SDWebImageManager.md

1. 在初始化过程中+[SDWebImageManager sharedManager]，然后执行-[init],然后首先读取defaultXXX,如果为null,然后再配置自己的类。

+ (id<SDImageCache>)defaultImageCache {
    return _defaultImageCache;
}

+ (void)setDefaultImageCache:(id<SDImageCache>)defaultImageCache {
    if (defaultImageCache && ![defaultImageCache conformsToProtocol:@protocol(SDImageCache)]) {
        return;
    }
    _defaultImageCache = defaultImageCache;
}

+ (id<SDImageLoader>)defaultImageLoader {
    return _defaultImageLoader;
}

+ (void)setDefaultImageLoader:(id<SDImageLoader>)defaultImageLoader {
    if (defaultImageLoader && ![defaultImageLoader conformsToProtocol:@protocol(SDImageLoader)]) {
        return;
    }
    _defaultImageLoader = defaultImageLoader;
}

+ (nonnull instancetype)sharedManager {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

- (nonnull instancetype)init {
/// 读取默认值，没有的话再设置[SDImageCache sharedImageCache]
    id<SDImageCache> cache = [[self class] defaultImageCache];
    if (!cache) {
        cache = [SDImageCache sharedImageCache];
    }
    /// 读取默认值，没有的话再设置[SDWebImageDownloader sharedDownloader]
    id<SDImageLoader> loader = [[self class] defaultImageLoader];
    if (!loader) {
        loader = [SDWebImageDownloader sharedDownloader];
    }
    return [self initWithCache:cache loader:loader];
}

- (nonnull instancetype)initWithCache:(nonnull id<SDImageCache>)cache loader:(nonnull id<SDImageLoader>)loader {
    if ((self = [super init])) {
        _imageCache = cache;
        _imageLoader = loader;
        _failedURLs = [NSMutableSet new];
        SD_LOCK_INIT(_failedURLsLock);
        _runningOperations = [NSMutableSet new];
        SD_LOCK_INIT(_runningOperationsLock);
    }
    return self;
}

2. 自旋锁

#if SD_USE_OS_UNFAIR_LOCK
#define SD_LOCK(lock) os_unfair_lock_lock(&lock)
#else
#define SD_LOCK(lock) if (@available(iOS 10, tvOS 10, watchOS 3, macOS 10.12, *)) os_unfair_lock_lock(&lock); \
else OSSpinLockLock(&lock##_deprecated);
#endif

3. 保证在主线程更新UI

#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

4.