AFNetworking 3.0 源码解读（十）之 UIActivityIndicatorView/UIRefreshControl/UIImageView + AFNetworking...
我們應(yīng)該看到過(guò)很多類似這樣的例子：某個(gè)控件擁有加載網(wǎng)絡(luò)圖片的能力。但這究竟是怎么做到的呢？看完這篇文章就明白了。
前言
這篇我們會(huì)介紹 AFNetworking 中的3個(gè)UIKit中的分類。UIActivityIndicatorView UIRefreshControl UIImageView。讀完本篇就能夠明白控件是如何顯示網(wǎng)絡(luò)圖片的。那么如果你有興趣，可以嘗試讓一個(gè)控件的layer也能夠加載網(wǎng)絡(luò)圖片。
提供的功能
我們解讀源碼不僅僅是了解內(nèi)部實(shí)現(xiàn)原理，還要讓開(kāi)發(fā)者明白在這些分類中我能夠使用那些功能，因此在這個(gè) 提供的功能 小結(jié)中，我會(huì)把這3個(gè)分類提供的功能羅列出來(lái)，即使不看下邊的源碼解讀，也會(huì)有所收獲。
UIActivityIndicatorView+AFNetworking
This category adds methods to the UIKit framework's UIActivityIndicatorView class. The methods in this category provide support for automatically starting and stopping animation depending on the loading state of a session task.
這個(gè)分類增加了UIActivityIndicatorView的一個(gè)方法。這個(gè)方法能夠提供根據(jù)task自動(dòng)開(kāi)始和結(jié)束動(dòng)畫的功能
這個(gè)分類需要依賴 AFNetworking。需要監(jiān)聽(tīng)AFNetworking中的網(wǎng)絡(luò)狀態(tài)的通知。按照通常的想法是，只要我監(jiān)聽(tīng)了通知然后設(shè)置自己的狀態(tài)就完事了。然而這并不是好的設(shè)計(jì)。一個(gè)控件的某項(xiàng)新的功能應(yīng)該交給一個(gè)專門負(fù)責(zé)這個(gè)功能的人去完成，這才是好的設(shè)計(jì)。
因此我們給UIActivityIndicatorView擴(kuò)展了一個(gè)屬性af_notificationObserver,這個(gè)屬性是專門處理上邊說(shuō)的事件的管理者。
好吧，我們寫出偽代碼：
- (通知監(jiān)聽(tīng)者 *)af_notificationObserver {return 通知監(jiān)聽(tīng)者; } - (void)setAnimatingWithStateOfTask:(NSURLSessionTask *)task {監(jiān)聽(tīng)者根據(jù)task來(lái)做一些事; }這樣寫的好處是：當(dāng)我們想擴(kuò)展別的功能的時(shí)候，只需要在添加一個(gè)其他功能的負(fù)責(zé)人就可以，所有的邏輯都是負(fù)責(zé)人自己實(shí)現(xiàn)。這種思想簡(jiǎn)直完美。我們看 AFNetworking 中對(duì)上邊偽代碼的實(shí)現(xiàn)。相信大多數(shù)朋友應(yīng)該知道，往分類中添加屬性使用Runtime，不明白的可以看這篇 Objective-C runtime的常見(jiàn)應(yīng)用.
