版本記錄

前言

我們做APP發(fā)起網絡請求，都離不開一個非常有用的框架AFNetworking，可以說這個框架的知名度已經超過了蘋果的底層網絡請求部分，很多人可能不知道蘋果底層是如何發(fā)起網絡請求的，但是一定知道AFNetworking，接下來幾篇我們就一起詳細的解析一下這個框架。感興趣的可以看上面寫的幾篇。
1. AFNetworking源碼探究（一） —— 基本介紹
2. AFNetworking源碼探究（二） —— GET請求實現之NSURLSessionDataTask實例化（一）
3. AFNetworking源碼探究（三） —— GET請求實現之任務進度設置和通知監(jiān)聽（一）

回顧

上一篇主要包括AFURLSessionManagerTaskDelegate代理為任務設置進度和AFURLSessionManager為任務添加通知監(jiān)聽。這一篇主要說一下自定義類AFURLSessionManagerTaskDelegate和AFURLSessionManager中有關代理的轉發(fā)邏輯，這一篇的重點就在于這個轉發(fā)邏輯思想的理解，代碼的意義其次，讀完你就會感到大神寫代碼的高質量。

重寫respondsToSelector方法

主要對應下面這端代碼。

- (BOOL)respondsToSelector:(SEL)selector {
    if (selector == @selector(URLSession:task:willPerformHTTPRedirection:newRequest:completionHandler:)) {
        return self.taskWillPerformHTTPRedirection != nil;
    } else if (selector == @selector(URLSession:dataTask:didReceiveResponse:completionHandler:)) {
        return self.dataTaskDidReceiveResponse != nil;
    } else if (selector == @selector(URLSession:dataTask:willCacheResponse:completionHandler:)) {
        return self.dataTaskWillCacheResponse != nil;
    } else if (selector == @selector(URLSessionDidFinishEventsForBackgroundURLSession:)) {
        return self.didFinishEventsForBackgroundURLSession != nil;
    }

    return [[self class] instancesRespondToSelector:selector];
}

重寫respondsToSelector的作用就是判斷NSURLSession那幾個代理是否能響應，如果響應了就去調用。復寫了selector的方法，這幾個方法是在本類有實現的，但是如果外面的Block沒賦值的話，則返回NO，相當于沒有實現。

這樣如果沒實現這些我們自定義的Block也不會去回調這些代理。因為本身某些代理，只執(zhí)行了這些自定義的Block，如果Block都沒有賦值，那我們調用代理也沒有任何意義。

block的屬性和值的設置

上面說到了block下面我們看一下定義的block的屬性。

@property (readwrite, nonatomic, copy) AFURLSessionDidBecomeInvalidBlock sessionDidBecomeInvalid;
@property (readwrite, nonatomic, copy) AFURLSessionDidReceiveAuthenticationChallengeBlock sessionDidReceiveAuthenticationChallenge;
@property (readwrite, nonatomic, copy) AFURLSessionDidFinishEventsForBackgroundURLSessionBlock didFinishEventsForBackgroundURLSession;
@property (readwrite, nonatomic, copy) AFURLSessionTaskWillPerformHTTPRedirectionBlock taskWillPerformHTTPRedirection;
@property (readwrite, nonatomic, copy) AFURLSessionTaskDidReceiveAuthenticationChallengeBlock taskDidReceiveAuthenticationChallenge;
@property (readwrite, nonatomic, copy) AFURLSessionTaskNeedNewBodyStreamBlock taskNeedNewBodyStream;
@property (readwrite, nonatomic, copy) AFURLSessionTaskDidSendBodyDataBlock taskDidSendBodyData;
@property (readwrite, nonatomic, copy) AFURLSessionTaskDidCompleteBlock taskDidComplete;
@property (readwrite, nonatomic, copy) AFURLSessionDataTaskDidReceiveResponseBlock dataTaskDidReceiveResponse;
@property (readwrite, nonatomic, copy) AFURLSessionDataTaskDidBecomeDownloadTaskBlock dataTaskDidBecomeDownloadTask;
@property (readwrite, nonatomic, copy) AFURLSessionDataTaskDidReceiveDataBlock dataTaskDidReceiveData;
@property (readwrite, nonatomic, copy) AFURLSessionDataTaskWillCacheResponseBlock dataTaskWillCacheResponse;
@property (readwrite, nonatomic, copy) AFURLSessionDownloadTaskDidFinishDownloadingBlock downloadTaskDidFinishDownloading;
@property (readwrite, nonatomic, copy) AFURLSessionDownloadTaskDidWriteDataBlock downloadTaskDidWriteData;
@property (readwrite, nonatomic, copy) AFURLSessionDownloadTaskDidResumeBlock downloadTaskDidResume;

typedef void (^AFURLSessionDidBecomeInvalidBlock)(NSURLSession *session, NSError *error);
typedef NSURLSessionAuthChallengeDisposition (^AFURLSessionDidReceiveAuthenticationChallengeBlock)(NSURLSession *session, NSURLAuthenticationChallenge *challenge, NSURLCredential * __autoreleasing *credential);

