ios - 以编程方式更新高度约束

Question

我是自动布局的新手。我已经从 xib 文件完成了我的所有项目，但现在我遇到了一个问题，我必须以编程方式更新 View 的高度。我在下面尝试过，但现在可以工作了。

[[self view] addConstraint:[NSLayoutConstraint constraintWithItem:loginContainer attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0f constant:loginFrame.size.height]];

在控制台中显示

Unable to simultaneously satisfy constraints.
    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) 
(
    "<NSLayoutConstraint:0x78724530 V:[UIView:0x790cdfb0(170)]>",
    "<NSLayoutConstraint:0x787da210 V:[UIView:0x790cdfb0(400)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x78724530 V:[UIView:0x790cdfb0(170)]>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

Answer 1

您需要修改现有约束上的常量，而不是添加新约束。

使用 IBOutlet 连接到 Interface Builder 中的约束:

@property (nonatomic, weak) NSLayoutConstraint *heightConstraint;

然后，当您需要以编程方式设置它时，只需在约束上设置常量属性:

heightConstraint.constant = 100;

或

如果在 Interface Builder 中无法访问 nib，请在代码中查找约束:

NSLayoutConstraint *heightConstraint;
for (NSLayoutConstraint *constraint in myView.constraints) {
    if (constraint.firstAttribute == NSLayoutAttributeHeight) {
        heightConstraint = constraint;
        break;
    }
}
heightConstraint.constant = 100;

在 Swift 中:

if let constraint = (myView.constraints.filter{$0.firstAttribute == .width}.first) {
            constraint.constant = 100.0
        }