从NSURLResponse完成块中获取数据 [英] Getting data out of the NSURLResponse completion block

问题描述

看起来我还没有完全理解块的概念......

It looks like I didn't get the concept of blocks completely yet...

在我的代码中我必须从<$中获取JSON数据c $ c> asychronous block 从'外部'方法返回。我用谷歌搜索，发现如果用__block 定义一个变量，那么该变量的变量扩展到<$的v̶i̶s̶i̶b̶i̶l̶i̶t̶y̶ _mutability_ c $ c>阻止。

In my code I have to get out the JSON data from the asychronous block to be returned to from the 'outer' method. I googled and found that if defining a variable with __block, the v̶i̶s̶i̶b̶i̶l̶i̶t̶y̶ _mutability_ of that variable is extended to the block.

但由于某些原因，返回的json对象为nil。我想知道为什么？

But for some reason returned json object is nil.I wonder why?

- (NSMutableDictionary *)executeRequestUrlString:(NSString *)urlString
{
__block NSMutableDictionary *json = nil;
NSURL *url = [NSURL URLWithString:urlString];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];

[request setHTTPShouldHandleCookies:YES];
[request setHTTPMethod:@"GET"];
[request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-type"];

NSString *cookieString = [self.userDefaults objectForKey:SAVED_COOKIE];

[request addValue:cookieString forHTTPHeaderField:@"Cookie"];

[NSURLConnection sendAsynchronousRequest:request
                                   queue:[NSOperationQueue currentQueue]
                       completionHandler:^(NSURLResponse *response, NSData *data, NSError *error)
                       {

                           NSLog(@"dataAsString %@", [NSString stringWithUTF8String:[data bytes]]);

                           NSError *error1;
                           NSMutableDictionary * innerJson = [NSJSONSerialization
                                   JSONObjectWithData:data
                                              options:kNilOptions
                                                error:&error1];
                           json = innerJson;

                       }];

    return json;
}

推荐答案

首先，回答你的问题：

但由于某种原因，返回的json对象是 nil 。我想知道为什么？

您返回的变量在您返回时尚未设置。在 sendAsynchronousRequest：queue：completionHandler：方法返回后，您无法立即收集结果：调用必须在回调块并设置 json 变量。

The variable that you are returning has not been set at the time when you return it. You cannot harvest the results immediately after the sendAsynchronousRequest:queue:completionHandler: method has returned: the call has to finish the roundtrip before calling back your block and setting json variable.

现在快速说明如何处理它：您的方法是尝试将异步调用转换为同步调用。如果可以，尽量保持异步。不要期望返回 NSMutableDictionary * 的方法，而是创建一个方法，该方法接受自己的块，并在时将字典传递给该块。 sendAsynchronousRequest：方法完成：

Now a quick note on what to do about it: your method is attempting to convert an asynchronous call into a synchronous one. Try to keep it asynchronous if you can. Rather than expecting a method that returns a NSMutableDictionary*, make a method that takes a block of its own, and pass the dictionary to that block when the sendAsynchronousRequest: method completes:

- (void)executeRequestUrlString:(NSString *)urlString withBlock:(void (^)(NSDictionary *jsonData))block {
    // Prepare for the call
    ...
    // Make the call
    [NSURLConnection sendAsynchronousRequest:request
                                    queue:[NSOperationQueue currentQueue]
                        completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
        NSLog(@"dataAsString %@", [NSString stringWithUTF8String:[data bytes]]);
        NSError *error1;
        NSMutableDictionary * innerJson = [NSJSONSerialization
            JSONObjectWithData:data options:kNilOptions error:&error1
        ];
        block(innerJson); // Call back the block passed into your method
        }];

}

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