iOS11软件开发工具包无法访问应用程序的委托

Question

- (NSString *)getAuthorizationHeader{
    iKMAppDelegate *delegate = (iKMAppDelegate *)[UIApplication sharedApplication].delegate;
    NSString *header = [NSString stringWithFormat:@"Bearer %@", delegate.appDataObject.oauth2AcessToken];
    return header;
}

此方法将在XCode9中收到警告

[UIApplication delegate] must be called from main thread only

而且我不认为在主队列中的调度会对我的功能起作用。那么如何修复这个警告呢？

Answer 1

不需要在主线程上访问UIApplication的委托，但可以使用dispatch_sync轻松完成

- (NSString *)getAuthorizationHeader{
    __block iKMAppDelegate *delegate;
    if([NSThread isMainThread]) {
        delegate = (iKMAppDelegate *)[UIApplication sharedApplication].delegate;
    } else {
        dispatch_sync(dispatch_get_main_queue(), ^{
            delegate = (iKMAppDelegate *)[UIApplication sharedApplication].delegate;
        });
    }
    NSString *header = [NSString stringWithFormat:@"Bearer %@", delegate.appDataObject.oauth2AcessToken];
    return header;
}

与dispatch_async相反，dispatch_sync函数将一直等到传递给它的块完成后才返回。

对于dispatch_sync，有必要检查函数是否不是从主线程执行的，这将导致死锁。

Answer 2

您可以在调用后台函数之前获取指向AppDelegate的指针。并从那个指针中获取它。

<...Main thread in app...>
[[MyDelegateHolder holder] setDelegate: (iKMAppDelegate *)[UIApplication sharedApplication].delegate];

<...RunBackground task...>
[[MyDelegateHolder holder].delegate methodCall];

<...In MyDelegateHolder's delegate method  ....>
- (iKMAppDelegate*)delegate
{
  if (self.delegate == nil)
  {
    <Assert or...>
    runSyncOnMainThread(^(){//you should get delegate in sync method from the main thread
       self.delegate = (iKMAppDelegate *)[UIApplication sharedApplication].delegate;
     });
  }

  return self.delegate;
}

在我的示例中，MyDelegateHolder是一个单例。

如果您需要多线程访问，在不设置委托之前，不要忘记访问锁(或将字段标记为原子)。