SDWebImage 是一個眾所周知加載圖片的框架，對常用UI元素：UIImageView,UIButton和MKAnnotationView提供了Category類別擴展，實現一個異步查詢，下載并且支持緩存的功能，整個框架各個類分工明確，接口調用靈活，代碼風格很值得學習。
- 調用方式非常簡單，UI分類類似接口如下：
- (void)sd_setImageWithURL:(NSURL *)url;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;
- (void)sd_setImageWithURL:(NSURL *)url completed:(SDWebImageCompletionBlock)completedBlock;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder completed:(SDWebImageCompletionBlock)completedBlock;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options completed:(SDWebImageCompletionBlock)completedBlock;
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
上述幾種方法最終都會調用最后一個參數最齊全的方法：在block中拿到image等數據之后處理自己的邏輯。
-
思路流程圖
縱觀整個框架，其實核心類就三個 SDWebImageManger，SDWebImageDownloader，SDWebImageCache ，分類UIImageView+WebCache和UIButton+WebCache為下載圖片提供接口，SDWebImageManger負責協調SDWebImageCache完成緩存查詢，存儲，清除和SDWebImageDownloader圖片下載，緩存。
圖一 簡單流程圖.png
為了方便自己及他人更好的理解跟吸收，附上官方文檔說明圖：
圖二 官方文檔圖.png
圖三 官方文檔圖.png - 源碼解析
在引入正題之前先來了解幾個枚舉
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
/**
* By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
* 默認情況下，當一個 URL 下載失敗，該URL被列入黑名單，將不會繼續嘗試下載
* This flag disable this blacklisting.
* 此標志取消黑名單
*/
SDWebImageRetryFailed = 1 << 0,
/**
* By default, image downloads are started during UI interactions, this flags disable this feature,
* 默認情況下，在 UI 交互時也會啟動圖像下載，此標記取消這一功能
* leading to delayed download on UIScrollView deceleration for instance.
* 會延遲下載,UIScrollView停止滾動之后再繼續下載
* 下載事件監聽的運行循環模式是 NSDefaultRunLoopMode
*/
SDWebImageLowPriority = 1 << 1,
/**
* This flag disables on-disk caching
* 禁用磁盤緩存
*/
SDWebImageCacheMemoryOnly = 1 << 2,
/**
* This flag enables progressive download, the image is displayed progressively during download as a browser would do.
* 此標記允許漸進式下載，就像瀏覽器中那樣，下載過程中，圖像會逐步顯示出來
* By default, the image is only displayed once completely downloaded.
* 默認情況下，圖像只會在下載完后顯示
*/
SDWebImageProgressiveDownload = 1 << 3,
/**
* Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
* 即使圖像被緩存，遵守 HTPP 響應的緩存控制，如果需要，從遠程刷新圖像
* The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
* 磁盤緩存將由 NSURLCache 處理，而不是 SDWebImage，這會對性能有輕微的影響
* This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
* 此選項有助于處理同一個請求 URL 的圖像發生變化
* If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
* 如果緩存的圖像被刷新，會調用一次 completion block，并傳遞最終的圖像
*
* Use this flag only if you can't make your URLs static with embedded cache busting parameter.
* 僅在無法使用嵌入式緩存清理參數確定圖像 URL 時，使用此標記
*/
SDWebImageRefreshCached = 1 << 4,
/**
* In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
* 在 iOS 4+，當 App 進入后臺后仍然會繼續下載圖像。這是向系統請求額外的后臺時間以保證下載請求完成的
* extra time in background to let the request finish. If the background task expires the operation will be cancelled.
* 如果后臺任務過期，請求將會被取消
*/
SDWebImageContinueInBackground = 1 << 5,
/**
* Handles cookies stored in NSHTTPCookieStore by setting
* 通過設置
* NSMutableURLRequest.HTTPShouldHandleCookies = YES;
* 處理保存在 NSHTTPCookieStore 中的 cookies
*/
SDWebImageHandleCookies = 1 << 6,
/**
* Enable to allow untrusted SSL certificates.
* 允許不信任的 SSL 證書
* Useful for testing purposes. Use with caution in production.
* 可以出于測試目的使用，在正式產品中慎用
*/
SDWebImageAllowInvalidSSLCertificates = 1 << 7,
/**
* By default, images are loaded in the order in which they were queued. This flag moves them to
* 默認情況下，圖像會按照在隊列中的順序被加載，此標記會將它們移動到隊列前部立即被加載
* the front of the queue.
*/
SDWebImageHighPriority = 1 << 8,
/**
* By default, placeholder images are loaded while the image is loading. This flag will delay the loading
* 默認情況下，在加載圖像時，占位圖像已經會被加載。而此標記會延遲加載占位圖像，直到圖像已經完成加載
* of the placeholder image until after the image has finished loading.
