response<Bitmap> parseNetworkResponse(NetworkResponse response) {synchronized (sDecodeLock) {try {return doParse(response);} catch (OutOfMemoryError e) {return Response.error(new VolleyError(e));}}}private Response<Bitmap> doParse(NetworkResponse response) {byte[] data = response.data;BitmapFactory.Options decodeOptions = new BitmapFactory.Options();Bitmap bitmap;if (mMaxWidth == 0 && mMaxHeight == 0) {decodeOptions.inPreferredConfig = mDecodeConfig;bitmap = BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);} else {// 获取网络图片的真实尺寸.decodeOptions.inJustDecodeBounds = true;BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);int actualWidth = decodeOptions.outWidth;int actualHeight = decodeOptions.outHeight;int desiredWidth = getResizedDimension(mMaxWidth, mMaxHeight,actualWidth, actualHeight, mScaleType);int desireHeight = getResizedDimension(mMaxWidth, mMaxHeight,actualWidth, actualHeight, mScaleType);decodeOptions.inJustDecodeBounds = false;decodeOptions.inSampleSize =findBestSampleSize(actualWidth, actualHeight, desiredWidth, desireHeight);Bitmap tempBitmap = BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);if (tempBitmap != null && (tempBitmap.getWidth() > desiredWidth ||tempBitmap.getHeight() > desireHeight)) {bitmap = Bitmap.createScaledBitmap(tempBitmap, desiredWidth, desireHeight, true);tempBitmap.recycle();} else {bitmap = tempBitmap;}}if (bitmap == null) {return Response.error(new VolleyError(response));} else {return Response.success(bitmap, HttpHeaderParser.parseCacheHeaders(response));}}static int findBestSampleSize(int actualWidth, int actualHeight, int desiredWidth, int desireHeight) {double wr = (double) actualWidth / desiredWidth;double hr = (double) actualHeight / desireHeight;double ratio = Math.min(wr, hr);float n = 1.0f;while ((n * 2) <= ratio) {n *= 2;}return (int) n;}/** 根据ImageView的ScaleType设置图片的大小. */private static int getResizedDimension(int maxPrimary, int maxSecondary, int actualPrimary,int actualSecondary, ImageView.ScaleType scaleType) {// 如果没有设置ImageView的最大值,则直接返回网络图片的真实大小.if ((maxPrimary == 0) && (maxSecondary == 0)) {return actualPrimary;}// 如果ImageView的ScaleType为FIX_XY,则将其设置为图片最值.if (scaleType == ImageView.ScaleType.FIT_XY) {if (maxPrimary == 0) {return actualPrimary;}return maxPrimary;}if (maxPrimary == 0) {double ratio = (double)maxSecondary / (double)actualSecondary;return (int)(actualPrimary * ratio);}if (maxSecondary == 0) {return maxPrimary;}double ratio = (double) actualSecondary / (double) actualPrimary;int resized = maxPrimary;if (scaleType == ImageView.ScaleType.CENTER_CROP) {if ((resized * ratio) < maxSecondary) {resized = (int)(maxSecondary / ratio);}return resized;}if ((resized * ratio) > maxSecondary) {resized = (int)(maxSecondary / ratio);}return resized;}@Overrideprotected void deliverResponse(Bitmap response) {mListener.onResponse(response);}
}

因为Volley本身框架已经实现了对网络请求的本地缓存,所以ImageRequest做的主要事情就是解析字节流为Bitmap,再解析过程中,通过静态变量保证每次只解析一个Bitmap防止OOM,使用ScaleType和用户设置的MaxWidth和MaxHeight来设置图片大小.
总体来说,ImageRequest的实现非常简单,这里不做过多的讲解.ImageRequest的缺陷在于:

1. 需要用户进行过多的设置,包括图片的大小的最大值.
2. 没有图片的内存缓存,因为Volley的缓存是基于Disk的缓存,有对象反序列化的过程.

ImageLoader.java

鉴于以上两个缺点，Volley又提供了一个更牛逼的ImageLoader类.其中,最关键的就是增加了内存缓存.
再讲解ImageLoader的源码之前,需要先介绍一下ImageLoader的使用方法.和之前的Request请求不同,ImageLoader并不是new出来直接扔给RequestQueue进行调度,它的使用方法大体分为4步:

• 创建一个RequestQueue对象.

RequestQueue queue = Volley.newRequestQueue(context);

• 创建一个ImageLoader对象.ImageLoader构造函数接收两个参数,第一个是RequestQueue对象,第二个是ImageCache对象(也就是内存缓存类,我们先不给出具体实现,讲解完ImageLoader源码之后,我会提供一个利用LRU算法的ImageCache实现类)

ImageLoader imageLoader = new ImageLoader(queue, new ImageCache() {@Overridepublic void putBitmap(String url, Bitmap bitmap) {}@Overridepublic Bitmap getBitmap(String url) { return null; }
});

• 获取一个ImageListener对象.

