Question

I am implementing a simple iOS solitaire game that allows the user to drag the cards around in the usual way. The cards are represented with the UIView subclass CardView. All the card view’s are siblings which are subviews of SolitaireView. The following snippet tries to “bring a card to the front” so that it is above all the other views as it is being dragged:

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
   UITouch *touch = [touches anyObject];
   if (touch.view.tag == CARD_TAG) {
      CardView *cardView = (CardView*) touch.view;
      ...
      [self bringSubviewToFront:cardView];
      ...
   }
}

Unfortunately, the card’s z-order remains unchanged during the drag. In the images below, I am dragging the King. Notice how it is correctly on top the Nine in the left image, but is incorrectly under the Two (under the entire stack actually) in the right image:

I also tried alter the layer.zPosition property as well to no avail.
How can I bring the card view to the front during the drag? I am mystified.

Answer 1

Confirmed. bringSubviewToFront: causes layoutSubview to be invoked. Since my version of layoutSubviews sets the z-orders on all the views, this was undoing the z-order I was setting in the touchesBegan:withEvent code above. Apple should mention this side effect in the bringSubviewToFront documentation.

Instead of using a UIView subclass, I created a CALayer subclass named CardLayer. I handle the touch in my KlondikeView subclass as listed below. topZPosition is an instance var that tracks the highest zPosition of all cards. Note that modifying the zPosition is usually animated — I turn this off in the code below:

-(void)touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event {
   UITouch *touch = [touches anyObject];
   CGPoint touchPoint = [touch locationInView:self];
   CGPoint hitTestPoint = [self.layer convertPoint:touchPoint
                                           toLayer:self.layer.superlayer];
   CALayer *layer = [self.layer hitTest:hitTestPoint];

   if (layer == nil) return;

   if ([layer.name isEqual:@"card"]) {
     CardLayer *cardLayer = (CardLayer*) layer;
     Card *card = cardLayer.card;

     if ([self.solitaire isCardFaceUp:card]) {
        //...                                                                                                            
        [CATransaction begin]; // disable animation of z change                                                                  
        [CATransaction setValue:(id)kCFBooleanTrue
                         forKey:kCATransactionDisableActions];              
        cardLayer.zPosition = topZPosition++; // bring to highest z

        // ... if card fan, bring whole fan to top

        [CATransaction commit];
        //...                                                                                                            
     }
     // ...                                                                                                             
   }

}