- (AFActivityIndicatorViewNotificationObserver *)af_notificationObserver {AFActivityIndicatorViewNotificationObserver *notificationObserver = objc_getAssociatedObject(self, @selector(af_notificationObserver));if (notificationObserver == nil) {notificationObserver = [[AFActivityIndicatorViewNotificationObserver alloc] initWithActivityIndicatorView:self];objc_setAssociatedObject(self, @selector(af_notificationObserver), notificationObserver, OBJC_ASSOCIATION_RETAIN_NONATOMIC);}return notificationObserver; }- (void)setAnimatingWithStateOfTask:(NSURLSessionTask *)task {[[self af_notificationObserver] setAnimatingWithStateOfTask:task]; }我們來(lái)看看這個(gè)af_notificationObserver有什么話要說(shuō)呢？
- UIActivityIndicatorView *activityIndicatorView 既然讓我來(lái)管理UIActivityIndicatorView，那就必須拿到這個(gè)控件才行。
- initWithActivityIndicatorView: 我不可能憑空出現(xiàn)，通過(guò)這個(gè)方法創(chuàng)建我。
- setAnimatingWithStateOfTask: 我就是通過(guò)這個(gè)方法來(lái)操控UIActivityIndicatorView的。
這么看來(lái)，這個(gè)af_notificationObserver只需要上邊3個(gè)東東就足夠了，那么我們就剩下setAnimatingWithStateOfTask:這個(gè)方法的實(shí)現(xiàn)了。
- (void)setAnimatingWithStateOfTask:(NSURLSessionTask *)task {NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];// 移除 AFNetworking 的通知[notificationCenter removeObserver:self name:AFNetworkingTaskDidResumeNotification object:nil];[notificationCenter removeObserver:self name:AFNetworkingTaskDidSuspendNotification object:nil];[notificationCenter removeObserver:self name:AFNetworkingTaskDidCompleteNotification object:nil];// task != nilif (task) {// task的狀態(tài)不等于完成if (task.state != NSURLSessionTaskStateCompleted) {#pragma clang diagnostic push #pragma clang diagnostic ignored "-Wreceiver-is-weak" #pragma clang diagnostic ignored "-Warc-repeated-use-of-weak"// 狀態(tài)為運(yùn)行中就開(kāi)始，否則為停止if (task.state == NSURLSessionTaskStateRunning) {[self.activityIndicatorView startAnimating];} else {[self.activityIndicatorView stopAnimating];} #pragma clang diagnostic pop// 移除 AFNetworking 的通知[notificationCenter addObserver:self selector:@selector(af_startAnimating) name:AFNetworkingTaskDidResumeNotification object:task];[notificationCenter addObserver:self selector:@selector(af_stopAnimating) name:AFNetworkingTaskDidCompleteNotification object:task];[notificationCenter addObserver:self selector:@selector(af_stopAnimating) name:AFNetworkingTaskDidSuspendNotification object:task];}} }#pragma mark -- (void)af_startAnimating {dispatch_async(dispatch_get_main_queue(), ^{ #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wreceiver-is-weak"[self.activityIndicatorView startAnimating]; #pragma clang diagnostic pop}); }- (void)af_stopAnimating {dispatch_async(dispatch_get_main_queue(), ^{ #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wreceiver-is-weak"[self.activityIndicatorView stopAnimating]; #pragma clang diagnostic pop}); }UIImageView+AFNetworking
我們?cè)?AFImageDownloader 那篇文章中提到過(guò)，要異步顯示網(wǎng)絡(luò)上的圖片，就要把圖片數(shù)據(jù)緩存下來(lái)才行。因此，要賦予UIImageView這項(xiàng)功能，就需要使用 AFImageDownloader 來(lái)獲取圖片數(shù)據(jù)。
不知道大家發(fā)現(xiàn)沒(méi)有，像這張圖片中的這些方法，，我們只需要實(shí)現(xiàn)參數(shù)最多的那個(gè)方法就行了。這應(yīng)該就是所謂的 尾調(diào)函數(shù) 吧。
首先我們先看看UIImageView擴(kuò)展的一個(gè)屬性af_activeImageDownloadReceipt，這個(gè)屬性是圖片依據(jù)
@interface UIImageView (_AFNetworking) @property (readwrite, nonatomic, strong, setter = af_setActiveImageDownloadReceipt:) AFImageDownloadReceipt *af_activeImageDownloadReceipt; @end@implementation UIImageView (_AFNetworking)- (AFImageDownloadReceipt *)af_activeImageDownloadReceipt {return (AFImageDownloadReceipt *)objc_getAssociatedObject(self, @selector(af_activeImageDownloadReceipt)); }- (void)af_setActiveImageDownloadReceipt:(AFImageDownloadReceipt *)imageDownloadReceipt {objc_setAssociatedObject(self, @selector(af_activeImageDownloadReceipt), imageDownloadReceipt, OBJC_ASSOCIATION_RETAIN_NONATOMIC); }通過(guò)運(yùn)行時(shí)為@selector(af_activeImageDownloadReceipt) 設(shè)置了關(guān)聯(lián)值，同樣的原理。 sharedImageDownloader 也是這么設(shè)置的
