如何包装一个异步方法,该方法接受一个块并在目标c中将其转换为同步

Question

我想包装一个如下所示的异步API:

[someObject completeTaskWithCompletionHandler:^(NSString *result) {

}];

进入一个我可以像这样调用的同步方法

NSString *result = [someObject completeTaskSynchronously];

我该怎么做呢？我做了一些文档阅读和谷歌搜索,并尝试使用"dispatch_semaphore"尝试实现它,如下所示:

-(NSString *) completeTaskSynchronously {
   __block NSString *returnResult;
   self.semaphore = dispatch_semaphore_create(0);  
   [self completeTaskWithCompletionHandler:^(NSString *result) {
       resultResult = result;
       dispatch_semaphore_signal(self.semaphore);
   }];

   dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
   return resultResult;
}

但这似乎没有用,它基本上只是停在dispatch_semaphore_wait.执行永远不会到达执行_signal的内部块.任何人都有关于如何做到这一点的代码示例？我怀疑该块必须在主线程的其他线程上？此外,假设我无法访问异步方法背后的源代码.谢谢!

Answer 1

dispatch_semaphore_wait阻止示例中的主队列.您可以将异步任务分派到不同的队列:

__block NSString *returnResult;
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0UL);
dispatch_async(queue,^{
     result = [someObject completeTaskSynchronously];
});

或者使用其他系统,如NSRunLoop:

   __block finished = NO;
   [self completeTaskWithCompletionHandler:^(NSString *result) {
       resultResult = result;
       finished = YES;
   }];
    while (!finished) {
        // wait 1 second for the task to finish (you are wasting time waiting here)
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:1]];
    }

Answer 2

使用NSRunLoop是最容易做到的.

__block NSString* result = nil;
[self completeTaskWithCompletionHandler:^(NSString *resultstring) {
    result = resultstring;
}];
while (!result) {
    [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}

Answer 3

您可以尝试使用 NSOperations 来异步执行您的操作。