objective-c - 如何在 GCD block 中指定返回类型

Question

如何在 GCD block 中指定返回类型？在这里，我想将结果返回为 NSData ...

- (NSData *)returnData:(NSString *)urlString{
    dispatch_queue_t concurrentQueue = 
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_sync(concurrentQueue, ^{   // here return type showing error

        NSString *strUrl = urlString;
        if([strUrl length] == 0)
            return nil; // from this point i want to stop the execution of the block and return nil .

        // otherwise only continue the left operations



    });         
}

Answer 1

- (NSData *)returnData:(NSString *)urlString {
    dispatch_queue_t concurrentQueue = 
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    // Declare your local data outside the block.
    // `__block` specifies that the variable can be modified from within the block.
    __block NSData *localData = nil;

    dispatch_sync(concurrentQueue, ^{
        // Do something with `localData`...
        localData = somethingWonderful;
    });

    // `localData` now contains your wonderful data.
    return localData;
}

block 的格式(参数/返回类型)在函数的签名中指定。在这种情况下，dispatch_sync 使用了一个没有返回类型和参数的 block 。如果您希望使用更多变量，您需要在 block 外声明它们，如上面的代码所示。