我可以更改NSLayoutConstraint的multiplier属性吗？

我在一个超级视图中创建了两个视图，然后在视图之间添加了约束：

_indicatorConstrainWidth = [NSLayoutConstraint constraintWithItem:self.view1 attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view2 attribute:NSLayoutAttributeWidth multiplier:1.0f constant:0.0f];
[_indicatorConstrainWidth setPriority:UILayoutPriorityDefaultLow];
_indicatorConstrainHeight = [NSLayoutConstraint constraintWithItem:self.view1 attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.view2 attribute:NSLayoutAttributeHeight multiplier:1.0f constant:0.0f];
[_indicatorConstrainHeight setPriority:UILayoutPriorityDefaultLow];
[self addConstraint:_indicatorConstrainWidth];
[self addConstraint:_indicatorConstrainHeight];

现在，我想用动画更改乘子属性，但是我不知道如何更改乘子属性。（我在头文件NSLayoutConstraint.h的私有属性中找到了_coefficient，但它是私有的。）

如何更改多重属性？

我的解决方法是删除旧的约束，并为添加新的约束multipler。

如果只有两套乘数需要应用，则从iOS8开始，您可以添加两组约束，并确定应在任何时候激活的约束：

NSLayoutConstraint *standardConstraint, *zoomedConstraint;

// ...
// switch between constraints
standardConstraint.active = NO; // this line should always be the first line. because you have to deactivate one before activating the other one. or they will conflict.
zoomedConstraint.active = YES;
[self.view layoutIfNeeded]; // or using [UIView animate ...]

Swift 5.0版本

var standardConstraint: NSLayoutConstraint!
var zoomedConstraint: NSLayoutConstraint!

// ...

// switch between constraints
standardConstraint.isActive = false // this line should always be the first line. because you have to deactivate one before activating the other one. or they will conflict.
zoomedConstraint.isActive = true
self.view.layoutIfNeeded() // or using UIView.animate

这是Swift中的NSLayoutConstraint扩展，可轻松设置新的乘数：

在Swift 3.0+中

import UIKit
extension NSLayoutConstraint {
    /**
     Change multiplier constraint

     - parameter multiplier: CGFloat
     - returns: NSLayoutConstraint
    */
    func setMultiplier(multiplier:CGFloat) -> NSLayoutConstraint {

        NSLayoutConstraint.deactivate([self])

        let newConstraint = NSLayoutConstraint(
            item: firstItem,
            attribute: firstAttribute,
            relatedBy: relation,
            toItem: secondItem,
            attribute: secondAttribute,
            multiplier: multiplier,
            constant: constant)

        newConstraint.priority = priority
        newConstraint.shouldBeArchived = self.shouldBeArchived
        newConstraint.identifier = self.identifier

        NSLayoutConstraint.activate([newConstraint])
        return newConstraint
    }
}

演示用法：

@IBOutlet weak var myDemoConstraint:NSLayoutConstraint!

override func viewDidLoad() {
    let newMultiplier:CGFloat = 0.80
    myDemoConstraint = myDemoConstraint.setMultiplier(newMultiplier)

    //If later in view lifecycle, you may need to call view.layoutIfNeeded() 
}

该multiplier属性是只读的。您必须删除旧的NSLayoutConstraint并用新的NSLayoutConstraint对其进行修改。

但是，由于您知道要更改乘数，因此只需在需要更改时自己乘以常数即可更改常数，而这通常是更少的代码。

我用来更改现有布局约束的乘数的辅助函数。它创建并激活了新的约束，并停用了旧的约束。

struct MyConstraint {
  static func changeMultiplier(_ constraint: NSLayoutConstraint, multiplier: CGFloat) -> NSLayoutConstraint {
    let newConstraint = NSLayoutConstraint(
      item: constraint.firstItem,
      attribute: constraint.firstAttribute,
      relatedBy: constraint.relation,
      toItem: constraint.secondItem,
      attribute: constraint.secondAttribute,
      multiplier: multiplier,
      constant: constraint.constant)

    newConstraint.priority = constraint.priority

    NSLayoutConstraint.deactivate([constraint])
    NSLayoutConstraint.activate([newConstraint])

    return newConstraint
  }
}

用法，将乘数更改为1.2：

constraint = MyConstraint.changeMultiplier(constraint, multiplier: 1.2)

Andrew Schreiber的Objective-C版本答案

创建NSLayoutConstraint类的类别，然后将方法添加到.h文件中，如下所示

    #import <UIKit/UIKit.h>

@interface NSLayoutConstraint (Multiplier)
-(instancetype)updateMultiplier:(CGFloat)multiplier;
@end

在.m文件中

#import "NSLayoutConstraint+Multiplier.h"

@implementation NSLayoutConstraint (Multiplier)
-(instancetype)updateMultiplier:(CGFloat)multiplier {

    [NSLayoutConstraint deactivateConstraints:[NSArray arrayWithObjects:self, nil]];

   NSLayoutConstraint *newConstraint = [NSLayoutConstraint constraintWithItem:self.firstItem attribute:self.firstAttribute relatedBy:self.relation toItem:self.secondItem attribute:self.secondAttribute multiplier:multiplier constant:self.constant];
    [newConstraint setPriority:self.priority];
    newConstraint.shouldBeArchived = self.shouldBeArchived;
    newConstraint.identifier = self.identifier;
    newConstraint.active = true;

    [NSLayoutConstraint activateConstraints:[NSArray arrayWithObjects:newConstraint, nil]];
    //NSLayoutConstraint.activateConstraints([newConstraint])
    return newConstraint;
}
@end

