ios - 以编程方式调整约束是否需要先删除然后重新添加？

Question

我的代码中有一个函数可以调整某些按钮的布局。这在设置 .hidden 时调用。

   private func layoutButtons() {

        redButton.hidden = !redButtonEnabled
        redButtonLabel.hidden = !redButtonEnabled
        yellowButton.hidden = !yellowButtonEnabled
        yellowButtonLabel.hidden = !yellowButtonEnabled

        removeConstraint(yellowButtonTrailingContraint)
        if yellowButtonEnabled && !redButtonEnabled {
            yellowButtonTrailingContraint = NSLayoutConstraint(item: yellowButton, attribute: .Trailing, relatedBy: .Equal, toItem: self, attribute: .Trailing, multiplier: 1.0, constant: -horizontalMargin)
        } else {
            yellowButtonTrailingContraint = NSLayoutConstraint(item: yellowButton, attribute: .Trailing, relatedBy: .Equal, toItem: redButton, attribute: .Leading, multiplier: 1.0, constant: -horizontalMargin)
        }
        addConstraint(yellowButtonTrailingContraint)
    }

是否有必要在更改约束之前首先删除约束，然后像我上面所做的那样在之后重新添加它？在某处的示例中看到了这一点，但看起来有点奇怪。对此的指点将不胜感激。谢谢!

Answer 1

是的，删除约束是一种选择，但并不总是必要的。

有时您可以通过更改其常量值来编辑约束，这将更新布局。例如:

    var constraintHeight = NSLayoutConstraint(item: someView, attribute: .Height, relatedBy:.Equal, toItem:nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 100)
    someView.addConstraint(constraintHeight)
    ...
    //The Constraint can be edited later by changing the constant value
    constraintHeight.constant = 200
    someView.layoutIfNeeded()

或者您可以激活或停用它们，例如:

    var constraintHeight1 = NSLayoutConstraint(item: someView, attribute: .Height, relatedBy:.Equal, toItem:nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 100)
    var constraintHeight2 = NSLayoutConstraint(item: someView, attribute: .Height, relatedBy:.Equal, toItem:nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 200)
    constraintHeight1.active = true
    constraintHeight2.active = false

    someView.addConstraint(constraintHeight1)
    someView.addConstraint(constraintHeight2)

    ...
    //Later you can set the other constraint as active
    constraintHeight1.active = false
    constraintHeight2.active = true
    someView.layoutIfNeeded()

在任何给定点，只有事件约束将用于决定 View 的最终布局。因此，您几乎没有其他选择，但我们必须确保永远不会有两个相互冲突的约束处于事件状态，否则应用程序将崩溃。我们将不得不删除其中一个冲突约束或将其停用。希望对您有所帮助:]