runtime category 属性

- (NSMutableDictionary *)operationDictionary {
    NSMutableDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
    if (operations) {
        return operations;
    }
    operations = [NSMutableDictionary dictionary];
    objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    return operations;
}

通过宏定义安全线程

#define dispatch_main_sync_safe(block)\
    if ([NSThread isMainThread]) {\
        block();\
    } else {\
        dispatch_sync(dispatch_get_main_queue(), block);\
    }

#define dispatch_main_async_safe(block)\
    if ([NSThread isMainThread]) {\
        block();\
    } else {\
        dispatch_async(dispatch_get_main_queue(), block);\
    }

通过宏控制ARC和非ARC下queue的释放和属性标识

#if OS_OBJECT_USE_OBJC
    #undef SDDispatchQueueRelease
    #undef SDDispatchQueueSetterSementics
    #define SDDispatchQueueRelease(q)
    #define SDDispatchQueueSetterSementics strong
#else
#undef SDDispatchQueueRelease
#undef SDDispatchQueueSetterSementics
#define SDDispatchQueueRelease(q) (dispatch_release(q))
#define SDDispatchQueueSetterSementics assign
#endif

使用：
@property (SDDispatchQueueSetterSementics, nonatomic) dispatch_queue_t ioQueue;

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    SDDispatchQueueRelease(_ioQueue);
}

在调试阶段可使用NSAssert判断nil

// Invoking this method without a completedBlock is pointless
    NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");

SDWebImageManager简单实现单单例

/**
 * Returns global SDWebImageManager instance.
 *
 * @return SDWebImageManager shared instance
 */
+ (id)sharedManager {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

NSURL判断，容错NSString、NSNull

// 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;
    }

使用NS_OPTIONS、NS_ENUM、typedef、block

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.
     * This flag disable this blacklisting.
     */
    SDWebImageRetryFailed = 1 << 0,

    /**
     * By default, image downloads are started during UI interactions, this flags disable this feature,
     * leading to delayed download on UIScrollView deceleration for instance.
     */
    SDWebImageLowPriority = 1 << 1,

    /**
     * This flag disables on-disk caching
     */
    SDWebImageCacheMemoryOnly = 1 << 2,

    /**
     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
     * By default, the image is only displayed once completely downloaded.
     */
    SDWebImageProgressiveDownload = 1 << 3,

};

typedef void(^SDWebImageCompletionBlock)(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL);
typedef void(^SDWebImageCompletionWithFinishedBlock)(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL);
typedef NSString *(^SDWebImageCacheKeyFilterBlock)(NSURL *url);

if判断多使用短的这种有利于代码可读

  if (!doneBlock) {
        return nil;
    }

   if (!key) {
        doneBlock(nil, SDImageCacheTypeNone);
        return nil;
    }

    // First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        doneBlock(image, SDImageCacheTypeMemory);
        return nil;
    }

block传值后应用copy关键字

- (id)initWithRequest:(NSURLRequest *)request
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock {
    if ((self = [super init])) {
        _request = request;
        _shouldDecompressImages = YES;
        _shouldUseCredentialStorage = YES;
        _options = options;
        _progressBlock = [progressBlock copy];
        _completedBlock = [completedBlock copy];
        _cancelBlock = [cancelBlock copy];
        _executing = NO;
        _finished = NO;
        _expectedSize = 0;
        responseFromCached = YES; // Initially wrong until `connection:willCacheResponse:` is called or not called
    }
    return self;
}