autolayout学习笔记_2

一个在控制面板获取autolayout的_autolayoutTrace的小技巧
add the debugging code to ViewController.m:

@interface UIWindow (AutoLayoutDebug) 
+ (UIWindow *)keyWindow;
- (NSString *)_autolayoutTrace;
@end

And:

- (void)viewDidAppear:(BOOL)animated {
           [super viewDidAppear:animated];
           NSLog(@"%@", [[UIWindow keyWindow] _autolayoutTrace]);
 }
- (void)didRotateFromInterfaceOrientation: (UIInterfaceOrientation)fromInterfaceOrientation
{
          [super didRotateFromInterfaceOrientation: fromInterfaceOrientation];
          NSLog(@"%@", [[UIWindow keyWindow] _autolayoutTrace]); 
}

I like to have this tracing code in there so that it’s easy to spot ambiguous layouts. Interface Builder works hard to prevent you from making mistakes, but when you’re building up your constraints programmatically, you can do all sorts of things wrong.

Sometimes your layout may even appear to work just fine, but then you do an _autolayoutTrace and it actually says AMBIGUOUS LAYOUT. You want to catch such errors before you ship your app to customers. It’s better to be on the safe side, especially when you’re just starting out with Auto Layout.