typedef NSURLRequest * (^AFURLSessionTaskWillPerformHTTPRedirectionBlock)(NSURLSession *session, NSURLSessionTask *task, NSURLResponse *response, NSURLRequest *request);
typedef NSURLSessionAuthChallengeDisposition (^AFURLSessionTaskDidReceiveAuthenticationChallengeBlock)(NSURLSession *session, NSURLSessionTask *task, NSURLAuthenticationChallenge *challenge, NSURLCredential * __autoreleasing *credential);
typedef void (^AFURLSessionDidFinishEventsForBackgroundURLSessionBlock)(NSURLSession *session);

typedef NSInputStream * (^AFURLSessionTaskNeedNewBodyStreamBlock)(NSURLSession *session, NSURLSessionTask *task);
typedef void (^AFURLSessionTaskDidSendBodyDataBlock)(NSURLSession *session, NSURLSessionTask *task, int64_t bytesSent, int64_t totalBytesSent, int64_t totalBytesExpectedToSend);
typedef void (^AFURLSessionTaskDidCompleteBlock)(NSURLSession *session, NSURLSessionTask *task, NSError *error);

typedef NSURLSessionResponseDisposition (^AFURLSessionDataTaskDidReceiveResponseBlock)(NSURLSession *session, NSURLSessionDataTask *dataTask, NSURLResponse *response);
typedef void (^AFURLSessionDataTaskDidBecomeDownloadTaskBlock)(NSURLSession *session, NSURLSessionDataTask *dataTask, NSURLSessionDownloadTask *downloadTask);
typedef void (^AFURLSessionDataTaskDidReceiveDataBlock)(NSURLSession *session, NSURLSessionDataTask *dataTask, NSData *data);
typedef NSCachedURLResponse * (^AFURLSessionDataTaskWillCacheResponseBlock)(NSURLSession *session, NSURLSessionDataTask *dataTask, NSCachedURLResponse *proposedResponse);

typedef NSURL * (^AFURLSessionDownloadTaskDidFinishDownloadingBlock)(NSURLSession *session, NSURLSessionDownloadTask *downloadTask, NSURL *location);
typedef void (^AFURLSessionDownloadTaskDidWriteDataBlock)(NSURLSession *session, NSURLSessionDownloadTask *downloadTask, int64_t bytesWritten, int64_t totalBytesWritten, int64_t totalBytesExpectedToWrite);
typedef void (^AFURLSessionDownloadTaskDidResumeBlock)(NSURLSession *session, NSURLSessionDownloadTask *downloadTask, int64_t fileOffset, int64_t expectedTotalBytes);
typedef void (^AFURLSessionTaskProgressBlock)(NSProgress *);

typedef void (^AFURLSessionTaskCompletionHandler)(NSURLResponse *response, id responseObject, NSError *error);

下面看一下這些屬性的值的設定。在.h文件中有接口，并在.m中是實現，這里就舉一個例子，如下：

/**
 Sets a block to be executed when the managed session becomes invalid, as handled by the `NSURLSessionDelegate` method `URLSession:didBecomeInvalidWithError:`.

 @param block A block object to be executed when the managed session becomes invalid. The block has no return value, and takes two arguments: the session, and the error related to the cause of invalidation.
 */
- (void)setSessionDidBecomeInvalidBlock:(nullable void (^)(NSURLSession *session, NSError *error))block;

- (void)setSessionDidBecomeInvalidBlock:(void (^)(NSURLSession *session, NSError *error))block {
    self.sessionDidBecomeInvalid = block;
}

這么做的目的是為了我們這些用戶使用起來方便，調用set方法去設置這些Block，能很清晰的看到Block的各個參數與返回值。

代理轉發(fā)思想

我先整體和大家說一下AFN中有關代理的轉發(fā)邏輯。

AFURLSessionManager中是有關NSURLSession的代理，主要包含下面的幾個代理的實現。

上面的幾個代理是什么關系，相信看過我寫的SDWebImage的源碼分析的應該很清楚，他們是繼承的關系，就不多說了。而AFURLSessionManagerTaskDelegate實現了三個代理方法。

AFUrlSessionManager對這一大堆代理做了一些公共的處理，而轉發(fā)到AF自定義代理的3條，則負責把每個task對應的數據回調出去。

那么是如何轉發(fā)過去的呢，簡單的調用就是這樣的。

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:task];

    // delegate may be nil when completing a task in the background
    if (delegate) {
        [delegate URLSession:session task:task didCompleteWithError:error];

        [self removeDelegateForTask:task];
    }

    if (self.taskDidComplete) {
        self.taskDidComplete(session, task, error);
    }
}

通過[delegate URLSession:session task:task didCompleteWithError:error];就轉發(fā)了過去。這里首先就是從字典中找到該任務對應的代理AFURLSessionManagerTaskDelegate對象，然后就是轉發(fā)到自定義代理中。

大家如果讀過SDWebImage的源碼的話，就應該明白，那里也用到了代理的轉發(fā)，可見，這種轉發(fā)思想有它的優(yōu)點，值得我們去學習。

后記

本篇文章主要介紹了一種代理的轉發(fā)思想，從NSURLSession原生的代理轉發(fā)代理到AFURLSessionManagerTaskDelegate自定義的代理中。這一篇最主要的就是學習和掌握這種思想。