+ (AFImageDownloader *)sharedImageDownloader {#pragma clang diagnostic push #pragma clang diagnostic ignored "-Wgnu"return objc_getAssociatedObject(self, @selector(sharedImageDownloader)) ?: [AFImageDownloader defaultInstance]; #pragma clang diagnostic pop }+ (void)setSharedImageDownloader:(AFImageDownloader *)imageDownloader {objc_setAssociatedObject(self, @selector(sharedImageDownloader), imageDownloader, OBJC_ASSOCIATION_RETAIN_NONATOMIC); }在這里說(shuō)下這個(gè)objc_setAssociatedObject方法，其中第二個(gè)參數(shù)是一個(gè)地址，因此我們可以用@selector
或者自定義一個(gè)全局的const字段，取它的地址。 看下邊的例子，我為UIImageView擴(kuò)展了一個(gè)屬性abc。
我在使用的時(shí)候
UIImageView *imageView = [[UIImageView alloc] init]; [imageView setValue:@"qwer" forKey:@"abc"];NSString *str = [imageView valueForKey:@"abc"]; NSLog(@"%@",str);--
- (void)cancelImageDownloadTask {if (self.af_activeImageDownloadReceipt != nil) {[[self.class sharedImageDownloader] cancelTaskForImageDownloadReceipt:self.af_activeImageDownloadReceipt];[self clearActiveDownloadInformation];} }- (void)clearActiveDownloadInformation {self.af_activeImageDownloadReceipt = nil; }- (BOOL)isActiveTaskURLEqualToURLRequest:(NSURLRequest *)urlRequest {return [self.af_activeImageDownloadReceipt.task.originalRequest.URL.absoluteString isEqualToString:urlRequest.URL.absoluteString]; }來(lái)看這個(gè)核心方法，處理手法和之前的代碼如出一轍，值得學(xué)習(xí)的是，核心方法中的判斷比較詳細(xì)。
- (void)setImageWithURLRequest:(NSURLRequest *)urlRequestplaceholderImage:(UIImage *)placeholderImagesuccess:(void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, UIImage *image))successfailure:(void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, NSError *error))failure {// urlRequest 不正確if ([urlRequest URL] == nil) {// 取消下載任務(wù)[self cancelImageDownloadTask];// 賦值替代圖片self.image = placeholderImage;return;}// 如果當(dāng)前活動(dòng)的下載和本下載一樣，就返回if ([self isActiveTaskURLEqualToURLRequest:urlRequest]){return;}// 取消之前的下載任務(wù)[self cancelImageDownloadTask];// 取出downloaderAFImageDownloader *downloader = [[self class] sharedImageDownloader];// 取出緩存id <AFImageRequestCache> imageCache = downloader.imageCache;//Use the image from the image cache if it existsUIImage *cachedImage = [imageCache imageforRequest:urlRequest withAdditionalIdentifier:nil];if (cachedImage) {// 如果寫了success Block 就調(diào)動(dòng)block，但不會(huì)給image賦值if (success) {success(urlRequest, nil, cachedImage);} else {self.image = cachedImage;}[self clearActiveDownloadInformation];} else {// 沒(méi)有緩存的話，先設(shè)置替代圖片if (placeholderImage) {self.image = placeholderImage;}__weak __typeof(self)weakSelf = self;NSUUID *downloadID = [NSUUID UUID];AFImageDownloadReceipt *receipt;receipt = [downloaderdownloadImageForURLRequest:urlRequestwithReceiptID:downloadIDsuccess:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, UIImage * _Nonnull responseObject) {__strong __typeof(weakSelf)strongSelf = weakSelf;if ([strongSelf.af_activeImageDownloadReceipt.receiptID isEqual:downloadID]) {if (success) {success(request, response, responseObject);} else if(responseObject) {strongSelf.image = responseObject;}[strongSelf clearActiveDownloadInformation];}}failure:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, NSError * _Nonnull error) {__strong __typeof(weakSelf)strongSelf = weakSelf;if ([strongSelf.af_activeImageDownloadReceipt.receiptID isEqual:downloadID]) {if (failure) {failure(request, response, error);}[strongSelf clearActiveDownloadInformation];}}];self.af_activeImageDownloadReceipt = receipt;} }方法不是最重要的，重要是梳理出這一整套的邏輯和想法，下面我們就來(lái)分析分析。
總結(jié)
通過(guò)對(duì)上邊的方法的解讀，我們就很容易的給別的控件添加異步加載功能了。使用上邊的方法且改動(dòng)很少的代碼就能完成。
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AFNetworking 3.0 源碼解讀（一）之 AFNetworkReachabilityManager
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AFNetworking 3.0 源碼解讀（三）之 AFURLRequestSerialization
AFNetworking 3.0 源碼解讀（四）之 AFURLResponseSerialization
AFNetworking 3.0 源碼解讀（五）之 AFURLSessionManager
AFNetworking 3.0 源碼解讀（六）之 AFHTTPSessionManager
AFNetworking 3.0 源碼解讀（七）之 AFAutoPurgingImageCache
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