SDWebImage 源码解读（一）：WebCache Categories
SDWebImage 是咱们在 iOS 开发过程当中常常使用的框架之一，它能够很方便的帮助咱们异步下载存储在远程服务器上的图片。其主流程的代码调用流程以下：ios
在接下来的几篇文章里，咱们会跟随这张图的流程，逐步深刻阅读该框架的代码。objective-c
UIImageView+WebCache
readme 中的使用该框架的调用示例以下：swift
#import <SDWebImage/SDWebImage.h>
...
[imageView sd_setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
placeholderImage:[UIImage imageNamed:@"placeholder.png"]];
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该方法声明在 UIImageView+WebCache 分类中，以下所示：api
/** * Set the imageView `image` with an `url` and a placeholder. * * The download is asynchronous and cached. * * @param url The url for the image. * @param placeholder The image to be set initially, until the image request finishes. * @see sd_setImageWithURL:placeholderImage:options: */
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder NS_REFINED_FOR_SWIFT;
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UIImage
方法声明中的 UIImage 不是简单的类名，而是宏定义，声明在 SDWebImageCompat.h 文件中以下所示：安全
#if SD_MAC
#import <AppKit/AppKit.h>
#ifndef UIImage
#define UIImage NSImage
#endif
#ifndef UIImageView
#define UIImageView NSImageView
#endif
#ifndef UIView
#define UIView NSView
#endif
#ifndef UIColor
#define UIColor NSColor
#endif
#else
#if SD_UIKIT
#import <UIKit/UIKit.h>
#endif
#if SD_WATCH
#import <WatchKit/WatchKit.h>
#ifndef UIView
#define UIView WKInterfaceObject
#endif
#ifndef UIImageView
#define UIImageView WKInterfaceImage
#endif
#endif
#endif
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由于 SDWebImage 是一个跨平台（macOS/iOS/tvOS/watchOS）的框架，但为了不在代码中到处进行平台的判断，因而经过宏定义来避免这一点。而 SD_MAC 等平台标示的声明以下：服务器
// Seems like TARGET_OS_MAC is always defined (on all platforms).
// To determine if we are running on macOS, use TARGET_OS_OSX in Xcode 8
#if TARGET_OS_OSX
#define SD_MAC 1
#else
#define SD_MAC 0
#endif
// iOS and tvOS are very similar, UIKit exists on both platforms
// Note: watchOS also has UIKit, but it's very limited
#if TARGET_OS_IOS || TARGET_OS_TV
#define SD_UIKIT 1
#else
#define SD_UIKIT 0
#endif
#if TARGET_OS_IOS
#define SD_IOS 1
#else
#define SD_IOS 0
#endif
#if TARGET_OS_TV
#define SD_TV 1
#else
#define SD_TV 0
#endif
#if TARGET_OS_WATCH
#define SD_WATCH 1
#else
#define SD_WATCH 0
#endif
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NS_REFINED_FOR_SWIFT
应用 NS_REFINED_FOR_SWIFT 宏会公开现有的 Objective-C API，以便在精炼的 API 中重用。现有 API 在导入时使用双下划线（__）重命名，以帮助防止你在其余地方意外使用现有 API。多线程
下面的示例将 NS_REFINED_FOR_SWIFT 宏添加到 getRed(red:green:blue:alpha:) 方法：app
@interface Color : NSObject
- (void)getRed:(nullable CGFloat *)red
green:(nullable CGFloat *)green
blue:(nullable CGFloat *)blue
alpha:(nullable CGFloat *)alpha NS_REFINED_FOR_SWIFT;
@end
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如下规则肯定如何导入 API 的现有接口：框架
- 初始化方法由 Swift 导入，并在其第一个参数标签前加上双下划线（__）。
- 若是 getter 或 setter 方法被标记为 NS_REFINED_FOR_SWIFT，则 Swift 将对象下标方法做为方法导入，其中双下划线（__）前置于其基本名称，而不是 Swift 下标。
- 导入其余方法时，在其基本名称前加上双下划线（__）。
更多详细信息能够参考这份文档：Improving Objective-C API Declarations for Swiftdom
书接上文，sd_setImageWithURL:placeholderImage: 方法的实现以下：
- (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder {
[self sd_setImageWithURL:url placeholderImage:placeholder options:0 progress:nil completed:nil];
}
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其调用了内部参数最多的方法，以下所示：
- (void)sd_setImageWithURL:(nullable NSURL *)url placeholderImage:(nullable UIImage *)placeholder options:(SDWebImageOptions)options progress:(nullable SDImageLoaderProgressBlock)progressBlock completed:(nullable SDExternalCompletionBlock)completedBlock {
[self sd_setImageWithURL:url placeholderImage:placeholder options:options context:nil progress:progressBlock completed:completedBlock];
}
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
[self sd_internalSetImageWithURL:url
placeholderImage:placeholder
options:options
context:context
setImageBlock:nil
progress:progressBlock
completed:^(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL) {
if (completedBlock) {
