Gituhb項目
Volley源碼中文注釋項目我已經上傳到github,歡迎大家fork和start.
為什么寫這篇博客
本來文章是維護在github上的,但是我在分析ImageLoader源碼過程中與到了一個問題,希望大家能幫助解答.
Volley獲取網絡圖片
本來想分析Universal Image Loader的源碼,但是發(fā)現(xiàn)Volley已經實現(xiàn)了網絡圖片的加載功能.其實,網絡圖片的加載也是分幾個步驟:
1. 獲取網絡圖片的url.
2. 判斷該url對應的圖片是否有本地緩存.
3. 有本地緩存,直接使用本地緩存圖片,通過異步回調給ImageView進行設置.
4. 無本地緩存,就先從網絡拉取,保存在本地后,再通過異步回調給ImageView進行設置.
我們通過Volley源碼,看一下Volley是否是按照這個步驟實現(xiàn)網絡圖片加載的.
ImageRequest.java
按照Volley的架構,我們首先需要構造一個網絡圖片請求,Volley幫我們封裝了ImageRequest類,我們來看一下它的具體實現(xiàn):
/** 網絡圖片請求類. */@SuppressWarnings("unused")public class ImageRequest extends Request<Bitmap> { /** 默認圖片獲取的超時時間(單位:毫秒) */ public static final int DEFAULT_IMAGE_REQUEST_MS = 1000; /** 默認圖片獲取的重試次數(shù). */ 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解析同步鎖,保證同一時間只有一個Bitmap被load到內存進行解析,防止OOM. */ private static final Object sDecodeLock = new Object(); /** * 構造一個網絡圖片請求. * @param url 圖片的url地址. * @param listener 請求成功用戶設置的回調接口. * @param maxWidth 圖片的最大寬度. * @param maxHeight 圖片的最大高度. * @param scaleType 圖片縮放類型. * @param decodeConfig 解析bitmap的配置. * @param errorListener 請求失敗用戶設置的回調接口. */ 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; } /** 設置網絡圖片請求的優(yōu)先級. */ @Override public Priority getPriority() { return Priority.LOW; } @Override protected 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 { // 獲取網絡圖片的真實尺寸. 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; } /** 根據(jù)ImageView的ScaleType設置圖片的大小. */ private static int getResizedDimension(int maxPrimary, int maxSecondary, int actualPrimary, int actualSecondary, ImageView.ScaleType scaleType) { // 如果沒有設置ImageView的最大值,則直接返回網絡圖片的真實大小. if ((maxPrimary == 0) && (maxSecondary == 0)) { return actualPrimary; } // 如果ImageView的ScaleType為FIX_XY,則將其設置為圖片最值. 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; } @Override protected void deliverResponse(Bitmap response) { mListener.onResponse(response); }}因為Volley本身框架已經實現(xiàn)了對網絡請求的本地緩存,所以ImageRequest做的主要事情就是解析字節(jié)流為Bitmap,再解析過程中,通過靜態(tài)變量保證每次只解析一個Bitmap防止OOM,使用ScaleType和用戶設置的MaxWidth和MaxHeight來設置圖片大小.
總體來說,ImageRequest的實現(xiàn)非常簡單,這里不做過多的講解.ImageRequest的缺陷在于:
1.需要用戶進行過多的設置,包括圖片的大小的最大值.
2.沒有圖片的內存緩存,因為Volley的緩存是基于Disk的緩存,有對象反序列化的過程.
ImageLoader.java
鑒于以上兩個缺點，Volley又提供了一個更牛逼的ImageLoader類.其中,最關鍵的就是增加了內存緩存.
再講解ImageLoader的源碼之前,需要先介紹一下ImageLoader的使用方法.和之前的Request請求不同,ImageLoader并不是new出來直接扔給RequestQueue進行調度,它的使用方法大體分為4步:
•創(chuàng)建一個RequestQueue對象.
RequestQueue queue = Volley.newRequestQueue(context);
•創(chuàng)建一個ImageLoader對象.
ImageLoader構造函數(shù)接收兩個參數(shù),第一個是RequestQueue對象,第二個是ImageCache對象(也就是內存緩存類,我們先不給出具體實現(xiàn),講解完ImageLoader源碼之后,我會提供一個利用LRU算法的ImageCache實現(xiàn)類)
ImageLoader imageLoader = new ImageLoader(queue, new ImageCache() { @Override public void putBitmap(String url, Bitmap bitmap) {} @Override public Bitmap getBitmap(String url) { return null; }}); •獲取一個ImageListener對象.
