我可以更改NSLayoutConstraint的multiplier属性吗？

Question

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。

Answer 1

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

import UIKit

extension NSLayoutConstraint {

    func setMultiplier(multiplier:CGFloat) -> NSLayoutConstraint {

        NSLayoutConstraint.deactivateConstraints([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.activateConstraints([newConstraint])

        return newConstraint

    }

}

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() 

}

Answer 2

我用来更改现有布局约束的乘数的辅助函数。它创建并激活了一个新的约束，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)