如何在运行时更改约束优先级 [英] How can I change constraints priority in run time

问题描述

我有一个具有动态高度的视图，我试图在运行时更改此视图高度优先级。

I have a view which has dynamic height and I am trying to change this view height priority in run time.

这是我的代码部分;

if (index == 0) {

    surveyViewHeightConstraint.constant = 0;
    surveyViewHeightConstraint.priority = 1000;

} else if (index == 1) {

    surveyViewHeightConstraint.constant = 163;
    surveyViewHeightConstraint.priority = 500;

}

我正在通过按钮操作更改索引。当我运行此代码时，我收到此错误：

I am changing index with a button action. When I run this code, I am getting this error:

*** Assertion failure in -[NSLayoutConstraint setPriority:], /SourceCache/Foundation/Foundation-1141.1/Layout.subproj/NSLayoutConstraint.m:174

我的错误是什么在这里？

What is my mistake in here?

推荐答案

如 NSLayoutConstraint 类引用：

优先级可能不会从非必需更改为必需，或从必需更改为非需要。如果OS X中的 NSLayoutPriorityRequired 或iOS中的 UILayoutPriorityRequired 的优先级更改为较低优先级，则会引发异常，或者在将约束添加到视图后将较低优先级更改为所需的优先级。即使在视图上安装了约束，也允许从一个可选优先级更改为另一个可选优先级。

Priorities may not change from nonrequired to required, or from required to nonrequired. An exception will be thrown if a priority of NSLayoutPriorityRequired in OS X or UILayoutPriorityRequired in iOS is changed to a lower priority, or if a lower priority is changed to a required priority after the constraints is added to a view. Changing from one optional priority to another optional priority is allowed even after the constraint is installed on a view.

使用优先级999而不是1000从技术上讲，这不是绝对必要的，但它将比其他任何东西都更优先。

Use priority 999 instead of 1000. It won't be absolutely required technically speaking, but it'll be a higher priority than anything else.

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