尊重原创 http://write.blog.youkuaiyun.com/postedit/26002961
通过前面浅析（一）和浅析（二）的分析，相信大家对于Volley有了初步的认识，但是如果想更深入的理解，还需要靠大家多多看源码。
这篇文章中我们主要来研究一下使用Volley框架请求大量图片的原理，在Android的应用中，通过http请求获取的数据主要有三类：
1、json
2、xml
3、Image
其中json和xml的获取其实原理很简单，使用Volley获取感觉有点大财小用了，了解Volley获取图片的原理才是比较有意义的，因为里面涉及到很多知识点，比如获取大量图片如何防止OOM。
那么我们就开始研究源码吧。
(1) ImageLoader.java
通过它的名字我们就知道是用来加载Image的工具类
/**
通过调用ImageLoader的get方法就可以获取到图片，然后通过一个Listener回调，将图片设置到ImgeView中（这个方法务必在主线程中调用）
*/
public class ImageLoader {
/** 前面已经接触过，请求队列（其实不是真实的队列，里面包含了本地队列和网络队列） */
private final RequestQueue mRequestQueue;
/** 图片缓冲，这个缓存不是前面提到的磁盘缓存，这个是内存缓存，我们可以通过LruCache实现这个接口 */
private final ImageCache mCache;
/**
* 用于存放具有相同cacheKey的请求
*/
private final HashMap<String, BatchedImageRequest> mInFlightRequests =
new HashMap<String, BatchedImageRequest>();
/** 用于存放具有相同Key,并且返回了数据的请求*/
private final HashMap<String, BatchedImageRequest> mBatchedResponses =
new HashMap<String, BatchedImageRequest>();
/** Handler to the main thread. */
private final Handler mHandler = new Handler(Looper.getMainLooper());
/** Runnable for in-flight response delivery. */
private Runnable mRunnable;
/**
* Simple cache adapter interface. If provided to the ImageLoader, it
* will be used as an L1 cache before dispatch to Volley. Implementations
* must not block. Implementation with an LruCache is recommended.
*/
public interface ImageCache {
public Bitmap getBitmap(String url);
public void putBitmap(String url, Bitmap bitmap);
}
/**
* 构造函数需要传入一个RequestQueue对象和一个内存缓存对象
* @param queue The RequestQueue to use for making image requests.
* @param imageCache The cache to use as an L1 cache.
*/
public ImageLoader(RequestQueue queue, ImageCache imageCache) {
mRequestQueue = queue;
mCache = imageCache;
}
/**
* 用于图片获取成功或者失败的回调
* @param imageView 需要设置图片的ImageView.
* @param defaultImageResId 默认显示图片.
* @param errorImageResId 出错时显示的图片.
*/
public static ImageListener getImageListener(final ImageView view,
final int defaultImageResId, final int errorImageResId) {
return new ImageListener() {
@Override
public void onErrorResponse(VolleyError error) {
//出错并且设置了出错图片，那么显示出错图片
if (errorImageResId != 0) {
view.setImageResource(errorImageResId);
}
}
@Override
public void onResponse(ImageContainer response, boolean isImmediate) {
if (response.getBitmap() != null) {
//成功获取到了数据，则显示
view.setImageBitmap(response.getBitmap());
} else if (defaultImageResId != 0) {
//数据为空，那么显示默认图片
view.setImageResource(defaultImageResId);
}
}
};
}
/**
* 判断图片是否已经缓存,不同尺寸的图片的cacheKey是不一样的
* @param requestUrl 图片的url
* @param maxWidth 请求图片的宽度.
* @param maxHeight 请求图片的高度.
* @return 返回true则缓存.
*/
public boolean isCached(String requestUrl, int maxWidth, int maxHeight) {
throwIfNotOnMainThread();
String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight);
return mCache.getBitmap(cacheKey) != null;
}
/**
* 这个方法时个核心方法，我们主要通过它来获取图片
*
* @param requestUrl The URL of the image to be loaded.
* @param defaultImage Optional default image to return until the actual image is loaded.
*/
public ImageContainer get(String requestUrl, final ImageListener listener) {
return get(requestUrl, listener, 0, 0);
}
/**
* 这个方法比上面方法多了两个参数，如果传入则图片大小会做相应处理，如果不传默认为0,图片大小不做处理
* @param requestUrl The url of the remote image
* @param imageListener The listener to call when the remote image is loaded
* @param maxWidth The maximum width of the returned image.
* @param maxHeight The maximum height of the returned image.
* @return A container object that contains all of the properties of the request, as well as
* the currently available image (default if remote is not loaded).
