相信對於廣大的iOS開發者，對SDWebImage並不會陌生，這個框架通過給UIImageView和UIButton新增分類，實現一個非同步下載圖片並且支援快取的功能。整個框架的介面非常簡潔，每個類的分工都很明確，是很值得大家學習的。
在使用這個框架的時候，只需要提供一個下載的url和佔點陣圖就可以在回撥裡拿到下載後的圖片：
[imageview sd_setImageWithURL:[NSURL URLWithString:@"pic.jpg"] placeholderImage:[UIImage imageNamed:@"placeholder"] completed:^(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL) {
imageview.image = image;
NSLog(@"圖片載入完成");
}];
複製程式碼而且我們還可以不設定佔點陣圖片，也可以不使用回撥的block，非常靈活:
//圖片下載完成後直接顯示下載後的圖片
[imageview sd_setImageWithURL:[NSURL URLWithString:@"pic.jpg"]];
複製程式碼在最開始先簡單介紹這個框架：
這個框架的核心類是SDWebImageManger，在外部有UIImageView+WebCache 和 UIButton+WebCache 為下載圖片的操作提供介面。內部有SDWebImageManger負責處理和協調 SDWebImageDownloader 和 SDWebImageCache：SDWebImageDownloader負責具體的下載任務，SDWebImageCache負責關於快取的工作：新增，刪除，查詢快取。
首先我們大致看一下這個框架的呼叫流程圖：
從這個流程圖裡可以大致看出，該框架分為兩個層：UIKit層（負責接收下載引數）和工具層（負責下載操作和快取）。
OK～基本流程大概清楚了，我們看一下每個層具體實現吧～
UIKit層
該框架最外層的類是UIImageView +WebCache，我們將圖片的URL，佔點陣圖片直接給這個類。下面是這個類的公共介面：
// ============== UIImageView + WebCache.h ============== //
- (void)sd_setImageWithURL:(nullable NSURL *)url;
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder;
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options;
- (void)sd_setImageWithURL:(nullable NSURL *)url
completed:(nullable SDExternalCompletionBlock)completedBlock;
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
completed:(nullable SDExternalCompletionBlock)completedBlock;
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
completed:(nullable SDExternalCompletionBlock)completedBlock;
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock;
複製程式碼可以看出，這個類提供的介面非常靈活，可以根據我們自己的需求來呼叫其中某一個方法，而這些方法到最後都會走到：
// ============== UIView+ WebCache.m ============== //
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock;
複製程式碼而這個方法裡面，呼叫的是UIView+WebCache分類的：
// ============== UIView+ WebCache.m ============== //
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
operationKey:(nullable NSString *)operationKey
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock;
複製程式碼為什麼不是UIImageView+WebCache而要上一層到UIView的分類裡呢？ 因為SDWebImage框架也支援UIButton的下載圖片等方法，所以需要在它們的父類：UIView裡面統一一個下載方法。
簡單看一下這個方法的實現（省略的程式碼用...代替）：
// ============== UIView+ WebCache.m ============== //
//valid key：UIImageView || UIButton
NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
//UIView+WebCacheOperation 的 operationDictionary
//下面這行程式碼是保證沒有當前正在進行的非同步下載操作, 使它不會與即將進行的操作發生衝突
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
//新增臨時的佔點陣圖（在不延遲新增佔點陣圖的option下）
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
});
}
//如果url存在
if (url) {
...
__weak __typeof(self)wself = self;
//SDWebImageManager下載圖片
id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
...
