iOS - 如何在活動視圖仍然呈現動畫時顯示UIView？

Question

我有代碼顯示一個「加載...」視圖與活動輪，當我的應用程序加載某些屏幕。如果加載需要特別長的時間（比如4秒以上），那麼我想顯示一條額外的消息，提示「抱歉，這麼長時間，請耐心等待」。我這樣做的方式是在4秒延遲上使用NSTimer，它調用一個方法來創建一個等待視圖，然後在加載視圖上覆蓋這個新視圖，使得這些單詞不重疊。如果頁面在小於4秒內加載，則加載視圖被隱藏，等待視圖不會被觸發並且用戶繼續他或她的快樂方式。

當我測試屏幕加載時間超過4秒，我似乎無法得到額外的視圖來顯示。這是我的代碼：

// this method is triggered elsewhere in my code 
// right before the code to start loading a screen 
- (void)showActivityViewer 
{  
    tooLong = YES; 
    waitTimer = [NSTimer scheduledTimerWithTimeInterval:4.0 
                target:self 
               selector:@selector(createWaitAlert) 
               userInfo:nil 
                repeats:NO]; 
    [[NSRunLoop currentRunLoop] addTimer: waitTimer forMode: NSDefaultRunLoopMode]; 

    loadingView = [LoadingView createLoadingView:self.view.bounds.size.width  :self.view.bounds.size.height];  
    [self.view addSubview: loadingView]; 

    // this next line animates the activity wheel which is a subview of loadingView 
    [[[loadingView subviews] objectAtIndex:0] startAnimating]; 
} 

- (void)createWaitAlert 
{ 
    [waitTimer invalidate]; 
    if (tooLong) 
    { 
     UIView *waitView = [LoadingView createWaitView:self.view.bounds.size.width :self.view.bounds.size.height]; 
     [self.view addSubview:waitView]; 
    } 
} 

// this method gets triggered elsewhere in my code 
// when the screen has finished loading and is ready to display 
- (void)hideActivityViewer 
{ 
    tooLong = NO; 
    [[[loadingView subviews] objectAtIndex:0] stopAnimating]; 
    [loadingView removeFromSuperview]; 
    loadingView = nil; 
} 

Answer 1

你確定你是從主線程執行這個嗎？試試這個：

- (void)showActivityViewer 
{  
    tooLong = YES; 

    dispatch_async(dispatch_queue_create(@"myQueue", NULL), ^{ 
     [NSThread sleepForTimeInterval:4]; 
     [self createWaitAlert]; 
    }); 

    loadingView = [LoadingView createLoadingView:self.view.bounds.size.width  :self.view.bounds.size.height];  
    [self.view addSubview: loadingView]; 

    // this next line animates the activity wheel which is a subview of loadingView 
    [[[loadingView subviews] objectAtIndex:0] startAnimating]; 
} 

- (void)createWaitAlert 
{ 
    dispatch_async(dispatch_get_main_queue(), ^{ 
     if (tooLong) 
     { 
      UIView *waitView = [LoadingView createWaitView:self.view.bounds.size.width :self.view.bounds.size.height]; 
      [self.view addSubview:waitView]; 
     } 
    }); 
}