*/
SDWebImageDelayPlaceholder = 1 << 9,
/**
* We usually don't call transformDownloadedImage delegate method on animated images,
* 通常不會在可動畫的圖像上調用 transformDownloadedImage 代理方法，因為大多數轉換代碼會破壞動畫文件
* as most transformation code would mangle it.
* Use this flag to transform them anyway.
* 使用此標記嘗試轉換
*/
SDWebImageTransformAnimatedImage = 1 << 10,
/**
* By default, image is added to the imageView after download. But in some cases, we want to
* have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
* Use this flag if you want to manually set the image in the completion when success
*/
SDWebImageAvoidAutoSetImage = 1 << 11
};
typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
//同SDWebImageLowPriority 意義相同
SDWebImageDownloaderLowPriority = 1 << 0,
//同SDWebImageProgressiveDownload 意義相同
SDWebImageDownloaderProgressiveDownload = 1 << 1,
/**
* By default, request prevent the use of NSURLCache. With this flag, NSURLCache
* is used with default policies.
默認情況下，請求阻止NSURLCache的使用，使用此標記，NSURLCache使用時是默認政策
*/
SDWebImageDownloaderUseNSURLCache = 1 << 2,
/**
* Call completion block with nil image/imageData if the image was read from NSURLCache
* (to be combined with `SDWebImageDownloaderUseNSURLCache`).
從NSURLCache中獲取的image，回調completion時參數imge／imageData傳nil
*/
SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
/**
* In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
* extra time in background to let the request finish. If the background task expires the operation will be cancelled.
同SDWebImageContinueInBackground 意義相同
*/
SDWebImageDownloaderContinueInBackground = 1 << 4,
/**
* Handles cookies stored in NSHTTPCookieStore by setting
* NSMutableURLRequest.HTTPShouldHandleCookies = YES;
設置NSMutableURLRequest.HTTPShouldHandleCookies = YES，允許在NSHTTPCookieStore中操作cookies
*/
SDWebImageDownloaderHandleCookies = 1 << 5,
/**
* Enable to allow untrusted SSL certificates.
測試環境允許使用過不信任SSL證書，生產環境要小心
* Useful for testing purposes. Use with caution in production.
*/
SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
/**
* Put the image in the high priority queue.
圖片下載操作至于高優先級隊列
*/
SDWebImageDownloaderHighPriority = 1 << 7,
};
typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
/**
* Default value. All download operations will execute in queue style (first-in-first-out).
操作默認執行方式，所有下載操作按照隊列方式執行
*/
SDWebImageDownloaderFIFOExecutionOrder,
/**
* All download operations will execute in stack style (last-in-first-out).
所有下載操作按照棧的方式執行
*/
SDWebImageDownloaderLIFOExecutionOrder
};
下面是下載過程的代碼
1..SDWebImageManager 執行下載操作
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
options:(SDWebImageOptions)options
progress:(SDWebImageDownloaderProgressBlock)progressBlock
completed:(SDWebImageCompletionWithFinishedBlock)completedBlock {
// Invoking this method without a completedBlock is pointless
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");
// Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, XCode won't
// throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
// Prevents app crashing on argument type error like sending NSNull instead of NSURL
if (![url isKindOfClass:NSURL.class]) {
url = nil;
}
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;
//1.判斷傳入的url是否是請求失敗的url
BOOL isFailedUrl = NO;
@synchronized (self.failedURLs) {
isFailedUrl = [self.failedURLs containsObject:url];
}
//針對url（如果沒有／請求失敗的url沒有設置SDWebImageRetryFailed選項）
if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
dispatch_main_sync_safe(^{
NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil];
completedBlock(nil, error, SDImageCacheTypeNone, YES, url);
});
return operation;
}
// 將當前此次下載圖片的operation添加到操作緩存中
@synchronized (self.runningOperations) {
[self.runningOperations addObject:operation];
}
//通過url找到對應cachekey
NSString *key = [self cacheKeyForURL:url];
//2.imageCache根據cachekey去查找內存中是否存儲image
operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) {
// 查到之后如果operation取消，就從操作緩存中移除
if (operation.isCancelled) {
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
return;
}
//2.1 imageCache 對查找結果處理 （沒有圖片||SDWebImageRefreshCached設置屬性）&&（代理沒有實現方法||代理實現方法返回BOOL）
if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
//2.1.1 存在image && SDWebImageRefreshCached 直接調用block進行回調
if (image && options & SDWebImageRefreshCached) {
dispatch_main_sync_safe(^{
// If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
// AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
completedBlock(image, nil, cacheType, YES, url);
});
}
