在块外返回变量值-Objective-C，iOS [英] Returning a variable value outside a block - Objective-C, iOS

问题描述

我在下面的代码中有一个到URL的连接，并获取了一些标头响应，例如http代码响应和最终URL(用于重定向的情况):

I have the code below that a an connection to an URL and fetches some header responses like http code response and final URL (for redirection case):

- (NSString *)test
{
    __block NSString *cod = @"x";
    NSString *urlString = @"http://www.google.com";
    NSURL *url = [NSURL URLWithString:urlString];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
                                                           cachePolicy:NSURLRequestReloadIgnoringCacheData
                                                       timeoutInterval:15.0f];

[request setHTTPMethod:@"HEAD"];

[NSURLConnection sendAsynchronousRequest:request
                                   queue:[NSOperationQueue mainQueue]
                       completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
                           NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;
                           NSURL *resolvedURL = [httpResponse URL];
                           NSString *code = [NSString stringWithFormat:@"%ld", (long)[httpResponse statusCode]];
                           NSLog(@"%@", resolvedURL);
                           NSLog(@"%@", code);

                           cod = @"y"; // the idea was use something like 'cod = code', then return 'code' at end.. But it dont works too.
                       }];
return cod; }

可以看到，我已将cod变量声明为__block类型，并设置了x值. 在块内部，我用y值设置了cod，但是在方法结束时我得到了cod的x值. 我试图使用类似cod = code的方法，然后返回cod，尊重对象类型，但是我在block内分配的任何内容，我都无法获得它之外的值. 我在做什么错了?

As can see, I have declared cod variable as __block type and set with x value. Inside block, I've set cod with y value, but at end of method I got x value for cod. I've tried to use something like cod = code then returns cod, respecting objects type, but anything that I assign inside block, I can't get the value outside it. What am I doing wrong?

推荐答案

查看方法名称:

[NSURLConnection sendAsynchronousRequest:...

哇！所以它是异步的.到完成块被调用时，您的test方法将已经返回.您无法从同步方法返回异步调用的结果，因为它没有意义.使您的类适应网络操作的异步特性，使用回调，委托等.总而言之，重新设计代码.

Whoops! So it's asynchronous. By the time the completion block is called, your test method will have already returned. You can't return the result of an asynchronous call from a synchronous method, because it doesn't make sense. Adapt your class to be coherent with the async nature of networking operations, use callbacks, delegation, etc. All in all, redesign your code.

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