1.接口文件

图片下载器的访问方法

/** 设置用于下载图片的图片下载器*/+ (void)setSharedImageDownloader:(AFImageDownloader *)imageDownloader;/** 获取用于下载图片的图片下载器 */+ (AFImageDownloader *)sharedImageDownloader;复制代码

为按钮设置图片的方法

/** 为按钮设置指定状态和指定图片链接的图片 */- (void)setImageForState:(UIControlState)state                 withURL:(NSURL *)url;/** 为按钮设置指定状态、指定图片链接和指定占位图的图片 */- (void)setImageForState:(UIControlState)state                 withURL:(NSURL *)url        placeholderImage:(nullable UIImage *)placeholderImage;/** 为按钮设置指定状态、指定图片链接、指定占位图以及指定成功与失败回调block的图片 */- (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;复制代码

为按钮设置背景图片的方法

/** 为按钮设置指定状态和指定图片链接的背景图片 */- (void)setBackgroundImageForState:(UIControlState)state                           withURL:(NSURL *)url;/** 为按钮设置指定状态、指定图片链接和指定占位图的背景图片 */- (void)setBackgroundImageForState:(UIControlState)state                           withURL:(NSURL *)url                  placeholderImage:(nullable UIImage *)placeholderImage;/** 为按钮设置指定状态、指定图片链接、指定占位图以及指定成功与失败回调block的背景图片 */- (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;复制代码

取消图片加载方法

/** 取消按钮在指定状态下的所有图片下载任务 */- (void)cancelImageDownloadTaskForState:(UIControlState)state;/** 取消按钮在指定状态下的所有背景图片下载任务 */- (void)cancelBackgroundImageDownloadTaskForState:(UIControlState)state;复制代码

2.实现文件

2.1.UIButton+_AFNetworking私有分类

2.1.1.image相关

静态字符

// 普通static char AFImageDownloadReceiptNormal;// 高亮static char AFImageDownloadReceiptHighlighted;// 已选static char AFImageDownloadReceiptSelected;// 禁用static char AFImageDownloadReceiptDisabled;复制代码

静态方法

/** 这个方法通过传入的控件状态返回对应的静态字符 */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;    }}复制代码

属性的访问方法

/** 通过Runtime的关联对象为分类添加属性的getter */- (AFImageDownloadReceipt *)af_imageDownloadReceiptForState:(UIControlState)state {    return (AFImageDownloadReceipt *)objc_getAssociatedObject(self, af_imageDownloadReceiptKeyForState(state));}/** 通过Runtime的关联对象为分类添加属性的setter */- (void)af_setImageDownloadReceipt:(AFImageDownloadReceipt *)imageDownloadReceipt                           forState:(UIControlState)state{    objc_setAssociatedObject(self, af_imageDownloadReceiptKeyForState(state), imageDownloadReceipt, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}复制代码

2.1.2.backgroundImage相关

静态字符

// 普通static char AFBackgroundImageDownloadReceiptNormal;// 高亮static char AFBackgroundImageDownloadReceiptHighlighted;// 已选static char AFBackgroundImageDownloadReceiptSelected;// 禁用static char AFBackgroundImageDownloadReceiptDisabled;复制代码

静态方法

/** 这个方法通过传入的控件状态返回对应的静态字符 */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;    }}复制代码

属性的访问方法

/** 通过Runtime的关联对象为分类添加属性的getter */- (AFImageDownloadReceipt *)af_backgroundImageDownloadReceiptForState:(UIControlState)state {    return (AFImageDownloadReceipt *)objc_getAssociatedObject(self, af_backgroundImageDownloadReceiptKeyForState(state));}/** 通过Runtime的关联对象为分类添加属性的setter */- (void)af_setBackgroundImageDownloadReceipt:(AFImageDownloadReceipt *)imageDownloadReceipt                                     forState:(UIControlState)state{    objc_setAssociatedObject(self, af_backgroundImageDownloadReceiptKeyForState(state), imageDownloadReceipt, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}复制代码

