我有一个具有动态高度的 View ，我正在尝试在运行时更改此 View 高度优先级。

这是我的部分代码；

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:

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

我的错误是什么？

最佳答案

如 NSLayoutConstraint class reference 中所述:

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，但它的优先级比其他任何东西都高。

https://stackoverflow.com/questions/31186187/