// download if no image or requested to refresh anyway, and download allowed by delegate
SDWebImageDownloaderOptions downloaderOptions = 0;
if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
if (image && options & SDWebImageRefreshCached) {
// force progressive off if image already cached but forced refreshing
downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
// ignore image read from NSURLCache if image if cached but force refreshing
downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}
//2.1.2 image不存在 imageDownloader開始下載圖片
id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
//在下載回調中處理
//1..操作取消，不做處理
if (!strongOperation || strongOperation.isCancelled) {
// Do nothing if the operation was cancelled
// See #699 for more details
// if we would call the completedBlock, there could be a race condition between this block and another completedBlock for the same object, so if this one is called second, we will overwrite the new data
}
//2..下載錯誤,回調block
else if (error) {
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(nil, error, SDImageCacheTypeNone, finished, url);
}
});
if ( error.code != NSURLErrorNotConnectedToInternet
&& error.code != NSURLErrorCancelled
&& error.code != NSURLErrorTimedOut
&& error.code != NSURLErrorInternationalRoamingOff
&& error.code != NSURLErrorDataNotAllowed
&& error.code != NSURLErrorCannotFindHost
&& error.code != NSURLErrorCannotConnectToHost) {
@synchronized (self.failedURLs) {
[self.failedURLs addObject:url];
}
}
}
//3.. 下載成功
else {
//3..1..SDWebImageRetryFailed 設置之后將請求失敗的url從failedURLs數組中移除,默認情況是url一旦請求不成功，默認加入failedURLs，不在發送請求
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
//3..2..下載之后進行緩存
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
//緩存圖片分為以下三種情況
//3..2..1.. (SDWebImageRefreshCached && 緩存image存在 && downloadimage不存在) 不做任何處理
if (options & SDWebImageRefreshCached && image && !downloadedImage) {
// Image refresh hit the NSURLCache cache, do not call the completion block
}
//3..2..2..（downloadimage存在 && SDWebImageTransformAnimatedImage && 圖片下載之后代理是否要轉換圖片）
else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
//異步緩存圖片
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
//downloadimage經代理操作的transformedImage
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
// imageCache 進行緩存圖片（圖片，是否重新計算大小，大小，cachekey，是否只存在disk中）
[self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:(imageWasTransformed ? nil : data) forKey:key toDisk:cacheOnDisk];
}
//上述異步操作完成之后回調block
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
});
}
//3..2..3..直接將downloadimage進行存儲
else {
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
}
dispatch_main_sync_safe(^{
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
}
});
}
}
//4.. 下載成功之后并且完成以后，將此次下載的operation從下載緩存中移除
if (finished) {
@synchronized (self.runningOperations) {
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
}
}];
//2.1.3 操作取消
operation.cancelBlock = ^{
[subOperation cancel];
@synchronized (self.runningOperations) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation) {
[self.runningOperations removeObject:strongOperation];
}
}
};
}
//2.2 緩存image存在
else if (image) {
dispatch_main_sync_safe(^{
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation && !strongOperation.isCancelled) {
completedBlock(image, nil, cacheType, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
//2.3 其他情況，緩存image不存在，同時delegate不允許下載
else {
// Image not in cache and download disallowed by delegate
dispatch_main_sync_safe(^{
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (strongOperation && !weakOperation.isCancelled) {
completedBlock(nil, nil, SDImageCacheTypeNone, YES, url);
}
});
@synchronized (self.runningOperations) {
[self.runningOperations removeObject:operation];
}
}
}];
return operation;
}
2.. SDImageCache執行查找操作，其中有幾個關鍵的屬性
@property (strong, nonatomic) NSCache *memCache;// 系統緩存類，根據cachekey查找對象
@property (strong, nonatomic) NSString *diskCachePath;// 用于存儲磁盤文件path
@property (strong, nonatomic) NSMutableArray *customPaths;// 自定義path
@property (SDDispatchQueueSetterSementics, nonatomic) dispatch_queue_t ioQueue;
//計算image花費的內存
FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
return image.size.height * image.size.width * image.scale * image.scale;
}
// 保質期是一周
static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7;
//cache 根據cachekey查找image
- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock {
此處...非空判斷
//1. 從內存中查找 NSCache中根據key查找object
// First check the in-memory cache...