completedBlock(image, error, cacheType, imageURL);
}
}];
}
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UIImageView+WebCache 分类里声明的全部方法最后都会调用 sd_internalSetImageWithURL:placeholderImage:options:context:setImageBlock:progress:completed: 方法，改方法声明在 UIView+WebCache.h 中。
UIView+WebCache
UIView+WebCache 是 UIView 的分类，全部的下载图片的方法都会汇总到这个点进行调用。方法声明以下：
/** * Set the imageView `image` with an `url` and optionally a placeholder image. * * The download is asynchronous and cached. * * @param url The url for the image. * @param placeholder The image to be set initially, until the image request finishes. * @param options The options to use when downloading the image. @see SDWebImageOptions for the possible values. * @param context A context contains different options to perform specify changes or processes, see `SDWebImageContextOption`. This hold the extra objects which `options` enum can not hold. * @param setImageBlock Block used for custom set image code. If not provide, use the built-in set image code (supports `UIImageView/NSImageView` and `UIButton/NSButton` currently) * @param progressBlock A block called while image is downloading * @note the progress block is executed on a background queue * @param completedBlock A block called when operation has been completed. * This block has no return value and takes the requested UIImage as first parameter and the NSData representation as second parameter. * In case of error the image parameter is nil and the third parameter may contain an NSError. * * The forth parameter is an `SDImageCacheType` enum indicating if the image was retrieved from the local cache * or from the memory cache or from the network. * * The fith parameter normally is always YES. However, if you provide SDWebImageAvoidAutoSetImage with SDWebImageProgressiveLoad options to enable progressive downloading and set the image yourself. This block is thus called repeatedly with a partial image. When image is fully downloaded, the * block is called a last time with the full image and the last parameter set to YES. * * The last parameter is the original image URL */
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock;
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方法的实现以下：
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
context:(nullable SDWebImageContext *)context
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDImageLoaderProgressBlock)progressBlock
completed:(nullable SDInternalCompletionBlock)completedBlock {
context = [context copy]; // copy to avoid mutable object
NSString *validOperationKey = context[SDWebImageContextSetImageOperationKey]; // SDWebImageContext 本质上就是个字典
if (!validOperationKey) {
validOperationKey = NSStringFromClass([self class]); // 若是 validOperationKey 为 nil，就经过当前对象的类名建立一个
}
self.sd_latestOperationKey = validOperationKey;
[self sd_cancelImageLoadOperationWithKey:validOperationKey]; // 取消上一次的图片加载操做
self.sd_imageURL = url;
if (!(options & SDWebImageDelayPlaceholder)) { // 若是 options 里有 SDWebImageDelayPlaceholder，马上设置占位图
dispatch_main_async_safe(^{
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock cacheType:SDImageCacheTypeNone imageURL:url];
});
}
if (url) {
// reset the progress
NSProgress *imageProgress = objc_getAssociatedObject(self, @selector(sd_imageProgress));
if (imageProgress) {
imageProgress.totalUnitCount = 0;
imageProgress.completedUnitCount = 0;
}
#if SD_UIKIT || SD_MAC
// check and start image indicator
[self sd_startImageIndicator];
id<SDWebImageIndicator> imageIndicator = self.sd_imageIndicator;
#endif
SDWebImageManager *manager = context[SDWebImageContextCustomManager]; // 从 context 中取出 SDWebImageManager
if (!manager) {
manager = [SDWebImageManager sharedManager];
}
// 设置更新进度 block
SDImageLoaderProgressBlock combinedProgressBlock = ^(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL) {
if (imageProgress) {
imageProgress.totalUnitCount = expectedSize;
imageProgress.completedUnitCount = receivedSize;
}
#if SD_UIKIT || SD_MAC