ImageListener listener = ImageLoader.getImageListener(imageView, R.drawable.default_imgage, R.drawable.failed_image);

• 调用ImageLoader的get方法加载网络图片.

imageLoader.get(mImageUrl, listener, maxWidth, maxHeight, scaleType);

有了ImageLoader的使用方法,我们结合使用方法来看一下ImageLoader的源码:

@SuppressWarnings({"unused", "StringBufferReplaceableByString"})
public class ImageLoader {/*** 关联用来调用ImageLoader的RequestQueue.*/private final RequestQueue mRequestQueue;/** 图片内存缓存接口实现类. */private final ImageCache mCache;/** 存储同一时间执行的相同CacheKey的BatchedImageRequest集合. */private final HashMap<String, BatchedImageRequest> mInFlightRequests =new HashMap<String, BatchedImageRequest>();private final HashMap<String, BatchedImageRequest> mBatchedResponses =new HashMap<String, BatchedImageRequest>();/** 获取主线程的Handler. */private final Handler mHandler = new Handler(Looper.getMainLooper());private Runnable mRunnable;/** 定义图片K1缓存接口,即将图片的内存缓存工作交给用户来实现. */public interface ImageCache {Bitmap getBitmap(String url);void putBitmap(String url, Bitmap bitmap);}/** 构造一个ImageLoader. */public ImageLoader(RequestQueue queue, ImageCache imageCache) {mRequestQueue = queue;mCache = imageCache;}/** 构造网络图片请求成功和失败的回调接口. */public static ImageListener getImageListener(final ImageView view, final int defaultImageResId,final int errorImageResId) {return new ImageListener() {@Overridepublic void onResponse(ImageContainer response, boolean isImmediate) {if (response.getBitmap() != null) {view.setImageBitmap(response.getBitmap());} else if (defaultImageResId != 0) {view.setImageResource(defaultImageResId);}}@Overridepublic void onErrorResponse(VolleyError error) {if (errorImageResId != 0) {view.setImageResource(errorImageResId);}}};}public ImageContainer get(String requestUrl, ImageListener imageListener,int maxWidth, int maxHeight, ScaleType scaleType) {// 判断当前方法是否在UI线程中执行.如果不是,则抛出异常.throwIfNotOnMainThread();final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight, scaleType);// 从L1级缓存中根据key获取对应的Bitmap.Bitmap cacheBitmap = mCache.getBitmap(cacheKey);if (cacheBitmap != null) {// L1缓存命中,通过缓存命中的Bitmap构造ImageContainer,并调用imageListener的响应成功接口.ImageContainer container = new ImageContainer(cacheBitmap, requestUrl, null, null);// 注意:因为目前是在UI线程中,因此这里是调用onResponse方法,并非回调.imageListener.onResponse(container, true);return container;}ImageContainer imageContainer =new ImageContainer(null, requestUrl, cacheKey, imageListener);// L1缓存命中失败,则先需要对ImageView设置默认图片.然后通过子线程拉取网络图片,进行显示.imageListener.onResponse(imageContainer, true);// 检查cacheKey对应的ImageRequest请求是否正在运行.BatchedImageRequest request = mInFlightRequests.get(cacheKey);if (request != null) {// 相同的ImageRequest正在运行,不需要同时运行相同的ImageRequest.// 只需要将其对应的ImageContainer加入到BatchedImageRequest的mContainers集合中.// 当正在执行的ImageRequest结束后,会查看当前有多少正在阻塞的ImageRequest,// 然后对其mContainers集合进行回调.request.addContainer(imageContainer);return imageContainer;}// L1缓存没命中,还是需要构造ImageRequest,通过RequestQueue的调度来获取网络图片// 获取方法可能是:L2缓存(ps:Disk缓存)或者HTTP网络请求.Request<Bitmap> newRequest =makeImageRequest(requestUrl, maxWidth, maxHeight, scaleType, cacheKey);mRequestQueue.add(newRequest);mInFlightRequests.put(cacheKey, new BatchedImageRequest(newRequest, imageContainer));return imageContainer;}/** 构造L1缓存的key值. */private String getCacheKey(String url, int maxWidth, int maxHeight, ScaleType scaleType) {return new StringBuilder(url.length() + 12).append("#W").append(maxWidth).append("#H").append(maxHeight).append("#S").append(scaleType.ordinal()).append(url).toString();}public boolean isCached(String requestUrl, int maxWidth, int maxHeight) {return isCached(requestUrl, maxWidth, maxHeight, ScaleType.CENTER_INSIDE);}private boolean isCached(String requestUrl, int maxWidth, int maxHeight, ScaleType scaleType) {throwIfNotOnMainThread();String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight, scaleType);return mCache.getBitmap(cacheKey) != null;}/** 当L1缓存没有命中时,构造ImageRequest,通过ImageRequest和RequestQueue获取图片. */protected Request<Bitmap> makeImageRequest(final String requestUrl, int maxWidth, int maxHeight,ScaleType scaleType, final String cacheKey) {return new ImageRequest(requestUrl, new Response.Listener<Bitmap>() {@Overridepublic void onResponse(Bitmap response) {onGetImageSuccess(cacheKey, response);}}, maxWidth, maxHeight, scaleType, Bitmap.Config.RGB_565, new Response.ErrorListener() {@Overridepublic void onErrorResponse(VolleyError error) {onGetImageError(cacheKey, error);}});}/** 图片请求失败回调.