//dispatch_main_sync_safe : 保證block能在主執行緒進行
dispatch_main_async_safe(^{
if (!sself) {
return;
}
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
//image，而且不自動替換 placeholder image
completedBlock(image, error, cacheType, url);
return;
} else if (image) {
//存在image，需要馬上替換 placeholder image
[sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
} else {
//沒有image，在圖片下載完之後顯示 placeholder image
if ((options & SDWebImageDelayPlaceholder)) {
[sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
//在操作快取字典（operationDictionary）裡新增operation，表示當前的操作正在進行
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
} else {
//如果url不存在，就在completedBlock裡傳入error（url為空）
dispatch_main_async_safe(^{
[self sd_removeActivityIndicator];
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
```
> 值得一提的是，在這一層，使用一個字典``operationDictionary``專門用作儲存操作的快取，隨時新增，刪除操作任務。
而這個字典是``UIView+WebCacheOperation``分類的關聯物件，它的存取方法使用執行時來操作：
```objc
// ============== UIView+WebCacheOperation.m ============== //
//獲取關聯物件：operations（用來存放操作的字典）
- (SDOperationsDictionary *)operationDictionary {
SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
//存放操作的字典
if (operations) {
return operations;
}
//如果沒有，就新建一個
operations = [NSMutableDictionary dictionary];
objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
return operations;
}
複製程式碼為什麼不直接在
UIImageView+WebCache裡直接關聯這個物件呢？我覺得這裡作者應該是遵從物件導向的單一職責原則（SRP：Single responsibility principle），就連類都要履行這個職責，何況分類呢？這裡作者專門創造一個分類UIView+WebCacheOperation來管理操作快取（字典）。
到這裡，UIKit層上面的東西都講完了，現在開始正式講解工具層。
工具層
上文提到過，SDWebImageManager同時管理SDImageCache和SDWebImageDownloader兩個類，它是這一層的老大哥。在下載任務開始的時候，SDWebImageManager首先訪問SDImageCache來查詢是否存在快取，如果有快取，直接返回快取的圖片。如果沒有快取，就命令SDWebImageDownloader來下載圖片，下載成功後，存入快取，顯示圖片。以上是SDWebImageManager大致的工作流程。
在詳細講解SDWebImageManager是如何下載圖片之前，我們先看一下這個類的幾個重要的屬性：
// ============== SDWebImageManager.h ============== //
@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;//管理快取
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader //下載器*imageDownloader;
@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs;//記錄失效url的名單
@property (strong, nonatomic, nonnull) NSMutableArray<SDWebImageCombinedOperation *> *runningOperations;//記錄當前正在執行的操作
複製程式碼SDWebImageManager下載圖片的方法只有一個：
[SDWebImageManager.sharedManager loadImageWithURL:options:progress:completed:]
複製程式碼看一下這個方法的具體實現：
// ============== SDWebImageManager.m ============== //
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
...
//在SDImageCache裡查詢是否存在快取的圖片
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
...
//（沒有快取圖片） || （即使有快取圖片，也需要更新快取圖片） || （代理沒有響應imageManager:shouldDownloadImageForURL:訊息，預設返回yes，需要下載圖片）|| （imageManager:shouldDownloadImageForURL:返回yes，需要下載圖片）
if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
//1. 存在快取圖片 && 即使有快取圖片也要下載更新圖片
if (cachedImage && options & SDWebImageRefreshCached) {
[self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
}
// 2. 如果不存在快取圖片
...
//開啟下載器下載
//subOperationToken 用來標記當前的下載任務，便於被取消
SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
__strong __typeof(weakOperation) strongOperation = weakOperation;
if (!strongOperation || strongOperation.isCancelled) {
// 1. 如果任務被取消，則什麼都不做，避免和其他的completedBlock重複
} else if (error) {
//2. 如果有錯誤
//2.1 在completedBlock裡傳入error
[self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];
//2.2 在錯誤url名單中新增當前的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];
}
}
}
else {
//3. 下載成功
//3.1 如果需要下載失敗後重新下載，則將當前url從失敗url名單裡移除
if ((options & SDWebImageRetryFailed)) {
@synchronized (self.failedURLs) {
[self.failedURLs removeObject:url];
}
}
//3.2 進行快取
BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
//（即使快取存在，也要重新整理圖片） && 快取圖片 && 不存在下載後的圖片：不做操作
} else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
//（下載圖片成功 && （沒有動圖||處理動圖） && （下載之後，快取之前處理圖片） dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
if (transformedImage && finished) {
BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
// pass nil if the image was transformed, so we can recalculate the data from the image
//快取圖片
[self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
}
//將圖片傳入completedBlock
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
});
} else {
//(圖片下載成功並結束)
if (downloadedImage && finished) {
[self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
}
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
}
}
//如果完成，從當前執行的操作列表裡移除當前操作
