UIButton+AFNetworking原始碼學習

思路與UIImageView+AFNetworking原始碼學習一致，只是多了不同的狀態：UIControlState，和不同的圖片設定：setImage和setBackgroundImage。

下面只列出與UIImageView的不同之處：
類別定義如下：

@interface UIButton (AFNetworking)

+ (void)setSharedImageDownloader:(AFImageDownloader *)imageDownloader;
+ (AFImageDownloader *)sharedImageDownloader;

//setImage with state
- (void)setImageForState:(UIControlState)state
                 withURL:(NSURL *)url;
- (void)setImageForState:(UIControlState)state
                 withURL:(NSURL *)url
        placeholderImage:(nullable UIImage *)placeholderImage;
- (void)setImageForState:(UIControlState)state
          withURLRequest:(NSURLRequest *)urlRequest
        placeholderImage:(nullable UIImage *)placeholderImage
                 success:(nullable void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, UIImage *image))success
                 failure:(nullable void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, NSError *error))failure;

//setBackgroundImage with state
- (void)setBackgroundImageForState:(UIControlState)state
                           withURL:(NSURL *)url;
- (void)setBackgroundImageForState:(UIControlState)state
                           withURL:(NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholderImage;
- (void)setBackgroundImageForState:(UIControlState)state
                    withURLRequest:(NSURLRequest *)urlRequest
                  placeholderImage:(nullable UIImage *)placeholderImage
                           success:(nullable void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, UIImage *image))success
                           failure:(nullable void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, NSError *error))failure;

//cancle image download with state
- (void)cancelImageDownloadTaskForState:(UIControlState)state;
//cancle backgroundImage download with state
- (void)cancelBackgroundImageDownloadTaskForState:(UIControlState)state;

@end

receipt的管理：

@interface UIButton (_AFNetworking)
@end

@implementation UIButton (_AFNetworking)

#pragma mark - setImage時對應不同state的receipt：af_imageDownloadReceiptKeyForState

static char AFImageDownloadReceiptNormal;
static char AFImageDownloadReceiptHighlighted;
static char AFImageDownloadReceiptSelected;
static char AFImageDownloadReceiptDisabled;

static const char * af_imageDownloadReceiptKeyForState(UIControlState state) {
    switch (state) {
        case UIControlStateHighlighted:
            return &AFImageDownloadReceiptHighlighted;
        case UIControlStateSelected:
            return &AFImageDownloadReceiptSelected;
        case UIControlStateDisabled:
            return &AFImageDownloadReceiptDisabled;
        case UIControlStateNormal:
        default:
            return &AFImageDownloadReceiptNormal;
    }
}

- (AFImageDownloadReceipt *)af_imageDownloadReceiptForState:(UIControlState)state {
    return (AFImageDownloadReceipt *)objc_getAssociatedObject(self, af_imageDownloadReceiptKeyForState(state));
}

- (void)af_setImageDownloadReceipt:(AFImageDownloadReceipt *)imageDownloadReceipt
                           forState:(UIControlState)state
{
    objc_setAssociatedObject(self, af_imageDownloadReceiptKeyForState(state), imageDownloadReceipt, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

#pragma mark - setBackgroundImage時，對應不同state的receipt：af_backgroundImageDownloadReceiptForState

static char AFBackgroundImageDownloadReceiptNormal;
static char AFBackgroundImageDownloadReceiptHighlighted;
static char AFBackgroundImageDownloadReceiptSelected;
static char AFBackgroundImageDownloadReceiptDisabled;

static const char * af_backgroundImageDownloadReceiptKeyForState(UIControlState state) {
    switch (state) {
        case UIControlStateHighlighted:
            return &AFBackgroundImageDownloadReceiptHighlighted;
        case UIControlStateSelected:
            return &AFBackgroundImageDownloadReceiptSelected;
        case UIControlStateDisabled:
            return &AFBackgroundImageDownloadReceiptDisabled;
        case UIControlStateNormal:
        default:
            return &AFBackgroundImageDownloadReceiptNormal;
    }
}

- (AFImageDownloadReceipt *)af_backgroundImageDownloadReceiptForState:(UIControlState)state {
    return (AFImageDownloadReceipt *)objc_getAssociatedObject(self, af_backgroundImageDownloadReceiptKeyForState(state));
}

- (void)af_setBackgroundImageDownloadReceipt:(AFImageDownloadReceipt *)imageDownloadReceipt
                                     forState:(UIControlState)state
{
    objc_setAssociatedObject(self, af_backgroundImageDownloadReceiptKeyForState(state), imageDownloadReceipt, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end