从内部块返回方法对象 [英] Returning method object from inside block

问题描述

我想知道如何正确执行以下操作：我有一个返回 NSData 对象的方法。它从 UIDocument 获取 NSData 对象。 NSData 对象可能会变大，所以我想确保它在响应开始之前已完全加载。因此，我想从块本身返回方法的值。所以类似这样：

I am wondering how to do the following correctly: I have a method that is to return an NSData object. It gets the NSData object from a UIDocument. The NSData object can get large, so I want to make sure it is fully loaded before the response starts. I would therefore like to return the value of the method from within the block itself. So something like this:

- (NSData*)getMyData {
  MyUIDocument *doc = [[MyUIDocument alloc] initWithFileURL:fileURL];
  [doc openWithCompletionHandler:^(BOOL success) {

    if (success) {
      return doc.myResponseData; // this is to be the return for the method not the block
    }
  }];
}

这会导致错误，因为返回显然是指块的返回。

This causes an error because the return apparently refers to the block's return.

如何在不必使线程阻塞等待/循环的情况下完成此操作？

How can I accomplish this without having to make a thread blocking wait/while loop?

谢谢。

推荐答案

你做不到。接受这样一个事实，即您尝试执行的操作是异步操作，并在 getMyData 方法中添加完成块参数，该方法在调用内部完成处理程序时调用。 （并从方法签名中删除 return ）：

You can't. Embrace the fact that what you're trying to do is asynchronous and add a completion block parameter to your getMyData method which is called when the inner completion handler is called. (And remove the return from the method signature):

- (void)getMyDataWithCompletion:(void(^)(NSData *data))completion {
    MyUIDocument *doc = [[MyUIDocument alloc] initWithFileURL:fileURL];
    [doc openWithCompletionHandler:^(BOOL success) {
        completion((success ? doc.myResponseData : nil));
    }];
}

swift中存在同样的问题，你可以添加一个类似的完成块： / p>

The same problem exists in swift and you can add a similar completion block:

func getMyData(completion: ((data: NSData?) -> Void) {
    data = ...
    completion(data)
}

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