if ([imageIndicator respondsToSelector:@selector(updateIndicatorProgress:)]) {
double progress = 0;
if (expectedSize != 0) {
progress = (double)receivedSize / expectedSize;
}
progress = MAX(MIN(progress, 1), 0); // 0.0 - 1.0
dispatch_async(dispatch_get_main_queue(), ^{
[imageIndicator updateIndicatorProgress:progress];
});
}
#endif
if (progressBlock) {
progressBlock(receivedSize, expectedSize, targetURL);
}
};
@weakify(self);
id <SDWebImageOperation> operation = [manager loadImageWithURL:url options:options context:context progress:combinedProgressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { // 加载图片的开始
@strongify(self);
if (!self) { return; }
// if the progress not been updated, mark it to complete state
if (imageProgress && finished && !error && imageProgress.totalUnitCount == 0 && imageProgress.completedUnitCount == 0) {
imageProgress.totalUnitCount = SDWebImageProgressUnitCountUnknown;
imageProgress.completedUnitCount = SDWebImageProgressUnitCountUnknown;
}
#if SD_UIKIT || SD_MAC
// check and stop image indicator
if (finished) {
[self sd_stopImageIndicator];
}
#endif
BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage);
BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) ||
(!image && !(options & SDWebImageDelayPlaceholder)));
SDWebImageNoParamsBlock callCompletedBlockClojure = ^{
if (!self) { return; }
if (!shouldNotSetImage) {
[self sd_setNeedsLayout];
}
if (completedBlock && shouldCallCompletedBlock) {
completedBlock(image, data, error, cacheType, finished, url);
}
};
// case 1a: we got an image, but the SDWebImageAvoidAutoSetImage flag is set
// OR
// case 1b: we got no image and the SDWebImageDelayPlaceholder is not set
if (shouldNotSetImage) {
dispatch_main_async_safe(callCompletedBlockClojure);
return;
}
UIImage *targetImage = nil;
NSData *targetData = nil;
if (image) {
// case 2a: we got an image and the SDWebImageAvoidAutoSetImage is not set
targetImage = image;
targetData = data;
} else if (options & SDWebImageDelayPlaceholder) {
// case 2b: we got no image and the SDWebImageDelayPlaceholder flag is set
targetImage = placeholder;
targetData = nil;
}
#if SD_UIKIT || SD_MAC
// check whether we should use the image transition
SDWebImageTransition *transition = nil;
if (finished && (options & SDWebImageForceTransition || cacheType == SDImageCacheTypeNone)) {
transition = self.sd_imageTransition;
}
#endif
dispatch_main_async_safe(^{
#if SD_UIKIT || SD_MAC
[self sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:transition cacheType:cacheType imageURL:imageURL];
#else
[self sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock cacheType:cacheType imageURL:imageURL];
#endif
callCompletedBlockClojure();
});
}];
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
} else {
#if SD_UIKIT || SD_MAC
[self sd_stopImageIndicator];
#endif
dispatch_main_async_safe(^{
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey : @"Image url is nil"}];
completedBlock(nil, nil, error, SDImageCacheTypeNone, YES, url);
}
});
}
}
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具体的代码分析已经写在代码注释里的，代码中有以下一些值得学习的点：
SDWebImageContext
SDWebImageContext 声明以下，本质上是泛型字典：
typedef NSDictionary<SDWebImageContextOption, id> SDWebImageContext;
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其 key 声明以下：
/** A String to be used as the operation key for view category to store the image load operation. This is used for view instance which supports different image loading process. If nil, will use the class name as operation key. (NSString *) */
FOUNDATION_EXPORT SDWebImageContextOption _Nonnull const SDWebImageContextSetImageOperationKey;
SDWebImageContextOption const SDWebImageContextSetImageOperationKey = @"setImageOperationKey";
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使用泛型很好的避免了使用普通字典会出现的硬编码问题。
sd_cancelImageLoadOperationWithKey
sd_cancelImageLoadOperationWithKey 方法用于取消图片的加载操做，其声明和实现以下：
/** * Cancel all operations for the current UIView and key * * @param key key for identifying the operations */