*/
public ImageContainer get(String requestUrl, ImageListener imageListener,
int maxWidth, int maxHeight) {
// only fulfill requests that were initiated from the main thread.
throwIfNotOnMainThread();
//获取key,其实就是url,width,height按照某种格式拼接
final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight);
// 首先从缓存里面取图片
Bitmap cachedBitmap = mCache.getBitmap(cacheKey);
if (cachedBitmap != null) {
// 如果缓存命中，则直接放回
ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null);
imageListener.onResponse(container, true);
return container;
}
// 没有命中，则创建一个ImageContainer,注意此时图片数据传入的null,
ImageContainer imageContainer =
new ImageContainer(null, requestUrl, cacheKey, imageListener);
// 这就是为什么在onResponse中我们需要判断图片数据是否为空，此时就是为空的
imageListener.onResponse(imageContainer, true);
// 判断同一个key的请求是否已经存在
BatchedImageRequest request = mInFlightRequests.get(cacheKey);
if (request != null) {
// 如果存在，则直接加入request中，没有必要对一个key发送多个请求
request.addContainer(imageContainer);
return imageContainer;
}
// 发送一个请求，并加入RequestQueue
Request<?> newRequest =
new ImageRequest(requestUrl, new Listener<Bitmap>() {
@Override
public void onResponse(Bitmap response) {
//成功获取到图片
onGetImageSuccess(cacheKey, response);
}
}, maxWidth, maxHeight,
Config.RGB_565, new ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
onGetImageError(cacheKey, error);
}
});
mRequestQueue.add(newRequest);
VolleyLog.e("-------------->"+newRequest.getSequence());
//加入到HashMap中，表明这个key已经存在一个请求
mInFlightRequests.put(cacheKey,
new BatchedImageRequest(newRequest, imageContainer));
return imageContainer;
}
/**
* Handler for when an image was successfully loaded.
* @param cacheKey The cache key that is associated with the image request.
* @param response The bitmap that was returned from the network.
*/
private void onGetImageSuccess(String cacheKey, Bitmap response) {
// 获取图片成功，放入缓存
mCache.putBitmap(cacheKey, response);
// 将cacheKey对应的请求从mInFlightRequests中移除
BatchedImageRequest request = mInFlightRequests.remove(cacheKey);
if (request != null) {
// Update the response bitmap.
request.mResponseBitmap = response;
// Send the batched response
batchResponse(cacheKey, request);
}
}
/**
* Handler for when an image failed to load.
* @param cacheKey The cache key that is associated with the image request.
*/
private void onGetImageError(String cacheKey, VolleyError error) {
// Notify the requesters that something failed via a null result.
// Remove this request from the list of in-flight requests.
BatchedImageRequest request = mInFlightRequests.remove(cacheKey);
if (request != null) {
// Set the error for this request
request.setError(error);
// Send the batched response
batchResponse(cacheKey, request);
}
}
/**
* Container object for all of the data surrounding an image request.
*/
public class ImageContainer {
/**
* 保存从网络获取的图片
*/
private Bitmap mBitmap;
private final ImageListener mListener;
/** The cache key that was associated with the request */
private final String mCacheKey;
/** The request URL that was specified */
private final String mRequestUrl;
/**
* Constructs a BitmapContainer object.
* @param bitmap The final bitmap (if it exists).
* @param requestUrl The requested URL for this container.
* @param cacheKey The cache key that identifies the requested URL for this container.
*/
public ImageContainer(Bitmap bitmap, String requestUrl,
String cacheKey, ImageListener listener) {
mBitmap = bitmap;
mRequestUrl = requestUrl;
mCacheKey = cacheKey;
mListener = listener;
}
/**
* 取消一个图片请求
*/
public void cancelRequest() {
if (mListener == null) {
return;
}
//判断此key对应的请求有没有
BatchedImageRequest request = mInFlightRequests.get(mCacheKey);
if (request != null) {
/**如果存在,request中mContainers中的这个Container,如果mContainers的size为0，那么
removeContainerAndCancelIfNecessary返回true
*/
boolean canceled = request.removeContainerAndCancelIfNecessary(this);
if (canceled) {
//如果返回true,那么说明没有任何一个ImageView对这个请求感兴趣，需要移除它
mInFlightRequests.remove(mCacheKey);
}
} else {
// 判断是否这个request已经成功返回了
request = mBatchedResponses.get(mCacheKey);
if (request != null) {
request.removeContainerAndCancelIfNecessary(this);
if (request.mContainers.size() == 0) {
//如果已经成功返回，并且没有ImageView对他感兴趣，那么删除它
mBatchedResponses.remove(mCacheKey);
}
}
}
}
/**
* Returns the bitmap associated with the request URL if it has been loaded, null otherwise.
*/
public Bitmap getBitmap() {
return mBitmap;
}
/**
* Returns the requested URL for this container.