ImageListener listener = ImageLoader.getImageListener(imageView, R.drawable.default_imgage, R.drawable.failed_image);
•調用ImageLoader的get方法加載網絡圖片.
imageLoader.get(mImageUrl, listener, maxWidth, maxHeight, scaleType);
有了ImageLoader的使用方法,我們結合使用方法來看一下ImageLoader的源碼:
@SuppressWarnings({"unused", "StringBufferReplaceableByString"})public class ImageLoader { /** * 關聯(lián)用來調用ImageLoader的RequestQueue. */ private final RequestQueue mRequestQueue; /** 圖片內存緩存接口實現(xiàn)類. */ private final ImageCache mCache; /** 存儲同一時間執(zhí)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>(); /** 獲取主線程的Handler. */ private final Handler mHandler = new Handler(Looper.getMainLooper()); private Runnable mRunnable; /** 定義圖片K1緩存接口,即將圖片的內存緩存工作交給用戶來實現(xiàn). */ public interface ImageCache { Bitmap getBitmap(String url); void putBitmap(String url, Bitmap bitmap); } /** 構造一個ImageLoader. */ public ImageLoader(RequestQueue queue, ImageCache imageCache) { mRequestQueue = queue; mCache = imageCache; } /** 構造網絡圖片請求成功和失敗的回調接口. */ public static ImageListener getImageListener(final ImageView view, final int defaultImageResId, final int errorImageResId) { return new ImageListener() { @Override public void onResponse(ImageContainer response, boolean isImmediate) { if (response.getBitmap() != null) { view.setImageBitmap(response.getBitmap()); } else if (defaultImageResId != 0) { view.setImageResource(defaultImageResId); } } @Override public void onErrorResponse(VolleyError error) { if (errorImageResId != 0) { view.setImageResource(errorImageResId); } } }; } public ImageContainer get(String requestUrl, ImageListener imageListener, int maxWidth, int maxHeight, ScaleType scaleType) { // 判斷當前方法是否在UI線程中執(zhí)行.如果不是,則拋出異常. throwIfNotOnMainThread(); final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight, scaleType); // 從L1級緩存中根據(jù)key獲取對應的Bitmap. Bitmap cacheBitmap = mCache.getBitmap(cacheKey); if (cacheBitmap != null) { // L1緩存命中,通過緩存命中的Bitmap構造ImageContainer,并調用imageListener的響應成功接口. ImageContainer container = new ImageContainer(cacheBitmap, requestUrl, null, null); // 注意:因為目前是在UI線程中,因此這里是調用onResponse方法,并非回調. imageListener.onResponse(container, true); return container; } ImageContainer imageContainer = new ImageContainer(null, requestUrl, cacheKey, imageListener); // L1緩存命中失敗,則先需要對ImageView設置默認圖片.然后通過子線程拉取網絡圖片,進行顯示. imageListener.onResponse(imageContainer, true); // 檢查cacheKey對應的ImageRequest請求是否正在運行. BatchedImageRequest request = mInFlightRequests.get(cacheKey); if (request != null) { // 相同的ImageRequest正在運行,不需要同時運行相同的ImageRequest. // 只需要將其對應的ImageContainer加入到BatchedImageRequest的mContainers集合中. // 當正在執(zhí)行的ImageRequest結束后,會查看當前有多少正在阻塞的ImageRequest, // 然后對其mContainers集合進行回調. request.addContainer(imageContainer); return imageContainer; } // L1緩存沒命中,還是需要構造ImageRequest,通過RequestQueue的調度來獲取網絡圖片 // 獲取方法可能是:L2緩存(ps:Disk緩存)或者HTTP網絡請求. Request<Bitmap> newRequest = makeImageRequest(requestUrl, maxWidth, maxHeight, scaleType, cacheKey); mRequestQueue.add(newRequest); mInFlightRequests.put(cacheKey, new BatchedImageRequest(newRequest, imageContainer)); return imageContainer; } /** 構造L1緩存的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; } /** 當L1緩存沒有命中時,構造ImageRequest,通過ImageRequest和RequestQueue獲取圖片. */ protected Request<Bitmap> makeImageRequest(final String requestUrl, int maxWidth, int maxHeight, ScaleType scaleType, final String cacheKey) { return new ImageRequest(requestUrl, new Response.Listener<Bitmap>() { @Override public void onResponse(Bitmap response) { onGetImageSuccess(cacheKey, response); } }, maxWidth, maxHeight, scaleType, Bitmap.Config.RGB_565, new Response.ErrorListener() { @Override public void onErrorResponse(VolleyError error) { onGetImageError(cacheKey, error); } }); } /** 圖片請求失敗回調.運行在UI線程中. */ private void onGetImageError(String cacheKey, VolleyError error) { BatchedImageRequest request = mInFlightRequests.remove(cacheKey); if (request != null) { request.setError(error); batchResponse(cacheKey, request); } } /** 圖片請求成功回調.