如何让UISearchBar的ScopeBar扩展至导航栏全屏宽度？

默认情况下，作为navigationItem.titleView的UISearchBar会被左右侧的BarButtonItem限制宽度，导致ScopeBar无法铺满导航栏全屏。以下是两种可行的解决方法：

方案一：通过AutoLayout约束让SearchBar占满导航栏宽度 #

直接给SearchBar添加与导航栏边缘对齐的约束，强制它铺满整个导航栏，ScopeBar会自动跟随扩展至全屏宽度：

import UIKit

class ResultSearchViewController: UIViewController {
    
    @IBOutlet weak var rightBarButton: UIBarButtonItem!
    @IBOutlet var searchBar: UISearchBar!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        navigationItem.titleView = searchBar
        searchBar.showsScopeBar = true
        
        // 禁用自动布局转换，添加自定义约束
        searchBar.translatesAutoresizingMaskIntoConstraints = false
        guard let navBar = navigationController?.navigationBar else { return }
        NSLayoutConstraint.activate([
            searchBar.leadingAnchor.constraint(equalTo: navBar.leadingAnchor),
            searchBar.trailingAnchor.constraint(equalTo: navBar.trailingAnchor),
            searchBar.topAnchor.constraint(equalTo: navBar.topAnchor),
            searchBar.bottomAnchor.constraint(equalTo: navBar.bottomAnchor)
        ])
    }
}

此方法无需修改返回按钮，SearchBar的输入区域会自动避开左右按钮位置，同时保证ScopeBar铺满全屏。

方案二：自定义返回按钮，彻底释放导航栏宽度 #

如果方案一的自动调整不符合预期，可隐藏系统默认返回按钮，自定义返回按钮后让SearchBar完全占据导航栏：

import UIKit

class ResultSearchViewController: UIViewController {
    
    @IBOutlet weak var rightBarButton: UIBarButtonItem!
    @IBOutlet var searchBar: UISearchBar!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 隐藏系统默认返回按钮
        navigationItem.hidesBackButton = true
        // 创建自定义返回按钮
        let backButton = UIButton(type: .system)
        backButton.setImage(UIImage(systemName: "chevron.left"), for: .normal)
        backButton.setTitle("返回", for: .normal)
        backButton.addTarget(self, action: #selector(backToPrevious), for: .touchUpInside)
        navigationItem.leftBarButtonItem = UIBarButtonItem(customView: backButton)
        
        // 设置SearchBar为titleView并启用ScopeBar
        navigationItem.titleView = searchBar
        searchBar.showsScopeBar = true
        
        // 添加约束让SearchBar占满导航栏
        searchBar.translatesAutoresizingMaskIntoConstraints = false
        guard let navBar = navigationController?.navigationBar else { return }
        NSLayoutConstraint.activate([
            searchBar.leadingAnchor.constraint(equalTo: navBar.leadingAnchor),
            searchBar.trailingAnchor.constraint(equalTo: navBar.trailingAnchor),
            searchBar.topAnchor.constraint(equalTo: navBar.topAnchor),
            searchBar.bottomAnchor.constraint(equalTo: navBar.bottomAnchor)
        ])
    }
    
    @objc private func backToPrevious() {
        navigationController?.popViewController(animated: true)
    }
}

该方法完全控制导航栏布局，确保ScopeBar不会被任何元素遮挡，同时保留返回功能。

内容的提问来源于stack exchange，提问作者haruka