*/
public String getRequestUrl() {
return mRequestUrl;
}
}
/**
* 对Request的一个包装，将所有有共同key的请求放入一个LinkedList中
*/
private class BatchedImageRequest {
/** The request being tracked */
private final Request<?> mRequest;
/** The result of the request being tracked by this item */
private Bitmap mResponseBitmap;
/** Error if one occurred for this response */
private VolleyError mError;
/** 存放具有共同key的ImageContainer*/
private final LinkedList<ImageContainer> mContainers = new LinkedList<ImageContainer>();
/**
* Constructs a new BatchedImageRequest object
* @param request The request being tracked
* @param container The ImageContainer of the person who initiated the request.
*/
public BatchedImageRequest(Request<?> request, ImageContainer container) {
mRequest = request;
mContainers.add(container);
}
/**
* Set the error for this response
*/
public void setError(VolleyError error) {
mError = error;
}
/**
* Get the error for this response
*/
public VolleyError getError() {
return mError;
}
/**
* Adds another ImageContainer to the list of those interested in the results of
* the request.
*/
public void addContainer(ImageContainer container) {
mContainers.add(container);
}
/**
* 移除一个ImageContainer，如果此时size==0，那么需要从mInFlightRequests中移除该BatchedImageRequest
* @param container The container to remove from the list
* @return True if the request was canceled, false otherwise.
*/
public boolean removeContainerAndCancelIfNecessary(ImageContainer container) {
mContainers.remove(container);
if (mContainers.size() == 0) {
mRequest.cancel();
return true;
}
return false;
}
}
/**
* 当请求返回后，将BatchedImageRequest放入到mBatchedResponses,然后将结果发送给所有具有相同key的ImageContainer,ImageContainer通过里面的Listener发送到ImageView,从而显示出来
* @param cacheKey The cacheKey of the response being delivered.
* @param request The BatchedImageRequest to be delivered.
* @param error The volley error associated with the request (if applicable).
*/
private void batchResponse(String cacheKey, BatchedImageRequest request) {
mBatchedResponses.put(cacheKey, request);
// If we don't already have a batch delivery runnable in flight, make a new one.
// Note that this will be used to deliver responses to all callers in mBatchedResponses.
if (mRunnable == null) {
mRunnable = new Runnable() {
@Override
public void run() {
for (BatchedImageRequest bir : mBatchedResponses.values()) {
for (ImageContainer container : bir.mContainers) {
// If one of the callers in the batched request canceled the request
// after the response was received but before it was delivered,
// skip them.
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 runnable.
mHandler.postDelayed(mRunnable, mBatchResponseDelayMs);
}
}
/**
* 获取一个请求的key,拼接规则就是使用#讲几个连接起来
* @param url The URL of the request.
* @param maxWidth The max-width of the output.
* @param maxHeight The max-height of the output.
*/
private static String getCacheKey(String url, int maxWidth, int maxHeight) {
return new StringBuilder(url.length() + 12).append("#W").append(maxWidth)
.append("#H").append(maxHeight).append(url).toString();
}
}
1、通过ImageLoader的get方法获取图片，如果我们只想获取原始图片，不用关心大小，则只用传入url和Listener,如果需要设置图片大小，那么传入你需要设置大大小
2、get方法中，先回去缓存中查找，如果命中，那么就直接放回，如果没有命中，那么就判断mInFlightRequests中是否有相同key的BatchedImageRequest，如果有则直接将ImageConainer加入BatchedImageRequest的mContainres中，因为对于同一个key没有必要发送两次请求
3、如果在mInFlightRequest中没有此key,那么需要创建一个ImageRequest对象，并加入RequestQueue中，并使用ImageRequest创建一个BatchedImageRequest加入mInFlightRequest
4、当请求返回后，将BatchedImageRequest从mInFlightRequest中移除，加入mBatchedResponses中，将返回结果返回给所有的ImageContainer
5、如果一个ImageContainer在收到返回结果之前就被cancel掉，那么需要将它从mInFlightRequest的mContainers中移除，如果移除后mContainers的size为0，说明这个请求只有一次，取消了就没有必要请求，需要把BatchedImageRequestmInFlightRequest中移走，从如果不等于0，说明这个请求被其他的ImageContainr需要，不能取消
如果我们仅仅是获取少量图片，Volley框架为我们提供了一个NetworkImageView,这个类继承自ImageView,使用时，我们只需要调用setImageUrl即可，下面就来看其实现机制
(2) NetworkImageView.java
public class NetworkImageView extends ImageView {
/** 需要加载图片的url */
private String mUrl;
/**
* 默认显示图片的id
*/
private int mDefaultImageId;
/**
* 错误图片的id
*/
private int mErrorImageId;
/** ImageLoader对象，其实就是用该对象去获取图片，所以了解了ImageLoader后，这个类很好理解 */
private ImageLoader mImageLoader;
/**把这个对象当成url和Listener的封装即可 */
private ImageContainer mImageContainer;
public NetworkImageView(Context context) {
this(context, null);
}
public NetworkImageView(Context context, AttributeSet attrs) {
this(context, attrs, 0);
}
public NetworkImageView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
/**
* 设置Url
*
* @param url The URL that should be loaded into this ImageView.
