Animation with Auto Layout

要讓 view 依照 autolayout 去動畫只要在變更 constaint 後在 UIView:animationWithDuration:中加入 [self.view layoutIfNeeded]即可。

self.userNameLabelWidth.constant = 20; // from 100 to 20
[UIView animateWithDuration:0.25 animations:^{
    [self.view layoutIfNeeded];
}];