UIImage *image = [self imageFromMemoryCacheForKey:key];
if (image) {
doneBlock(image, SDImageCacheTypeMemory);
return nil;
}
//2.從磁盤查找，主要是根據key找到文件路徑，取出其文件路徑下的image
NSOperation *operation = [NSOperation new];
dispatch_async(self.ioQueue, ^{
if (operation.isCancelled) {
return;
}
@autoreleasepool {
UIImage *diskImage = [self diskImageForKey:key];
//diskimage && 從磁盤中發現image之后要存儲到內存中
if (diskImage && self.shouldCacheImagesInMemory) {
//存儲image需要花費的內存（圖片的width*height*scale*scale）
NSUInteger cost = SDCacheCostForImage(diskImage);
//存進內存
[self.memCache setObject:diskImage forKey:key cost:cost];
}
//成功存儲之后進行回調
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, SDImageCacheTypeDisk);
});
}
});
return operation;
}
//其中涉及得到的方法，請自行查看，此處省略
3.. SDWebImageDownloader執行下載操作
//下載圖片
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageDownloaderCompletedBlock)completedBlock {
__block SDWebImageDownloaderOperation *operation;
__weak __typeof(self)wself = self;
[self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^{
// 如果是第一次創建，實際回調的是createblock
//限制request的時間限制
NSTimeInterval timeoutInterval = wself.downloadTimeout;
if (timeoutInterval == 0.0) {
timeoutInterval = 15.0;
}
// In order to prevent from potential duplicate caching (NSURLCache + SDImageCache) we disable the cache for image requests if told otherwise
//構建請求的request 設置request的屬性
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
request.HTTPShouldUsePipelining = YES;
//request的allHTTPHeaderFields 根據headersFilter 中的block中的url跟dic 推出dic
if (wself.headersFilter) {
request.allHTTPHeaderFields = wself.headersFilter(url, [wself.HTTPHeaders copy]);
}
else {
request.allHTTPHeaderFields = wself.HTTPHeaders;
}
//針對 request構建operation
operation = [[wself.operationClass alloc] initWithRequest:request
inSession:self.session
options:options
progress:^(NSInteger receivedSize, NSInteger expectedSize) {
SDWebImageDownloader *sself = wself;
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_sync(sself.barrierQueue, ^{
//url對應的value
callbacksForURL = [sself.URLCallbacks[url] copy];
});
for (NSDictionary *callbacks in callbacksForURL) {
dispatch_async(dispatch_get_main_queue(), ^{
SDWebImageDownloaderProgressBlock callback = callbacks[kProgressCallbackKey];
if (callback) callback(receivedSize, expectedSize);
});
}
}
completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {
SDWebImageDownloader *sself = wself;
if (!sself) return;
__block NSArray *callbacksForURL;
dispatch_barrier_sync(sself.barrierQueue, ^{
callbacksForURL = [sself.URLCallbacks[url] copy];
if (finished) {
[sself.URLCallbacks removeObjectForKey:url];
}
});
for (NSDictionary *callbacks in callbacksForURL) {
SDWebImageDownloaderCompletedBlock callback = callbacks[kCompletedCallbackKey];
if (callback) callback(image, data, error, finished);
}
}
cancelled:^{
SDWebImageDownloader *sself = wself;
if (!sself) return;
dispatch_barrier_async(sself.barrierQueue, ^{
[sself.URLCallbacks removeObjectForKey:url];
});
}];
operation.shouldDecompressImages = wself.shouldDecompressImages;
if (wself.urlCredential) {
operation.credential = wself.urlCredential;
} else if (wself.username && wself.password) {
operation.credential = [NSURLCredential credentialWithUser:wself.username password:wself.password persistence:NSURLCredentialPersistenceForSession];
}
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
// 下載到operation加入到下載隊列中進行操作
[wself.downloadQueue addOperation:operation];
//設置operaton操作的操作順序
if (wself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
[wself.lastAddedOperation addDependency:operation];
wself.lastAddedOperation = operation;
}
}];
return operation;
}
//其中涉及得到的方法，請自行查看，此處省略
- SDWebImage的特性
為UIImageView,UIButton和MKAnnotationView進行了類別擴展，添加了web圖片和緩存管理；
是一個異步圖片下載器；
異步的內存+硬盤緩沖以及自動的緩沖過期處理；
后臺圖片解壓縮功能；
可以保證相同的url（圖片的檢索key）不會被重復多次下載；
可以保證假的無效url不會不斷嘗試去加載；
保證主線程不會被阻塞；
性能高；
使用GCD和ARC； - SDWebImage注意點??：
附上一個大神總結的：http://www.lxweimin.com/p/277b688b0384
最后，SDWebImage博大精深，日后還需潛心鉆研，這篇博客只是為了記錄自己學習的一個過程，日后復習方便，如有不對，望提供寶貴意見，謝謝～
另附上參考文章鏈接：
http://www.lxweimin.com/p/277b688b0384
http://blog.csdn.net/cordova/article/details/54708029
http://www.lxweimin.com/p/93696717b4a3
