使用WKWebView替换UIWebView

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本文详细介绍iOS8后推出的WKWebView，对比UIWebView，展示其在性能、稳定性及功能上的优势。涵盖基本使用方法，包括加载网页、状态回调、WKUIDelegate协议、动态加载JS代码等。

开发App的过程中，常常会遇到在App内部加载网页，通常用UIWebView加载。这个自iOS2开始使用的网页加载器一直是开发的心病：加载速度慢，占用内存多，优化困难。如果加载网页多，还可能因为过量占用内存而给系统kill掉。各种优化的方法效果也不那么明显(点击查看常用优化方法)。

iOS8以后，苹果推出了新框架Wekkit，提供了替换UIWebView的组件WKWebView。各种UIWebView的问题没有了，速度更快了，占用内存少了，一句话，WKWebView是App内部加载网页的最佳选择！

先看下 WKWebView的特性：

在性能、稳定性、功能方面有很大提升（最直观的体现就是加载网页是占用的内存，模拟器加载百度与开源中国网站时，WKWebView占用23M，而UIWebView占用85M）；
允许JavaScript的Nitro库加载并使用（UIWebView中限制）；
支持了更多的HTML5特性；
高达60fps的滚动刷新率以及内置手势；
将UIWebViewDelegate与UIWebView重构成了14类与3个协议（查看苹果官方文档）；

然后从以下几个方面说下WKWebView的基本用法：

加载网页
加载的状态回调
新的WKUIDelegate协议
动态加载并运行JS代码
webView 执行JS代码
JS调用App注册过的方法

一、加载网页

加载网页或HTML代码的方式与UIWebView相同，代码示例如下：

<code class="objectivec"> WKWebView *webView = [[WKWebView alloc] initWithFrame:<span class="hljs-keyword">self</span><span class="hljs-variable">.view</span><span class="hljs-variable">.bounds</span>];
[webView loadRequest:[<span class="hljs-built_in">NSURLRequest</span> requestWithURL:[<span class="hljs-built_in">NSURL</span> URLWithString:<span class="hljs-string">@"http://www.baidu.com"</span>]]];
[<span class="hljs-keyword">self</span><span class="hljs-variable">.view</span> addSubview:webView];</code>

二、加载的状态回调（WKNavigationDelegate）

用来追踪加载过程（页面开始加载、加载完成、加载失败）的方法：

<code class="objectivec"><span class="hljs-comment">// 页面开始加载时调用</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView didStartProvisionalNavigation:(WKNavigation *)navigation;
<span class="hljs-comment">// 当内容开始返回时调用</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView didCommitNavigation:(WKNavigation *)navigation;
<span class="hljs-comment">// 页面加载完成之后调用</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView didFinishNavigation:(WKNavigation *)navigation;
<span class="hljs-comment">// 页面加载失败时调用</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView didFailProvisionalNavigation:(WKNavigation *)navigation;</code>

页面跳转的代理方法：

<code class="objectivec"><span class="hljs-comment">// 接收到服务器跳转请求之后调用</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView didReceiveServerRedirectForProvisionalNavigation:(WKNavigation *)navigation;
<span class="hljs-comment">// 在收到响应后，决定是否跳转</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView decidePolicyForNavigationResponse:(WKNavigationResponse *)navigationResponse decisionHandler:(<span class="hljs-keyword">void</span> (^)(WKNavigationResponsePolicy))decisionHandler;
<span class="hljs-comment">// 在发送请求之前，决定是否跳转</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView decidePolicyForNavigationAction:(WKNavigationAction *)navigationAction decisionHandler:(<span class="hljs-keyword">void</span> (^)(WKNavigationActionPolicy))decisionHandler;</code>

三、新的`WKUIDelegate`协议

这个协议主要用于WKWebView处理web界面的三种提示框(警告框、确认框、输入框)，下面是警告框的例子:

<code class="java"><span class="hljs-comment">/**
 *  web界面中有弹出警告框时调用
 *
 *  <span class="hljs-doctag">@param</span> webView           实现该代理的webview
 *  <span class="hljs-doctag">@param</span> message           警告框中的内容
 *  <span class="hljs-doctag">@param</span> frame             主窗口
 *  <span class="hljs-doctag">@param</span> completionHandler 警告框消失调用
 */</span>
- (<span class="hljs-keyword">void</span>)webView:(WKWebView *)webView runJavaScriptAlertPanelWithMessage:(NSString *)message initiatedByFrame:(<span class="hljs-keyword">void</span> (^)())completionHandler;</code>

四、动态加载并运行`JS`代码

用于在客户端内部加入JS代码，并执行，示例如下：

<code class="objectivec"><span class="hljs-comment">// 图片缩放的js代码</span>
<span class="hljs-built_in">NSString</span> *js = <span class="hljs-string">@"var count = document.images.length;for (var i = 0; i < count; i++) {var image = document.images[i];image.style.width=320;};window.alert('找到' + count + '张图');"</span>;
<span class="hljs-comment">// 根据JS字符串初始化WKUserScript对象</span>
WKUserScript *script = [[WKUserScript alloc] initWithSource:js injectionTime:WKUserScriptInjectionTimeAtDocumentEnd forMainFrameOnly:<span class="hljs-literal">YES</span>];
<span class="hljs-comment">// 根据生成的WKUserScript对象，初始化WKWebViewConfiguration</span>
WKWebViewConfiguration *config = [[WKWebViewConfiguration alloc] init];
[config<span class="hljs-variable">.userContentController</span> addUserScript:script];
_webView = [[WKWebView alloc] initWithFrame:<span class="hljs-keyword">self</span><span class="hljs-variable">.view</span><span class="hljs-variable">.bounds</span> configuration:config];
[_webView loadHTMLString:<span class="hljs-string">@"<head></head><imgea src='http://www.nsu.edu.cn/v/2014v3/img/background/3.jpg' />"</span>baseURL:<span class="hljs-literal">nil</span>];
[<span class="hljs-keyword">self</span><span class="hljs-variable">.view</span> addSubview:_webView];</code>

五、webView 执行`JS`代码

用户调用用JS写过的代码，一般指服务端开发的：

<code class="objectivec"><span class="hljs-comment">//javaScriptString是JS方法名，completionHandler是异步回调block</span>
[<span class="hljs-keyword">self</span><span class="hljs-variable">.webView</span> evaluateJavaScript:javaScriptString completionHandler:completionHandler];</code>

六、`JS`调用App注册过的方法

再WKWebView里面注册供JS调用的方法，是通过WKUserContentController类下面的方法：

<code class="objectivec">- (<span class="hljs-keyword">void</span>)addScriptMessageHandler:(<span class="hljs-keyword">id</span> <WKScriptMessageHandler>)scriptMessageHandler name:(<span class="hljs-built_in">NSString</span> *)name;</code>

scriptMessageHandler是代理回调，JS调用name方法后，OC会调用scriptMessageHandler指定的对象。

JS在调用OC注册方法的时候要用下面的方式：

<code class="css"><span class="hljs-tag">window</span><span class="hljs-class">.webkit</span><span class="hljs-class">.messageHandlers</span>.<<span class="hljs-tag">name</span>><span class="hljs-class">.postMessage</span>(<<span class="hljs-tag">messageBody</span>>)</code>

注意，name(方法名)是放在中间的，messageBody只能是一个对象，如果要传多个值，需要封装成数组，或者字典。整个示例如下：

<code class="objectivec"><span class="hljs-comment">//OC注册供JS调用的方法</span>
[[_webView configuration]<span class="hljs-variable">.userContentController</span> addScriptMessageHandler:<span class="hljs-keyword">self</span> name:<span class="hljs-string">@"closeMe"</span>];