运行在UI线程中. */private void onGetImageError(String cacheKey, VolleyError error) {BatchedImageRequest request = mInFlightRequests.remove(cacheKey);if (request != null) {request.setError(error);batchResponse(cacheKey, request);}}/** 图片请求成功回调.运行在UI线程中. */protected void onGetImageSuccess(String cacheKey, Bitmap response) {// 增加L1缓存的键值对.mCache.putBitmap(cacheKey, response);// 同一时间内最初的ImageRequest执行成功后,回调这段时间阻塞的相同ImageRequest对应的成功回调接口.BatchedImageRequest request = mInFlightRequests.remove(cacheKey);if (request != null) {request.mResponseBitmap = response;// 将阻塞的ImageRequest进行结果分发.batchResponse(cacheKey, request);}}private void batchResponse(String cacheKey, BatchedImageRequest request) {mBatchedResponses.put(cacheKey, request);if (mRunnable == null) {mRunnable = new Runnable() {@Overridepublic void run() {for (BatchedImageRequest bir :  mBatchedResponses.values()) {for (ImageContainer container : bir.mContainers) {if (container.mListener == null) {continue;}if (bir.getError() == null) {container.mBitmap = bir.mResponseBitmap;container.mListener.onResponse(container, false);} else {container.mListener.onErrorResponse(bir.getError());}}}mBatchedResponses.clear();mRunnable = null;}};// Post the runnablemHandler.postDelayed(mRunnable, 100);}}private void throwIfNotOnMainThread() {if (Looper.myLooper() != Looper.getMainLooper()) {throw new IllegalStateException("ImageLoader must be invoked from the main thread.");}}/** 抽象出请求成功和失败的回调接口.默认可以使用Volley提供的ImageListener. */public interface ImageListener extends Response.ErrorListener {void onResponse(ImageContainer response, boolean isImmediate);}/** 网络图片请求的承载对象. */public class ImageContainer {/** ImageView需要加载的Bitmap. */private Bitmap mBitmap;/** L1缓存的key */private final String mCacheKey;/** ImageRequest请求的url. */private final String mRequestUrl;/** 图片请求成功或失败的回调接口类. */private final ImageListener mListener;public ImageContainer(Bitmap bitmap, String requestUrl, String cacheKey,ImageListener listener) {mBitmap = bitmap;mRequestUrl = requestUrl;mCacheKey = cacheKey;mListener = listener;}public void cancelRequest() {if (mListener == null) {return;}BatchedImageRequest request = mInFlightRequests.get(mCacheKey);if (request != null) {boolean canceled = request.removeContainerAndCancelIfNecessary(this);if (canceled) {mInFlightRequests.remove(mCacheKey);}} else {request = mBatchedResponses.get(mCacheKey);if (request != null) {request.removeContainerAndCancelIfNecessary(this);if (request.mContainers.size() == 0) {mBatchedResponses.remove(mCacheKey);}}}}public Bitmap getBitmap() {return mBitmap;}public String getRequestUrl() {return mRequestUrl;}}/*** CacheKey相同的ImageRequest请求抽象类.* 判定两个ImageRequest相同包括:* 1. url相同.* 2. maxWidth和maxHeight相同.* 3. 显示的scaleType相同.* 同一时间可能有多个相同CacheKey的ImageRequest请求,由于需要返回的Bitmap都一样,所以用BatchedImageRequest* 来实现该功能.同一时间相同CacheKey的ImageRequest只能有一个.* 为什么不使用RequestQueue的mWaitingRequestQueue来实现该功能?* 答:是因为仅靠URL是没法判断两个ImageRequest相等的.*/private class BatchedImageRequest {/** 对应的ImageRequest请求. */private final Request<?> mRequest;/** 请求结果的Bitmap对象. */private Bitmap mResponseBitmap;/** ImageRequest的错误. */private VolleyError mError;/** 所有相同ImageRequest请求结果的封装集合. */private final LinkedList<ImageContainer> mContainers = new LinkedList<ImageContainer>();public BatchedImageRequest(Request<?> request, ImageContainer container) {mRequest = request;mContainers.add(container);}public VolleyError getError() {return mError;}public void setError(VolleyError error) {mError = error;}public void addContainer(ImageContainer container) {mContainers.add(container);}public boolean removeContainerAndCancelIfNecessary(ImageContainer container) {mContainers.remove(container);if (mContainers.size() == 0) {mRequest.cancel();return true;}return false;}}
}