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key;
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {
if (key) {
// Cancel in progress downloader from queue
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
id<SDWebImageOperation> operation;
@synchronized (self) {
operation = [operationDictionary objectForKey:key];
}
if (operation) {
if ([operation conformsToProtocol:@protocol(SDWebImageOperation)]) {
[operation cancel];
}
@synchronized (self) {
[operationDictionary removeObjectForKey:key];
}
}
}
}
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具体的说就是经过 key 去字典里取得对应的 operation，而后 cancel 掉，而后移除。值得注意的是为了防止字典引起的多线程问题，这里使用了 @synchronized。
sd_operationDictionary 方法的实现以下：
- (SDOperationsDictionary *)sd_operationDictionary {
@synchronized(self) {
SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
if (operations) {
return operations;
}
operations = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory capacity:0];
objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
return operations;
}
}
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operation 属性是经过关联对象实现的，SDOperationsDictionary 本质上是一个 NSMapTable，NSMapTable 是一个 hashTable，与字典的区别在于能够选择以什么样的方式存储 key 和 value，以下所示：
// key is strong, value is weak because operation instance is retained by SDWebImageManager's runningOperations property
// we should use lock to keep thread-safe because these method may not be acessed from main queue
typedef NSMapTable<NSString *, id<SDWebImageOperation>> SDOperationsDictionary;
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dispatch_main_async_safe
dispatch_main_async_safe 是一个宏，其目的是为了方便快捷的保证线程的安全调用，其实现以下：
#ifndef dispatch_main_async_safe
#define dispatch_main_async_safe(block)\
if (dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL) == dispatch_queue_get_label(dispatch_get_main_queue())) {\
block();\
} else {\
dispatch_async(dispatch_get_main_queue(), block);\
}
#endif
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值得注意的是，判断当前队列是不是主队列的方法是经过 dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL) == dispatch_queue_get_label(dispatch_get_main_queue()) 作到的。
@weakify 和 @strongify
@weakify 和 @strongify 是两个用于处理 weak-strong-dance 的宏，具体更详尽的解释能够看这篇文章：
深刻研究 Block 用 weakSelf、strongSelf、@weakify、@strongify 解决循环引用
sd_setImageLoadOperation
sd_setImageLoadOperation 方法用于将 operation 存储起来，声明和实现以下：
/** * Set the image load operation (storage in a UIView based weak map table) * * @param operation the operation * @param key key for storing the operation */
- (void)sd_setImageLoadOperation:(nullable id<SDWebImageOperation>)operation forKey:(nullable NSString *)key;
- (void)sd_setImageLoadOperation:(nullable id<SDWebImageOperation>)operation forKey:(nullable NSString *)key {
if (key) {
[self sd_cancelImageLoadOperationWithKey:key];
if (operation) {
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
@synchronized (self) {
[operationDictionary setObject:operation forKey:key];
}
}
}
}
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设置图片
在下载图片完成或者须要设置占位图的时候，会走 sd_setImage:imageData:basedOnClassOrViaCustomSetImageBlock:cacheType:imageURL: 方法，实现以下：
- (void)sd_setImage:(UIImage *)image imageData:(NSData *)imageData basedOnClassOrViaCustomSetImageBlock:(SDSetImageBlock)setImageBlock cacheType:(SDImageCacheType)cacheType imageURL:(NSURL *)imageURL {
#if SD_UIKIT || SD_MAC
[self sd_setImage:image imageData:imageData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:nil cacheType:cacheType imageURL:imageURL];
#else
// watchOS does not support view transition. Simplify the logic
if (setImageBlock) {
setImageBlock(image, imageData, cacheType, imageURL);
} else if ([self isKindOfClass:[UIImageView class]]) {
UIImageView *imageView = (UIImageView *)self;
[imageView setImage:image];
}
#endif
}
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watchOS 由于不支持 transtion，直接进行图片设置或者调用 block，其余平台会走 sd_setImage:imageData:basedOnClassOrViaCustomSetImageBlock:transition:cacheType:imageURL: 方法，实现以下：