* @param imageLoader ImageLoader that will be used to make the request.
*/
public void setImageUrl(String url, ImageLoader imageLoader) {
mUrl = url;
mImageLoader = imageLoader;
// 这个方法我们后面分析
loadImageIfNecessary(false);
}
/**
* Sets the default image resource ID to be used for this view until the attempt to load it
* completes.
*/
public void setDefaultImageResId(int defaultImage) {
mDefaultImageId = defaultImage;
}
/**
* Sets the error image resource ID to be used for this view in the event that the image
* requested fails to load.
*/
public void setErrorImageResId(int errorImage) {
mErrorImageId = errorImage;
}
/**
* 这个方法在onLayout方法中传入true,其他地方传入false
* @param isInLayoutPass True if this was invoked from a layout pass, false otherwise.
*/
void loadImageIfNecessary(final boolean isInLayoutPass) {
int width = getWidth();
int height = getHeight();
boolean wrapWidth = false, wrapHeight = false;
if (getLayoutParams() != null) {
wrapWidth = getLayoutParams().width == LayoutParams.WRAP_CONTENT;
wrapHeight = getLayoutParams().height == LayoutParams.WRAP_CONTENT;
}
// if the view's bounds aren't known yet, and this is not a wrap-content/wrap-content
// view, hold off on loading the image.
boolean isFullyWrapContent = wrapWidth && wrapHeight;
if (width == 0 && height == 0 && !isFullyWrapContent) {
return;
}
// if the URL to be loaded in this view is empty, cancel any old requests and clear the
// currently loaded image.
if (TextUtils.isEmpty(mUrl)) {
if (mImageContainer != null) {
mImageContainer.cancelRequest();
mImageContainer = null;
}
setDefaultImageOrNull();
return;
}
// if there was an old request in this view, check if it needs to be canceled.
if (mImageContainer != null && mImageContainer.getRequestUrl() != null) {
if (mImageContainer.getRequestUrl().equals(mUrl)) {
//如果请求url相同，则直接return
return;
} else {
// 请求url不同，则cancel,并显示默认图片或者不显示图片
mImageContainer.cancelRequest();
setDefaultImageOrNull();
}
}
// Calculate the max image width / height to use while ignoring WRAP_CONTENT dimens.
int maxWidth = wrapWidth ? 0 : width;
int maxHeight = wrapHeight ? 0 : height;
//调用了get方法
ImageContainer newContainer = mImageLoader.get(mUrl,
new ImageListener() {
@Override
public void onErrorResponse(VolleyError error) {
if (mErrorImageId != 0) {
setImageResource(mErrorImageId);
}
}
@Override
public void onResponse(final ImageContainer response, boolean isImmediate) {
// If this was an immediate response that was delivered inside of a layout
// pass do not set the image immediately as it will trigger a requestLayout
// inside of a layout. Instead, defer setting the image by posting back to
// the main thread.
if (isImmediate && isInLayoutPass) {
post(new Runnable() {
@Override
public void run() {
onResponse(response, false);
}
});
return;
}
if (response.getBitmap() != null) {
setImageBitmap(response.getBitmap());
} else if (mDefaultImageId != 0) {
setImageResource(mDefaultImageId);
}
}
}, maxWidth, maxHeight);
// update the ImageContainer to be the new bitmap container.
mImageContainer = newContainer;
}
private void setDefaultImageOrNull() {
if(mDefaultImageId != 0) {
setImageResource(mDefaultImageId);
}
else {
setImageBitmap(null);
}
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
loadImageIfNecessary(true);
}
@Override
protected void onDetachedFromWindow() {
if (mImageContainer != null) {
// If the view was bound to an image request, cancel it and clear
// out the image from the view.
mImageContainer.cancelRequest();
setImageBitmap(null);
// also clear out the container so we can reload the image if necessary.
mImageContainer = null;
}
super.onDetachedFromWindow();
}
@Override
protected void drawableStateChanged() {
super.drawableStateChanged();
invalidate();
}
}
到目前为止Volley框架的源码分析差不多了，下一篇文章我打算使用一个GridView展示大量图片的例子来讲解Volley的使用.....
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