2.2.方法实现

属性的访问方法

/** 通过Runtime的关联对象为分类添加sharedImageDownloader属性的getter */+ (AFImageDownloader *)sharedImageDownloader {    return objc_getAssociatedObject(self, @selector(sharedImageDownloader)) ?: [AFImageDownloader defaultInstance];}/** 通过Runtime的关联对象为分类添加sharedImageDownloader属性的setter */+ (void)setSharedImageDownloader:(AFImageDownloader *)imageDownloader {    objc_setAssociatedObject(self, @selector(sharedImageDownloader), imageDownloader, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}复制代码

image相关接口方法实现

- (void)setImageForState:(UIControlState)state                 withURL:(NSURL *)url{    // 调用下面的方法    [self setImageForState:state withURL:url placeholderImage:nil];}- (void)setImageForState:(UIControlState)state                 withURL:(NSURL *)url        placeholderImage:(UIImage *)placeholderImage{    // 调用下面的方法    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];    [request addValue:@"image/*" forHTTPHeaderField:@"Accept"];    [self setImageForState:state withURLRequest:request placeholderImage:placeholderImage success:nil failure:nil];}- (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{    // 如果这个网络请求正在进行中就不重复执行了    if ([self isActiveTaskURLEqualToURLRequest:urlRequest forState:state]) {        return;    }    // 取消掉这个状态的图片下载任务    [self cancelImageDownloadTaskForState:state];    // 实例化图片下载器    AFImageDownloader *downloader = [[self class] sharedImageDownloader];    // 获取到图片缓存对象    id 
    
      imageCache = downloader.imageCache;    // 获取到缓存图片    UIImage *cachedImage = [imageCache imageforRequest:urlRequest withAdditionalIdentifier:nil];    // 如果有缓存    if (cachedImage) {        // 如果设置了成功回调block就调用，否则就为按钮设置图片        if (success) {            success(urlRequest, nil, cachedImage);        } else {            [self setImage:cachedImage forState:state];        }        [self af_setImageDownloadReceipt:nil forState:state];    // 如果没有缓存    } else {        // 如果有占位图先设置占位图        if (placeholderImage) {            [self setImage:placeholderImage forState:state];        }        // 开始下载        __weak __typeof(self)weakSelf = self;        NSUUID *downloadID = [NSUUID UUID];        AFImageDownloadReceipt *receipt;        receipt = [downloader                   downloadImageForURLRequest:urlRequest                   withReceiptID:downloadID                   success:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, UIImage * _Nonnull responseObject) {                       // 如果是当前按钮的下载回调                       __strong __typeof(weakSelf)strongSelf = weakSelf;                       if ([[strongSelf af_imageDownloadReceiptForState:state].receiptID isEqual:downloadID]) {                            // 如果设置了成功回调block就调用，否则就为按钮设置图片                           if (success) {                               success(request, response, responseObject);                           } else if(responseObject) {                               [strongSelf setImage:responseObject forState:state];                           }                           // 将保存下载封装对象的属性置nil                           [strongSelf af_setImageDownloadReceipt:nil forState:state];                       }                   }                   failure:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, NSError * _Nonnull error) {                       // 如果是当前按钮的下载回调                       __strong __typeof(weakSelf)strongSelf = weakSelf;                       if ([[strongSelf af_imageDownloadReceiptForState:state].receiptID isEqual:downloadID]) {                           // 失败回调block                           if (failure) {                               failure(request, response, error);                           }                           // 将保存下载封装对象的属性置nil                           [strongSelf  af_setImageDownloadReceipt:nil forState:state];                       }                   }];        // 用分类的属性保存图片下载封装对象        [self af_setImageDownloadReceipt:receipt forState:state];    }}- (void)cancelImageDownloadTaskForState:(UIControlState)state {     // 通过状态获取到图片下载封装对象    AFImageDownloadReceipt *receipt = [self af_imageDownloadReceiptForState:state];    if (receipt != nil) {        // 根据获取到的图片下载封装对象取消对应的图片下载任务        [[self.class sharedImageDownloader] cancelTaskForImageDownloadReceipt:receipt];        // 把保存图片下载封装对象的属性置nil        [self af_setImageDownloadReceipt:nil forState:state];    }}复制代码

backgroundImage相关接口方法实现

backgroundImage的实现和image的实现除了在设置图片时一个给backgroundImage，一个给image之外都相同。

- (void)setBackgroundImageForState:(UIControlState)state                           withURL:(NSURL *)url{    [self setBackgroundImageForState:state withURL:url placeholderImage:nil];}- (void)setBackgroundImageForState:(UIControlState)state                           withURL:(NSURL *)url                  placeholderImage:(nullable UIImage *)placeholderImage{    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];    [request addValue:@"image/*" forHTTPHeaderField:@"Accept"];    [self setBackgroundImageForState:state withURLRequest:request placeholderImage:placeholderImage success:nil failure:nil];}- (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{    if ([self isActiveBackgroundTaskURLEqualToURLRequest:urlRequest forState:state]) {        return;    }    [self cancelBackgroundImageDownloadTaskForState:state];    AFImageDownloader *downloader = [[self class] sharedImageDownloader];    id 
    