稍后在ViewController中，为要更新的约束创建出口。

@property (strong, nonatomic) IBOutlet NSLayoutConstraint *topConstraint;

并在下面的任意位置更新乘数。

self.topConstraint = [self.topConstraint updateMultiplier:0.9099];

您可以更改“ constant”属性，而只需一点数学就可以达到相同的目标。假设约束的默认乘数为1.0f。这是Xamarin C＃代码，可以轻松将其转换为Objective-C

private void SetMultiplier(nfloat multiplier)
{
    FirstItemWidthConstraint.Constant = -secondItem.Frame.Width * (1.0f - multiplier);
}

正如在其他答案中所解释的：您需要删除约束并创建新约束。

您可以通过为NSLayoutConstraintwith inout参数创建静态方法来避免返回新的约束，该方法允许您重新分配传递的约束

import UIKit

extension NSLayoutConstraint {

    static func setMultiplier(_ multiplier: CGFloat, of constraint: inout NSLayoutConstraint) {
        NSLayoutConstraint.deactivate([constraint])

        let newConstraint = NSLayoutConstraint(item: constraint.firstItem, attribute: constraint.firstAttribute, relatedBy: constraint.relation, toItem: constraint.secondItem, attribute: constraint.secondAttribute, multiplier: multiplier, constant: constraint.constant)

        newConstraint.priority = constraint.priority
        newConstraint.shouldBeArchived = constraint.shouldBeArchived
        newConstraint.identifier = constraint.identifier

        NSLayoutConstraint.activate([newConstraint])
        constraint = newConstraint
    }

}

用法示例：

@IBOutlet weak var constraint: NSLayoutConstraint!

override func viewDidLoad() {
    NSLayoutConstraint.setMultiplier(0.8, of: &constraint)
    // view.layoutIfNeeded() 
}

在尝试修改自己的代码后，上面的代码均对我无用。此代码在Xcode 10和Swift 4.2中有效

import UIKit
extension NSLayoutConstraint {
/**
 Change multiplier constraint

 - parameter multiplier: CGFloat
 - returns: NSLayoutConstraintfor 
*/i
func setMultiplier(multiplier:CGFloat) -> NSLayoutConstraint {

    NSLayoutConstraint.deactivate([self])

    let newConstraint = NSLayoutConstraint(
        item: firstItem,
        attribute: firstAttribute,
        relatedBy: relation,
        toItem: secondItem,
        attribute: secondAttribute,
        multiplier: multiplier,
        constant: constant)

    newConstraint.priority = priority
    newConstraint.shouldBeArchived = self.shouldBeArchived
    newConstraint.identifier = self.identifier

    NSLayoutConstraint.activate([newConstraint])
    return newConstraint
}
}



 @IBOutlet weak var myDemoConstraint:NSLayoutConstraint!

override func viewDidLoad() {
let newMultiplier:CGFloat = 0.80
myDemoConstraint = myDemoConstraint.setMultiplier(newMultiplier)

//If later in view lifecycle, you may need to call view.layoutIfNeeded() 
 }

是的，只要更改NSLayoutConstraint并像->那样使用它，就可以更改乘数值

   func setMultiplier(_ multiplier:CGFloat) -> NSLayoutConstraint {

        NSLayoutConstraint.deactivate([self])

        let newConstraint = NSLayoutConstraint(
            item: firstItem!,
            attribute: firstAttribute,
            relatedBy: relation,
            toItem: secondItem,
            attribute: secondAttribute,
            multiplier: multiplier,
            constant: constant)

        newConstraint.priority = priority
        newConstraint.shouldBeArchived = shouldBeArchived
        newConstraint.identifier = identifier

        NSLayoutConstraint.activate([newConstraint])
        return newConstraint
    }

            self.mainImageViewHeightMultiplier = self.mainImageViewHeightMultiplier.setMultiplier(375.0/812.0)

如许多其他答案所建议的那样，通过更改代码中的活动约束进行切换对我不起作用。因此，我创建了2个约束，一个已安装，另一个未安装，将两者都绑定到代码，然后通过删除一个并添加另一个来进行切换。

为了完整起见，要绑定约束，请使用鼠标右键将约束拖至代码，就像其他任何图形元素一样：

我将一个比例命名为iPad，另一个比例命名为iPhone。

然后，在viewDidLoad处添加以下代码

override open func viewDidLoad() {
   super.viewDidLoad()
   if ... {

       view.removeConstraint(proportionIphone)
       view.addConstraint(proportionIpad) 
   }     
}

我正在使用xCode 10和Swift 5.0

这是基于C＃中@Tianfu的答案的答案。需要激活和取消约束的其他答案对我不起作用。

    var isMapZoomed = false
@IBAction func didTapMapZoom(_ sender: UIButton) {
    let offset = -1.0*graphHeightConstraint.secondItem!.frame.height*(1.0 - graphHeightConstraint.multiplier)
    graphHeightConstraint.constant = (isMapZoomed) ? offset : 0.0

    isMapZoomed = !isMapZoomed
    self.view.layoutIfNeeded()
}

要更改乘数的值，请执行以下步骤：1.为所需约束（例如someConstraint）创建出口。2.更改乘数，例如someConstraint.constant * = 0.8或任何乘数。

就是这样，快乐的编码。

可以读到：

var multiplier: CGFloat
The multiplier applied to the second attribute participating in the constraint.

在此文档页面上。这不是意味着一个人应该能够修改乘数（因为它是一个变量）？