<span class="hljs-comment">//OC在JS调用方法做的处理</span>
- (<span class="hljs-keyword">void</span>)userContentController:(WKUserContentController *)userContentController didReceiveScriptMessage:(WKScriptMessage *)message
{
    <span class="hljs-built_in">NSLog</span>(<span class="hljs-string">@"JS 调用了 %@ 方法，传回参数 %@"</span>,message<span class="hljs-variable">.name</span>,message<span class="hljs-variable">.body</span>);
}

<span class="hljs-comment">//JS调用</span>
    window<span class="hljs-variable">.webkit</span><span class="hljs-variable">.messageHandlers</span><span class="hljs-variable">.closeMe</span><span class="hljs-variable">.postMessage</span>(null);</code>

如果你在self的dealloc打个断点，会发现self没有释放！这显然是不行的！谷歌后看到一种解决方法，如下：

<code class="objectivec"><span class="hljs-class"><span class="hljs-keyword">@interface</span> <span class="hljs-title">WeakScriptMessageDelegate</span> : <span class="hljs-title">NSObject</span><<span class="hljs-title">WKScriptMessageHandler</span>></span>

<span class="hljs-keyword">@property</span> (<span class="hljs-keyword">nonatomic</span>, <span class="hljs-keyword">weak</span>) <span class="hljs-keyword">id</span><WKScriptMessageHandler> scriptDelegate;

- (instancetype)initWithDelegate:(<span class="hljs-keyword">id</span><WKScriptMessageHandler>)scriptDelegate;

<span class="hljs-keyword">@end</span>

<span class="hljs-class"><span class="hljs-keyword">@implementation</span> <span class="hljs-title">WeakScriptMessageDelegate</span></span>

- (instancetype)initWithDelegate:(<span class="hljs-keyword">id</span><WKScriptMessageHandler>)scriptDelegate
{
    <span class="hljs-keyword">self</span> = [<span class="hljs-keyword">super</span> init];
    <span class="hljs-keyword">if</span> (<span class="hljs-keyword">self</span>) {
        _scriptDelegate = scriptDelegate;
    }
    <span class="hljs-keyword">return</span> <span class="hljs-keyword">self</span>;
}

- (<span class="hljs-keyword">void</span>)userContentController:(WKUserContentController *)userContentController didReceiveScriptMessage:(WKScriptMessage *)message
{
    [<span class="hljs-keyword">self</span><span class="hljs-variable">.scriptDelegate</span> userContentController:userContentController didReceiveScriptMessage:message];
}

<span class="hljs-keyword">@end</span></code>

思路是另外创建一个代理对象，然后通过代理对象回调指定的self，

<code class="swift"><span class="hljs-type">WKUserContentController</span> *userContentController = [[<span class="hljs-type">WKUserContentController</span> alloc] <span class="hljs-keyword">init</span>];    
[userContentController addScriptMessageHandler:[[<span class="hljs-type">WeakScriptMessageDelegate</span> alloc] initWithDelegate:<span class="hljs-keyword">self</span>] name:@<span class="hljs-string">"closeMe"</span>];</code>

运行代码，self释放了，WeakScriptMessageDelegate却没有释放啊啊啊！
还需在self的dealloc里面添加这样一句代码：

<code class="css"><span class="hljs-attr_selector">[[_webView configuration]</span><span class="hljs-class">.userContentController</span> <span class="hljs-rule"><span class="hljs-attribute">removeScriptMessageHandlerForName</span>:<span class="hljs-value">@<span class="hljs-string">"closeMe"</span>]</span></span>;</code>

OK，圆满解决问题！

目前，大多数App需要支持iOS7以上的版本，而WKWebView只在iOS8后才能用，所以需要一个兼容性方案，既iOS7下用UIWebView，iOS8后用WKWebView。这个库提供了这种兼容性方案：https://github.com/wangyangcc/IMYWebView