重大疑问

个人对Imageloader的源码有两个重大疑问?

• batchResponse方法的实现.

我很奇怪,为什么ImageLoader类里面要有一个HashMap来保存BatchedImageRequest集合呢?

    private final HashMap<String, BatchedImageRequest> mBatchedResponses =new HashMap<String, BatchedImageRequest>();

毕竟batchResponse是在特定的ImageRequest执行成功的回调中被调用的,调用代码如下:

    protected void onGetImageSuccess(String cacheKey, Bitmap response) {// 增加L1缓存的键值对.mCache.putBitmap(cacheKey, response);// 同一时间内最初的ImageRequest执行成功后,回调这段时间阻塞的相同ImageRequest对应的成功回调接口.BatchedImageRequest request = mInFlightRequests.remove(cacheKey);if (request != null) {request.mResponseBitmap = response;// 将阻塞的ImageRequest进行结果分发.batchResponse(cacheKey, request);}}

从上述代码可以看出,ImageRequest请求成功后,已经从mInFlightRequests中获取了对应的BatchedImageRequest对象.而同一时间被阻塞的相同的ImageRequest对应的ImageContainer都在BatchedImageRequest的mContainers集合中.
那我认为,batchResponse方法只需要遍历对应BatchedImageRequest的mContainers集合即可.
但是,ImageLoader源码中,我认为多余的构造了一个HashMap对象mBatchedResponses来保存BatchedImageRequest集合,然后在batchResponse方法中又对集合进行两层for循环各种遍历,实在是非常诡异,求指导.
诡异代码如下:

    private void batchResponse(String cacheKey, BatchedImageRequest request) {mBatchedResponses.put(cacheKey, request);if (mRunnable == null) {mRunnable = new Runnable() {@Overridepublic void run() {for (BatchedImageRequest bir :  mBatchedResponses.values()) {for (ImageContainer container : bir.mContainers) {if (container.mListener == null) {continue;}if (bir.getError() == null) {container.mBitmap = bir.mResponseBitmap;container.mListener.onResponse(container, false);} else {container.mListener.onErrorResponse(bir.getError());}}}mBatchedResponses.clear();mRunnable = null;}};// Post the runnablemHandler.postDelayed(mRunnable, 100);}}

我认为的代码实现应该是:

    private void batchResponse(String cacheKey, BatchedImageRequest request) {if (mRunnable == null) {mRunnable = new Runnable() {@Overridepublic void run() {for (ImageContainer container : request.mContainers) {if (container.mListener == null) {continue;}if (request.getError() == null) {container.mBitmap = request.mResponseBitmap;container.mListener.onResponse(container, false);} else {container.mListener.onErrorResponse(request.getError());}}mRunnable = null;}};// Post the runnablemHandler.postDelayed(mRunnable, 100);}}

• 使用ImageLoader默认提供的ImageListener,我认为存在一个缺陷,即图片闪现问题.当为ListView的item设置图片时,需要增加TAG判断.因为对应的ImageView可能已经被回收利用了.

自定义L1缓存类

首先说明一下,所谓的L1和L2缓存分别指的是内存缓存和硬盘缓存.
实现L1缓存,我们可以使用Android提供的Lru缓存类,示例代码如下:

import android.graphics.Bitmap;
import android.support.v4.util.LruCache;/** Lru算法的L1缓存实现类. */
@SuppressWarnings("unused")
public class ImageLruCache implements ImageLoader.ImageCache {private LruCache<String, Bitmap> mLruCache;public ImageLruCache() {this((int) Runtime.getRuntime().maxMemory() / 8);}public ImageLruCache(final int cacheSize) {createLruCache(cacheSize);}private void createLruCache(final int cacheSize) {mLruCache = new LruCache<String, Bitmap>(cacheSize) {@Overrideprotected int sizeOf(String key, Bitmap value) {return value.getRowBytes() * value.getHeight();}};}@Overridepublic Bitmap getBitmap(String url) {return mLruCache.get(url);}@Overridepublic void putBitmap(String url, Bitmap bitmap) {mLruCache.put(url, bitmap);}
}