      imageCache = downloader.imageCache;    //Use the image from the image cache if it exists    UIImage *cachedImage = [imageCache imageforRequest:urlRequest withAdditionalIdentifier:nil];    if (cachedImage) {        if (success) {            success(urlRequest, nil, cachedImage);        } else {            [self setBackgroundImage:cachedImage forState:state];        }        [self af_setBackgroundImageDownloadReceipt:nil forState:state];    } else {        if (placeholderImage) {            [self setBackgroundImage:placeholderImage forState:state];        }        __weak __typeof(self)weakSelf = self;        NSUUID *downloadID = [NSUUID UUID];        AFImageDownloadReceipt *receipt;        receipt = [downloader                   downloadImageForURLRequest:urlRequest                   withReceiptID:downloadID                   success:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, UIImage * _Nonnull responseObject) {                       __strong __typeof(weakSelf)strongSelf = weakSelf;                       if ([[strongSelf af_backgroundImageDownloadReceiptForState:state].receiptID isEqual:downloadID]) {                           if (success) {                               success(request, response, responseObject);                           } else if(responseObject) {                               [strongSelf setBackgroundImage:responseObject forState:state];                           }                           [strongSelf af_setBackgroundImageDownloadReceipt:nil forState:state];                       }                   }                   failure:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, NSError * _Nonnull error) {                       __strong __typeof(weakSelf)strongSelf = weakSelf;                       if ([[strongSelf af_backgroundImageDownloadReceiptForState:state].receiptID isEqual:downloadID]) {                           if (failure) {                               failure(request, response, error);                           }                           [strongSelf  af_setBackgroundImageDownloadReceipt:nil forState:state];                       }                   }];        [self af_setBackgroundImageDownloadReceipt:receipt forState:state];    }}- (void)cancelBackgroundImageDownloadTaskForState:(UIControlState)state {    AFImageDownloadReceipt *receipt = [self af_backgroundImageDownloadReceiptForState:state];    if (receipt != nil) {        [[self.class sharedImageDownloader] cancelTaskForImageDownloadReceipt:receipt];        [self af_setBackgroundImageDownloadReceipt:nil forState:state];    }}复制代码

私有方法

/** 判断图片下载请求是否已经正在进行中 */- (BOOL)isActiveTaskURLEqualToURLRequest:(NSURLRequest *)urlRequest forState:(UIControlState)state {    // 先根据状态获取到属性中保存的图片下载封装对象    AFImageDownloadReceipt *receipt = [self af_imageDownloadReceiptForState:state];    // 根据两者的链接是否相同进行比较    return [receipt.task.originalRequest.URL.absoluteString isEqualToString:urlRequest.URL.absoluteString];}/** 判断背景图片下载请求是否已经正在进行中 */- (BOOL)isActiveBackgroundTaskURLEqualToURLRequest:(NSURLRequest *)urlRequest forState:(UIControlState)state {    // 实现和上面的方法相同    AFImageDownloadReceipt *receipt = [self af_backgroundImageDownloadReceiptForState:state];    return [receipt.task.originalRequest.URL.absoluteString isEqualToString:urlRequest.URL.absoluteString];}复制代码

3.总结

当我们利用这个分类为按钮设置图片时

首先会根据要设置的控件状态和图片链接判断是否是重复下载，如果是重复下载就不继续向下进行了。

接着会取消在设置的控件状态下正在进行的下载操作（如果有的话），也就是说只有最新的这次图片设置才有效。

然后获取到图片下载器的图片缓存对象，从图片缓存对象中查找是否有缓存，如果有缓存的话会直接取出添加到按钮上，但是如果用户设置了成功回调block，就不会将图片添加到按钮上，而是会调用blokc，将结果传递给用户，让用户来处理。

如果没有缓存，就会先判断有没有占位图，如果有占位图的话，就先给按钮设置占位图。

接下来将UUID作为标识符开启图片下载任务，并将任务封装对象保存在属性中，属性的命名是根据图片要设置的控件状态相关联的。

下载任务执行完成后，无论成功还是失败，都会先根据设置的控件状态获取到对应的下载任务封装对象，对比下载任务封装对象的标识符和UUID判断是否是同一任务的回调，只用是同一任务的回调才继续执行

在下载成功的情况下，如果用户设置了成功回调block，就调用block，将结果传递给用户，让用户来处理，如果没有设置，就直接将下载好的图片添加到按钮的指定状态上，并将指定状态所对应的保存下载任务封装对象的属性置nil。

当下载失败时，如果用户设置了失败回调block，就调用block，将结果传递给用户，让用户来处理，如果没有设置，就将指定状态所对应的保存下载任务封装对象的属性置nil。

源码阅读系列：AFNetworking