if (finished) {
[self safelyRemoveOperationFromRunning:strongOperation];
}
}];
//取消的block
operation.cancelBlock = ^{
//取消當前的token
[self.imageDownloader cancel:subOperationToken];
__strong __typeof(weakOperation) strongOperation = weakOperation;
//從當前執行的操作列表裡移除當前操作
[self safelyRemoveOperationFromRunning:strongOperation];
};
} else if (cachedImage) {
//存在快取圖片
__strong __typeof(weakOperation) strongOperation = weakOperation;
//呼叫完成的block
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
//刪去當前的的下載操作（執行緒安全）
[self safelyRemoveOperationFromRunning:operation];
} else {
//沒有快取的圖片，而且下載被代理終止了
__strong __typeof(weakOperation) strongOperation = weakOperation;
// 呼叫完成的block
[self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
//刪去當前的下載操作
[self safelyRemoveOperationFromRunning:operation];
}
}];
return operation;
}
複製程式碼看完了SDWebImageManager的回撥處理，我們分別看一下
SDImageCache和SDWebImageDownloader內部具體是如何工作的。首先看一下SDImageCache：
SDImageCache
屬性
// ============== SDImageCache.m ============== //
@property (strong, nonatomic, nonnull) NSCache *memCache;//記憶體快取
@property (strong, nonatomic, nonnull) NSString *diskCachePath;//磁碟快取路徑
@property (strong, nonatomic, nullable) NSMutableArray<NSString *> *customPaths;//
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t //ioQueue唯一子執行緒;
複製程式碼核心方法：查詢快取
// ============== SDImageCache.m ============== //
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
if (!key) {
if (doneBlock) {
doneBlock(nil, nil, SDImageCacheTypeNone);
}
return nil;
}
//================檢視記憶體的快取=================//
UIImage *image = [self imageFromMemoryCacheForKey:key];
// 如果存在，直接呼叫block，將image，data，CaheType傳進去
if (image) {
NSData *diskData = nil;
//如果是gif，就拿到data，後面要傳到doneBlock裡。不是gif就傳nil
if ([image isGIF]) {
diskData = [self diskImageDataBySearchingAllPathsForKey:key];
}
if (doneBlock) {
doneBlock(image, diskData, SDImageCacheTypeMemory);
}
//因為圖片有快取可供使用，所以不用例項化NSOperation，直接範圍nil
return nil;
}
//================檢視磁碟的快取=================//
NSOperation *operation = [NSOperation new];
//唯一的子執行緒：self.ioQueue
dispatch_async(self.ioQueue, ^{
if (operation.isCancelled) {
// 在用之前就判斷operation是否被取消了，作者考慮的非常嚴謹
return;
}
@autoreleasepool {
NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
UIImage *diskImage = [self diskImageForKey:key];
if (diskImage && self.config.shouldCacheImagesInMemory) {
// cost 被用來計算快取中所有物件的代價。當記憶體受限或者所有快取物件的總代價超過了最大允許的值時，快取會移除其中的一些物件。
NSUInteger cost = SDCacheCostForImage(diskImage);
//存入記憶體快取中
[self.memCache setObject:diskImage forKey:key cost:cost];
}
if (doneBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
});
}
}
});
return operation;
}
複製程式碼SDWebImageDownloader
屬性
// ============== SDWebImageDownloader.m ============== //
@property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue;//下載佇列
@property (weak, nonatomic, nullable) NSOperation *lastAddedOperation;//最後新增的下載操作
@property (assign, nonatomic, nullable) Class operationClass;//操作類
@property (strong, nonatomic, nonnull) NSMutableDictionary<NSURL *, SDWebImageDownloaderOperation *> *URLOperations;//運算元組
@property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders;//HTTP請求頭
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t barrierQueue;//用來阻塞前面的下載執行緒（序列化）
複製程式碼核心方法：下載圖片
// ============== SDWebImageDownloader.m ============== //
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
__weak SDWebImageDownloader *wself = self;
return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
__strong __typeof (wself) sself = wself;
NSTimeInterval timeoutInterval = sself.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
//建立下載請求
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:(options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData) timeoutInterval:timeoutInterval];
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
request.HTTPShouldUsePipelining = YES;
if (sself.headersFilter) {
request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]);
}
else {
request.allHTTPHeaderFields = sself.HTTPHeaders;
}
//建立下載操作：SDWebImageDownloaderOperation用於請求網路資源的操作，它是一個 NSOperation 的子類
SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
operation.shouldDecompressImages = sself.shouldDecompressImages;
//url證書
if (sself.urlCredential) {
operation.credential = sself.urlCredential;
} else if (sself.username && sself.password) {
operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
