从 UIWebView 到 WKWebView

WKWebView 是 iOS 8 引入的一个 WebKit API，替换 iOS 的 UIWebView 和 OSX 的 WebView，WKWebView 使用 Nitro JavaScript 引擎，运行 JavaScript 和 Safari 一样快。WKWebView 和 UIWebView 使用上有很多区别，这篇主要介绍代理、标题、进度条展示的区别。

代理

UIWebView 是 delegate: UIWebViewDelegate，WKWebView 使用以下两个代理

1. navigationDelegate: WKNavigationDelegate
2. UIDelegate: WKUIDelegate

其中 navigationDelegate 代替 UIWebView 中的 delegate，下表是这两个代理方法的对比：

UIWebView	WKWebView
- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script;	- (void)evaluateJavaScript:(NSString *)javaScriptString completionHandler:(void (^ __nullable)(__nullable id, NSError * __nullable error))completionHandler;
- (void)webViewDidStartLoad:(UIWebView *)webView	- (void)webView:(WKWebView *)webView didCommitNavigation:(null_unspecified WKNavigation *)navigation
- (void)webViewDidFinishLoad:(UIWebView *)webView	- (void)webView:(WKWebView *)webView didFinishNavigation:(null_unspecified WKNavigation *)navigation
- (void)webView:(UIWebView *)webView didFailLoadWithError:(NSError *)error	- (void)webView:(WKWebView *)webView didFailNavigation:(null_unspecified WKNavigation *)navigation withError:(NSError *)error
- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType	- (void)webView:(WKWebView *)webView decidePolicyForNavigationAction:(WKNavigationAction *)navigationAction decisionHandler:(void (^)(WKNavigationActionPolicy))decisionHandler

比如，如果要拦截网页请求，UIWebView 使用以下代码：

- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType {
     NSURL *url = request.URL;
    NSString *host = [url host];
    if ([url.scheme isEqualToString:@"inapp"]) {

        /*
         * 做一些处理
         */
         
        return NO;
    }
    
    return YES;
}

在 WKWebView 中则用以下代码：

- (void)webView:(WKWebView *)webView decidePolicyForNavigationAction:(WKNavigationAction *)navigationAction decisionHandler:(void (^)(WKNavigationActionPolicy))decisionHandler {
    
    NSURL *url = navigationAction.request.URL;
    NSString *host = [url host];
    if ([url.scheme isEqualToString:@"inapp"]) {

        /*
         * 做一些处理
         */
         
        decisionHandler(WKNavigationActionPolicyCancel);
    } else {
    
        decisionHandler(WKNavigationActionPolicyAllow);
    }
}

获取 title

在 UIWebView 中

self.title = [webView stringByEvaluatingJavaScriptFromString:@"document.title"];

而 WKWebView 有 title 属性可直接获取网页标题，可通过 KVO 方式获取网页标题的动态变化。

self.title = webView.title;

estimatedProgress

WKWebView 增加了 estimatedProgress，获取网页加载进度，轻松显示进度条。可通过 KVO 的方式获取 estimatedProgress 的变化。首先在 viewDidLoad 中添加观察者

[self.webView addObserver:self forKeyPath:NSStringFromSelector(@selector(estimatedProgress)) options:NSKeyValueObservingOptionNew context:NULL];
[self.webView addObserver:self forKeyPath:NSStringFromSelector(@selector(title)) options:NSKeyValueObservingOptionNew context:NULL];

KVO 的实现：

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:NSStringFromSelector(@selector(estimatedProgress))] && object == self.webView) {
        NSLog(@"%f", self.webView.estimatedProgress);
        // estimatedProgress is a value from 0.0 to 1.0
        // Update your UI here accordingly

        [self.progressView setAlpha:1.0f];
        [self.progressView setProgress:self.webView.estimatedProgress animated:YES];
        
        if(self.webView.estimatedProgress >= 1.0f) {
            [UIView animateWithDuration:0.3 delay:0.3 options:UIViewAnimationOptionCurveEaseOut animations:^{
                [self.progressView setAlpha:0.0f];
            } completion:^(BOOL finished) {
                [self.progressView setProgress:0.0f animated:NO];
            }];
        }
        
    } else if ([keyPath isEqualToString:@"title"]) {
        if (object == self.webView) {
            self.title = self.webView.title;
        } else {
            [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
        }
    } else {
        
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

最后，在 dealloc 中移除观察者

- (void)dealloc {
    if ([self isViewLoaded]) {
        [self.webView removeObserver:self forKeyPath:NSStringFromSelector(@selector(estimatedProgress))];
        [self.webView removeObserver:self forKeyPath:NSStringFromSelector(@selector(title))];
    }
    
    [self.webView setNavigationDelegate:nil];
}

WKWebView 还增加了 Message Handlers，用于和网页的JS交互，不需要处理复杂的evaluateJavaScript。例如：

class NotificationScriptMessageHandler: NSObject, WKScriptMessageHandler {
    func userContentController(userContentController: WKUserContentController, didReceiveScriptMessage message: WKScriptMessage!) {
        print(message.body)
    }
}

let userContentController = WKUserContentController()
let handler = NotificationScriptMessageHandler()
userContentController.addScriptMessageHandler(handler, name: "notification")

则 JavaScript 端可通过以下代码即可调用原生的代码：

window.webkit.messageHandlers.notification.postMessage({body: "..."});

详情参考 NSHipster 这篇 WKWeb​View