UIActivityIndicatorView为nil崩溃求助：已关联Storyboard仍报错

嘿，我一眼就看出问题出在哪了——你重写了loadView方法，但没有正确加载Storyboard里的视图层级，导致你的activityIndicator这个Outlet根本没被初始化，自然就成nil了！

问题根源 #

默认情况下，当ViewController从Storyboard实例化时，系统会自动调用loadView来加载Storyboard中定义的view，同时完成所有Outlet的关联。但你现在重写了loadView，只创建了webView并直接赋值给self.view，完全跳过了Storyboard的视图加载流程，那原来在Storyboard里的activityIndicator当然找不到对应的父视图，也就没法完成关联了。

解决方案一：保留Storyboard布局（推荐） #

如果你还想继续用Storyboard里布置好的activityIndicator，只需要调整webView的初始化逻辑，不要直接覆盖self.view，而是把webView添加到Storyboard加载的view上：

import UIKit
import WebKit

class ViewController: UIViewController, WKUIDelegate, WKNavigationDelegate {
    var webView: WKWebView!
    @IBOutlet weak var activityIndicator: UIActivityIndicatorView!

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 初始化webView并添加到当前view中
        let webConfiguration = WKWebViewConfiguration()
        webView = WKWebView(frame: view.bounds, configuration: webConfiguration)
        webView.uiDelegate = self
        webView.navigationDelegate = self
        
        // 把webView放到最底层，确保activityIndicator能显示在上方
        view.insertSubview(webView, at: 0)
        
        // 设置自动布局让webView填满整个view
        webView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            webView.topAnchor.constraint(equalTo: view.topAnchor),
            webView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            webView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            webView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])
        
        // 加载测试网页
        guard let url = URL(string: "https://www.example.com") else { return }
        webView.load(URLRequest(url: url))
    }

    // 通过WKNavigationDelegate控制指示器状态
    func webView(_ webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
        activityIndicator.startAnimating()
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        activityIndicator.stopAnimating()
    }

    func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error) {
        activityIndicator.stopAnimating()
    }
}

解决方案二：完全代码创建视图 #

如果你想彻底抛弃Storyboard，用纯代码实现所有视图，那直接去掉Outlet，手动创建activityIndicator即可：

import UIKit
import WebKit

class ViewController: UIViewController, WKUIDelegate, WKNavigationDelegate {
    var webView: WKWebView!
    var activityIndicator: UIActivityIndicatorView!

    override func loadView() {
        let webConfiguration = WKWebViewConfiguration()
        webView = WKWebView(frame: .zero, configuration: webConfiguration)
        webView.uiDelegate = self
        webView.navigationDelegate = self
        self.view = webView
        
        // 代码创建activityIndicator
        activityIndicator = UIActivityIndicatorView(style: .large)
        activityIndicator.translatesAutoresizingMaskIntoConstraints = false
        activityIndicator.hidesWhenStopped = true
        webView.addSubview(activityIndicator)
        
        // 设置指示器居中布局
        NSLayoutConstraint.activate([
            activityIndicator.centerXAnchor.constraint(equalTo: webView.centerXAnchor),
            activityIndicator.centerYAnchor.constraint(equalTo: webView.centerYAnchor)
        ])
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        guard let url = URL(string: "https://www.example.com") else { return }
        webView.load(URLRequest(url: url))
    }

    // 同样实现导航代理方法控制指示器
    func webView(_ webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
        activityIndicator.startAnimating()
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        activityIndicator.stopAnimating()
    }

    func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error) {
        activityIndicator.stopAnimating()
    }
}

两种方案选哪种取决于你的开发习惯：第一种适合已经在Storyboard里完成了部分布局的场景，第二种适合纯代码驱动的项目。

内容的提问来源于stack exchange，提问作者mistakeNot