如何更改 uiview 的框架在 iOS 中使用自动布局？

Question

我在 XIB 中添加了 1 个视图，它设置为自动布局。但是在运行时，我想更改该视图的框架。我知道如果使用自动布局，需要在我的代码中设置translatesAutoresizingMaskIntoConstraints = YES。我已经这样做了，我为该视图创建了新框架，但它在控制台中显示了警告消息：

    Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSIBPrototypingLayoutConstraint:0x16e4b750 'IB auto generated at build time for view with fixed frame' V:|-(0)-[UIView:0x16ed3c30]   (Names: '|':UIView:0x16ee06b0 )>",
    "<NSAutoresizingMaskLayoutConstraint:0x16e19e30 h=-&- v=-&- UIView:0x16ed3c30.midY == UIView:0x16ee06b0.midY + 44>",
    "<NSAutoresizingMaskLayoutConstraint:0x16e19e60 h=-&- v=-&- UIView:0x16ed3c30.height == UIView:0x16ee06b0.height>"
)

Will attempt to recover by breaking constraint 
<NSIBPrototypingLayoutConstraint:0x16e4b750 'IB auto generated at build time for view with fixed frame' V:|-(0)-[UIView:0x16ed3c30]   (Names: '|':UIView:0x16ee06b0 )>

Break on objc_exception_throw to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

这是创建新框架的代码，我只希望 Y 的原点等于 0。

self.leftView.frame= CGRectMake(0, 0, 40, self.view.frame.size.height);

我该如何修复这个错误？请给一些建议。非常感谢。

Answer 1

如果我理解正确，我认为你需要在你的类中使用一个存储的 NSLayoutConstraint 变量，然后将约束的常量值更新为 0 以使你的 UIView 滑动到顶部。

另外，当您使用自动布局时，您通常会设置：

myView.translateAutoResizingMaskIntoConstraint = NO;

那是 NO 而不是 YES。

所以一个例子是这样的：

@interface MyViewController()
{
    NSLayoutConstraint *myViewTopConstraint;
}

@end

@implementation MyViewController

-(void)setupMyConstraintsMethod
{
    ...

    // ---------------------------------------------------------------
    // This constraint here says "myView" top edge should be pinned
    // to the top edge of the view controller's view but with an offset
    // of 50 pixels. 
    // ---------------------------------------------------------------
    NSLayoutConstraint *myViewTopConstraint = [NSLayoutConstraint constraintWithItem:self.myView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:50.0];

    [self.view addConstraint:myViewTopConstraint];
}

-(void)buttonPressed:(id)sender
{
    // ------------------------------------------------------------
    // now you want myView's top edge to be at the top of your
    // view controller's view, so you set the constant to 0
    // ------------------------------------------------------------
    myViewTopConstraint.constant = 0;

    // animates the sliding up of "myView"
    [UIView animateWithDuration:0.5 animations:^{
        [self.view layoutIfNeeded];
    }];
}

@end