如何从方法内的块返回变量？ [英] How do I return a variable from a block inside a method?

问题描述

假设我有这个方法，给定一个URL返回一个UIImage：

Say I have this method that given a URL returns a UIImage:

- (void)getUIImageFromURL:(NSURL *)URL {
    NSURLRequest *request = [NSURLRequest requestWithURL:URL];
    AFHTTPRequestOperation *imageOperation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    imageOperation.responseSerializer = [AFImageResponseSerializer serializer];

    [imageOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
        return (UIImage *)responseObject;
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
    }];

    [imageOperation start];
}

但它一直给我这个错误：

But it keeps giving me this error:

不兼容的块指针类型将'UIImage *（^）（AFHTTPRequestOperation * __ strong，_ strong id）'发送到'void（^）类型的参数（AFHTTPRequestOperation * _strong，__strong id）'

Incompatible block pointer types sending 'UIImage *(^)(AFHTTPRequestOperation *__strong, _strong id)' to parameter of type 'void (^)(AFHTTPRequestOperation *_strong, __strong id)'

我对块有些新意，所以也许我完全倒退了。我最好如何实现这样的方法？

I'm somewhat new to blocks, so perhaps I'm approaching this completely backwards. How best would I implement a method like this?

推荐答案

您无法从块内返回图像。这是一个异步API，不能以您尝试的方式使用。要么使用阻塞API，要么在下载映像之前阻塞该方法（一个糟糕的解决方案），要么实现对异步API的支持。例如，将完成块传递给 getImage 方法，并在下载操作的完成块中调用它。在此块中，使用图像执行所需操作。

You cannot return an image from inside a block. This is an asynchronous API and cannot be used in the manner you are attempting to. Either use a blocking API, where the method is blocking until the image is downloaded (a bad solution), or implement support for the asynchronous API. For instance, pass a completion block to your getImage method and call it with in the completion block of the download operation. In this block, do what you need with the image.

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