}
//優先順序
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
//在下載佇列裡新增下載操作，執行下載操作
[sself.downloadQueue addOperation:operation];
//如果後進先出
if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
// Emulate LIFO execution order by systematically adding new operations as last operation's dependency
//addDependency:引數opertaion倍新增到NSOperationQueue後，只有等該opertion結束後才能執行其他的operation，實現了後進先出
[sself.lastAddedOperation addDependency:operation];
sself.lastAddedOperation = operation;
}
return operation;
}];
}
複製程式碼這裡面還有一個addProgressCallback: progressBlock: completedBlock: forURL: createCallback:方法，用來儲存progressBlock和completedBlock。我們看一下這個方法的實現：
// ============== SDWebImageDownloader.m ============== //
- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
forURL:(nullable NSURL *)url
createCallback:(SDWebImageDownloaderOperation *(^)())createCallback {
// url 用來作為回撥字典的key，如果為空，立即返回失敗
if (url == nil) {
if (completedBlock != nil) {
completedBlock(nil, nil, nil, NO);
}
return nil;
}
__block SDWebImageDownloadToken *token = nil;
//序列化前面所有的操作
dispatch_barrier_sync(self.barrierQueue, ^{
//當前下載操作中取出SDWebImageDownloaderOperation例項
SDWebImageDownloaderOperation *operation = self.URLOperations[url];
if (!operation) {
//如果沒有，就初始化它
operation = createCallback();
self.URLOperations[url] = operation;
__weak SDWebImageDownloaderOperation *woperation = operation;
operation.completionBlock = ^{
SDWebImageDownloaderOperation *soperation = woperation;
if (!soperation) return;
if (self.URLOperations[url] == soperation) {
[self.URLOperations removeObjectForKey:url];
};
};
}
id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
//這裡 downloadOperationCancelToken 預設是一個字典，存放 progressBlock 和 completedBlock
token = [SDWebImageDownloadToken new];
token.url = url;
token.downloadOperationCancelToken = downloadOperationCancelToken;
});
return token;
}
複製程式碼這裡真正儲存兩個block的方法是addHandlersForProgress: completed:：
- (nullable id)addHandlersForProgress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
//例項化一個SDCallbacksDictionary，存放一個progressBlock 和 completedBlock
SDCallbacksDictionary *callbacks = [NSMutableDictionary new];
if (progressBlock) callbacks[kProgressCallbackKey] = [progressBlock copy];
if (completedBlock) callbacks[kCompletedCallbackKey] = [completedBlock copy];
dispatch_barrier_async(self.barrierQueue, ^{
//新增到快取中 self.callbackBlocks
[self.callbackBlocks addObject:callbacks];
});
return callbacks;
}
複製程式碼到這裡SDWebImage的核心方法都講解完畢了，其他沒有講到的部分以後會慢慢新增上去。
最後看一下一些比較零散的知識點：
1. 執行時存取關聯物件：
存：
objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
//將operations物件關聯給self，地址為&loadOperationKey，語義是OBJC_ASSOCIATION_RETAIN_NONATOMIC。
複製程式碼取：
SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
//將operations物件通過地址&loadOperationKey從self裡取出來
複製程式碼2. 陣列的寫操作需要加鎖（多執行緒訪問，避免覆寫）
//給self.runningOperations加鎖
//self.runningOperations陣列的新增操作
@synchronized (self.runningOperations) {
[self.runningOperations addObject:operation];
}
//self.runningOperations陣列的刪除操作
- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation {
@synchronized (self.runningOperations) {
if (operation) {
[self.runningOperations removeObject:operation];
}
}
}
複製程式碼3. 確保在主執行緒的巨集：
dispatch_main_async_safe(^{
//將下面這段程式碼放在主執行緒中
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
});
//巨集定義：
#define dispatch_main_async_safe(block)\
if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
block();\
} else {\
dispatch_async(dispatch_get_main_queue(), block);\
}
#endif
複製程式碼4. 設定不能為nil的引數
- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader {
if ((self = [super init])) {
_imageCache = cache;
_imageDownloader = downloader;
_failedURLs = [NSMutableSet new];
_runningOperations = [NSMutableArray new];
}
return self;
}
複製程式碼如果在引數裡新增了nonnull關鍵字，那麼編譯器就可以檢查傳入的引數是否為nil，如果是，則編譯器會有警告
5. 容錯，強制轉換型別
if ([url isKindOfClass:NSString.class]) {
url = [NSURL URLWithString:(NSString *)url];
}
複製程式碼在傳入的引數為NSString時（但是方法引數要求是NSURL），自動轉換為NSURL
貌似還有圖片解碼等內容沒有詳細看，以後會逐漸補充噠～
本文已經同步到我的個人技術部落格：傳送門
---------------------------- 2018年7月17日更新 ----------------------------
注意注意！！！
筆者在近期開通了個人公眾號，主要分享程式設計，讀書筆記，思考類的文章。
- 程式設計類文章：包括筆者以前釋出的精選技術文章，以及後續釋出的技術文章（以原創為主），並且逐漸脫離 iOS 的內容，將側重點會轉移到提高程式設計能力的方向上。
- 讀書筆記類文章：分享程式設計類，思考類，心理類，職場類書籍的讀書筆記。
- 思考類文章：分享筆者平時在技術上，生活上的思考。
因為公眾號每天釋出的訊息數有限制，所以到目前為止還沒有將所有過去的精選文章都發布在公眾號上，後續會逐步釋出的。
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