以编程方式向导航控制器添加自动布局约束

问题描述

我正在尝试将约束添加到我在导航栏上添加的 UI 标签.UI 标签显示一个正在运行的计时器

I am trying to add constraints to a UI label I added on a navigation bar. The UI label shows a running timer

这是我的代码，

self.label = [[UILabel alloc] initWithFrame:CGRectMake(550, 15, 150, 14)];
self.label.text = @"Label";
[self.label setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.navigationController.navigationBar addSubview:self.label];

[self.navigationController.navigationBar addConstraint:[NSLayoutConstraint constraintWithItem:self.label
                                                       attribute:NSLayoutAttributeTrailing
                                                       relatedBy:NSLayoutRelationEqual
                                                          toItem:self.navigationController.navigationBar
                                                       attribute:NSLayoutAttributeTrailing
                                                      multiplier:1.0f
                                                        constant:-20.0f]];

但我在运行应用程序时收到错误消息无法修改控制器管理的 UINavigationBar 的约束".

But I get the error 'Cannot modify constraints for UINavigationBar managed by a controller' when I run the app.

如何添加此约束?

推荐答案

简而言之:你不能.

导航控制器管理导航栏和可选导航工具栏的创建、配置和显示.允许自定义导航栏与外观相关的属性，但绝不能直接更改其框架、边界或 alpha 值……导航控制器使用导航项对象(UINavigationItem 的实例)动态构建导航栏的内容类)与导航堆栈上的视图控制器相关联

The navigation controller manages the creation, configuration, and display of the navigation bar and optional navigation toolbar. It is permissible to customize the navigation bar’s appearance-related properties but you must never change its frame, bounds, or alpha values directly ... A navigation controller builds the contents of the navigation bar dynamically using the navigation item objects (instances of the UINavigationItem class) associated with the view controllers on the navigation stack

来源:Apple Docs

您需要将 UINavigationBar 子类化，然后使用 initWithNavigationBarClass:toolbarClass: 实例化 UINavigationController 以做进一步的事情.

You would need to subclass the UINavigationBar and then instantiate the UINavigationController using initWithNavigationBarClass:toolbarClass: to do something further.

如果您正在寻找自定义导航栏的方法，UIKit 用户界面指南 是了解您可以做什么的良好开端.但是，如果它将是一个文本标签，请考虑设置栏的 title 属性，例如.

Though if you're looking for ways to customize the navbar, UIKit User Interface Guide is a good start to know what you may be able to do. But if it's going to be a text label, consider setting the bar's title property, for example.

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