運行在UI線程中. */ protected void onGetImageSuccess(String cacheKey, Bitmap response) { // 增加L1緩存的鍵值對. mCache.putBitmap(cacheKey, response); // 同一時間內最初的ImageRequest執(zhí)行成功后,回調這段時間阻塞的相同ImageRequest對應的成功回調接口. BatchedImageRequest request = mInFlightRequests.remove(cacheKey); if (request != null) { request.mResponseBitmap = response; // 將阻塞的ImageRequest進行結果分發(fā). batchResponse(cacheKey, request); } } private void batchResponse(String cacheKey, BatchedImageRequest request) { mBatchedResponses.put(cacheKey, request); if (mRunnable == null) { mRunnable = new Runnable() { @Override public 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 runnable mHandler.postDelayed(mRunnable, 100); } } private void throwIfNotOnMainThread() { if (Looper.myLooper() != Looper.getMainLooper()) { throw new IllegalStateException("ImageLoader must be invoked from the main thread."); } } /** 抽象出請求成功和失敗的回調接口.默認可以使用Volley提供的ImageListener. */ public interface ImageListener extends Response.ErrorListener { void onResponse(ImageContainer response, boolean isImmediate); } /** 網絡圖片請求的承載對象. */ public class ImageContainer { /** ImageView需要加載的Bitmap. */ private Bitmap mBitmap; /** L1緩存的key */ private final String mCacheKey; /** ImageRequest請求的url. */ private final String mRequestUrl; /** 圖片請求成功或失敗的回調接口類. */ 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請求抽象類. * 判定兩個ImageRequest相同包括: * 1. url相同. * 2. maxWidth和maxHeight相同. * 3. 顯示的scaleType相同. * 同一時間可能有多個相同CacheKey的ImageRequest請求,由于需要返回的Bitmap都一樣,所以用BatchedImageRequest * 來實現(xiàn)該功能.同一時間相同CacheKey的ImageRequest只能有一個. * 為什么不使用RequestQueue的mWaitingRequestQueue來實現(xiàn)該功能? * 答:是因為僅靠URL是沒法判斷兩個ImageRequest相等的. */ private class BatchedImageRequest { /** 對應的ImageRequest請求. */ private final Request<?> mRequest; /** 請求結果的Bitmap對象. */ private Bitmap mResponseBitmap; /** ImageRequest的錯誤. */ private VolleyError mError; /** 所有相同ImageRequest請求結果的封裝集合. */ 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; } }}重大疑問
個人對Imageloader的源碼有兩個重大疑問?
•batchResponse方法的實現(xiàn).
我很奇怪,為什么ImageLoader類里面要有一個HashMap來保存BatchedImageRequest集合呢?
private final HashMap<String, BatchedImageRequest> mBatchedResponses = new HashMap<String, BatchedImageRequest>();
畢竟batchResponse是在特定的ImageRequest執(zhí)行成功的回調中被調用的,調用代碼如下:
protected void onGetImageSuccess(String cacheKey, Bitmap response) { // 增加L1緩存的鍵值對. mCache.putBitmap(cacheKey, response); // 同一時間內最初的ImageRequest執(zhí)行成功后,回調這段時間阻塞的相同ImageRequest對應的成功回調接口. BatchedImageRequest request = mInFlightRequests.remove(cacheKey); if (request != null) { request.mResponseBitmap = response; // 將阻塞的ImageRequest進行結果分發(fā). batchResponse(cacheKey, request); } }從上述代碼可以看出,ImageRequest請求成功后,已經從mInFlightRequests中獲取了對應的BatchedImageRequest對象.而同一時間被阻塞的相同的ImageRequest對應的ImageContainer都在BatchedImageRequest的mContainers集合中.
那我認為,batchResponse方法只需要遍歷對應BatchedImageRequest的mContainers集合即可.
但是,ImageLoader源碼中,我認為多余的構造了一個HashMap對象mBatchedResponses來保存BatchedImageRequest集合,然后在batchResponse方法中又對集合進行兩層for循環(huán)各種遍歷,實在是非常詭異,求指導.
詭異代碼如下:
private void batchResponse(String cacheKey, BatchedImageRequest request) { mBatchedResponses.put(cacheKey, request); if (mRunnable == null) { mRunnable = new Runnable() { @Override public 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 runnable mHandler.postDelayed(mRunnable, 100); } }我認為的代碼實現(xiàn)應該是:
private void batchResponse(String cacheKey, BatchedImageRequest request) { if (mRunnable == null) { mRunnable = new Runnable() { @Override public 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 runnable mHandler.postDelayed(mRunnable, 100); } } •使用ImageLoader默認提供的ImageListener,我認為存在一個缺陷,即圖片閃現(xiàn)問題.當為ListView的item設置圖片時,需要增加TAG判斷.因為對應的ImageView可能已經被回收利用了.
自定義L1緩存類
首先說明一下,所謂的L1和L2緩存分別指的是內存緩存和硬盤緩存.
實現(xiàn)L1緩存,我們可以使用Android提供的Lru緩存類,示例代碼如下:
import android.graphics.Bitmap;import android.support.v4.util.LruCache;/** Lru算法的L1緩存實現(xiàn)類. */@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) { @Override protected int sizeOf(String key, Bitmap value) { return value.getRowBytes() * value.getHeight(); } }; } @Override public Bitmap getBitmap(String url) { return mLruCache.get(url); } @Override public void putBitmap(String url, Bitmap bitmap) { mLruCache.put(url, bitmap); }}以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持武林網。
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