#if SD_UIKIT || SD_MAC
- (void)sd_setImage:(UIImage *)image imageData:(NSData *)imageData basedOnClassOrViaCustomSetImageBlock:(SDSetImageBlock)setImageBlock transition:(SDWebImageTransition *)transition cacheType:(SDImageCacheType)cacheType imageURL:(NSURL *)imageURL {
UIView *view = self;
SDSetImageBlock finalSetImageBlock;
if (setImageBlock) {
finalSetImageBlock = setImageBlock;
} else if ([view isKindOfClass:[UIImageView class]]) {
UIImageView *imageView = (UIImageView *)view;
finalSetImageBlock = ^(UIImage *setImage, NSData *setImageData, SDImageCacheType setCacheType, NSURL *setImageURL) {
imageView.image = setImage;
};
}
#if SD_UIKIT
else if ([view isKindOfClass:[UIButton class]]) {
UIButton *button = (UIButton *)view;
finalSetImageBlock = ^(UIImage *setImage, NSData *setImageData, SDImageCacheType setCacheType, NSURL *setImageURL) {
[button setImage:setImage forState:UIControlStateNormal];
};
}
#endif
#if SD_MAC
else if ([view isKindOfClass:[NSButton class]]) {
NSButton *button = (NSButton *)view;
finalSetImageBlock = ^(UIImage *setImage, NSData *setImageData, SDImageCacheType setCacheType, NSURL *setImageURL) {
button.image = setImage;
};
}
#endif
if (transition) {
#if SD_UIKIT
[UIView transitionWithView:view duration:0 options:0 animations:^{
// 0 duration to let UIKit render placeholder and prepares block
if (transition.prepares) {
transition.prepares(view, image, imageData, cacheType, imageURL);
}
} completion:^(BOOL finished) {
[UIView transitionWithView:view duration:transition.duration options:transition.animationOptions animations:^{
if (finalSetImageBlock && !transition.avoidAutoSetImage) {
finalSetImageBlock(image, imageData, cacheType, imageURL);
}
if (transition.animations) {
transition.animations(view, image);
}
} completion:transition.completion];
}];
#elif SD_MAC
[NSAnimationContext runAnimationGroup:^(NSAnimationContext * _Nonnull prepareContext) {
// 0 duration to let AppKit render placeholder and prepares block
prepareContext.duration = 0;
if (transition.prepares) {
transition.prepares(view, image, imageData, cacheType, imageURL);
}
} completionHandler:^{
[NSAnimationContext runAnimationGroup:^(NSAnimationContext * _Nonnull context) {
context.duration = transition.duration;
context.timingFunction = transition.timingFunction;
context.allowsImplicitAnimation = (transition.animationOptions & SDWebImageAnimationOptionAllowsImplicitAnimation);
if (finalSetImageBlock && !transition.avoidAutoSetImage) {
finalSetImageBlock(image, imageData, cacheType, imageURL);
}
if (transition.animations) {
transition.animations(view, image);
}
} completionHandler:^{
if (transition.completion) {
transition.completion(YES);
}
}];
}];
#endif
} else {
if (finalSetImageBlock) {
finalSetImageBlock(image, imageData, cacheType, imageURL);
}
}
}
#endif
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SDWebImageTransition 是一个模型类，其中封装了进行过渡动画所需的全部参数，如 duration，timingFunction 等，其还经过分类提供了方便方法，能够快速建立动画效果，以下所示：
+ (SDWebImageTransition *)fadeTransition {
SDWebImageTransition *transition = [SDWebImageTransition new];
#if SD_UIKIT
transition.animationOptions = UIViewAnimationOptionTransitionCrossDissolve | UIViewAnimationOptionAllowUserInteraction;
#else
transition.animations = ^(__kindof NSView * _Nonnull view, NSImage * _Nullable image) {
CATransition *trans = [CATransition animation];
trans.type = kCATransitionFade;
[view.layer addAnimation:trans forKey:kCATransition];
};
#endif
return transition;
}
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至此，关于 WebCache Categories 的相关代码就所有阅读完毕了。
相关文章
- 1. SDWebImage源码解读
- 2. SDWebImage 源码解读（二）：Manager
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- 5. SDWebImage 源码解读（六）：SDImageIOCoder
- 6. SDWebImage 源码解读（五）：Decoder
- 7. SDWebImage 源码解读（四）：Downloader
- 8. SDWebImage 源码解读（三）：Cache
- 9. SDWebImage源码解析(一)
- 10. 【原】SDWebImage源码阅读（三）
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