0. OC 执行 JS 方法
- (void)evaluateJavaScript:(NSString *)javaScriptString completionHandler:(void (^)(id, NSError *error))completionHandler;
这个方法中webView调用JS，block只是成功或失败的回调
1. JS方法中
window.webkit.messageHandlers.webViewApp.postMessage(message);
作用是JS 向之前注册的 webViewApp 发送消息。
OC 端接到消息会调用 <WKScriptMessageHandler> 中下面代理方法
- (void)userContentController:(WKUserContentController *)userContentController didReceiveScriptMessage:(WKScriptMessage *)message

XYWKWebViewController  ---> 管理 webView 加载相关的代理方法
XYWKWebView            ---> 封装 webView 请求相关方法
XYScriptMessage        ---> 封装JS回调信息

/**
 *  加载本地HTML页面
 *
 *  @param htmlName html页面文件名称
 */
- (void)loadLocalHTMLWithFileName:(nonnull NSString *)htmlName

- (void)loadLocalHTMLWithFileName:(nonnull NSString *)htmlName {
    NSString *path = [[NSBundle mainBundle] bundlePath];
    NSURL *baseURL = [NSURL fileURLWithPath:path];
    NSString * htmlPath = [[NSBundle mainBundle] pathForResource:htmlName
                                                          ofType:@"html"];
    NSString * htmlCont = [NSString stringWithContentsOfFile:htmlPath
                                                    encoding:NSUTF8StringEncoding
                                                       error:nil];
    
    // WKWebView 的 loadHTMLString: 方法
    [self loadHTMLString:htmlCont baseURL:baseURL];
}

// 加载网络内容，根据是否有参数选不同方法
- (void)loadRequestWithRelativeUrl:(nonnull NSString *)relativeUrl;
- (void)loadRequestWithRelativeUrl:(nonnull NSString *)relativeUrl params:(nullable NSDictionary *)params;

- (void)loadRequestWithRelativeUrl:(NSString *)relativeUrl params:(NSDictionary *)params {
    
    NSURL *url = [self generateURL:relativeUrl params:params];
    
    [self loadRequest:[NSURLRequest requestWithURL:url]];
}
- (NSURL *)generateURL:(NSString*)baseURL params:(NSDictionary*)params {
    
    self.webViewRequestUrl = baseURL;
    self.webViewRequestParams = params;
    
    NSMutableDictionary *param = [NSMutableDictionary dictionaryWithDictionary:params];
    
    NSMutableArray* pairs = [NSMutableArray array];
    //可以在这里将token参数添加进去，这样就可以实现补全token功能    
    for (NSString* key in param.keyEnumerator) {
        NSString *value = [NSString stringWithFormat:@"%@",[param objectForKey:key]];
        NSString* escaped_value = (__bridge_transfer NSString *)CFURLCreateStringByAddingPercentEscapes(NULL,
                                                                              (__bridge CFStringRef)value,
                                                                              NULL,
                                                                              (CFStringRef)@"!*'\"();:@&=+$,/?%#[]% ",
                                                                              kCFStringEncodingUTF8);
        
        [pairs addObject:[NSString stringWithFormat:@"%@=%@", key, escaped_value]];
    }
    
    NSString *query = [pairs componentsJoinedByString:@"&"];
    baseURL = [baseURL stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
    
    NSString* url = @"";
    if ([baseURL containsString:@"?"]) {
        url = [NSString stringWithFormat:@"%@&%@",baseURL, query];
    }
    else {
        url = [NSString stringWithFormat:@"%@?%@",baseURL, query];
    }
    //绝对地址
    if ([url.lowercaseString hasPrefix:@"http"]) {
        return [NSURL URLWithString:url];
    }
    else {
        return [NSURL URLWithString:url relativeToURL:self.baseUrl];
    }
}

if (self.shouldShowProgress) {
   [self.webView addObserver:self forKeyPath:@"estimatedProgress" options:NSKeyValueObservingOptionNew context:NULL];
}
if (self.isUseWebPageTitle) {
   [self.webView addObserver:self forKeyPath:@"title" options:NSKeyValueObservingOptionNew context:NULL];
}

- (instancetype)initWithFrame:(CGRect)frame configuration:(WKWebViewConfiguration *)configuration {
    self = [super initWithFrame:frame configuration:configuration];
    if (self) {
    
        if (configuration) {
            //文档中说
            //Adds a script message handler.
            //Adding a script message handler with name name causes the JavaScript function window.webkit.messageHandlers.name.postMessage(messageBody) to be defined in all frames in all web views that use the user content controller.
        
            // 这里就是设置 网页中 JS Message handler
            // 通过 “name” 注册之后，JS 内部函数 window.webkit.messageHandlers.“name”.postMessage(messageBody) 就被定义到整个用户的Web内容的控制器中。
            //后面的JS调用OC也是通过 “name” 联系的
            [configuration.userContentController addScriptMessageHandler:self name:@"webViewApp"];
        }
        
        //默认允许系统自带的侧滑后退
        self.allowsBackForwardNavigationGestures = YES;
    }
    return self;
}

// JS 调用 OC 的回调。JS 向OC 发送消息会调用这个方法
- (void)userContentController:(WKUserContentController *)userContentController didReceiveScriptMessage:(WKScriptMessage *)message {
    
    NSLog(@"得到的 JS message 是 :%@",message.body);
    if ([message.body isKindOfClass:[NSDictionary class]]) {
        
        NSDictionary *body = (NSDictionary *)message.body;
        
        // 这里是对 JS 消息的一个处理，用自己定义的消息类型，封装并发送给代理去外部处理，具体格式需要工作中和H5共同制定
        
        XYScriptMessage *msg = [XYScriptMessage new];
        [msg setValuesForKeysWithDictionary:body];
        
        if (self.xy_messageHandlerDelegate && [self.xy_messageHandlerDelegate respondsToSelector:@selector(xy_webView:didReceiveScriptMessage:)]) {
            [self.xy_messageHandlerDelegate xy_webView:self didReceiveScriptMessage:msg];
        }
    }
    
}

/**
 *  WKWebView与JS调用时参数规范实体
 */
@interface XYScriptMessage : NSObject
/**
 *  方法名
 *  用来确定Native App的执行逻辑
 */
@property (nonatomic, copy) NSString *method;
/**
 *  方法参数
 *  json字符串
 */
@property (nonatomic, copy) NSDictionary *params;
/**
 *  回调函数名
 *  Native App执行完后回调的JS方法名
 */
@property (nonatomic, copy) NSString *callback;
@end

/**
 *  JS调用原生方法处理，其中方法名都需要和 H5 端相互协调
 */
- (void)xy_webView:(XYWKWebView *)webView didReceiveScriptMessage:(XYScriptMessage *)message {
    
    NSLog(@"webView method:%@",message.method);
    
    //返回上一页
    if ([message.method isEqualToString:@"tobackpage"]) {
        [self.navigationController popViewControllerAnimated:YES];
    }
    //打开新页面
    else if ([message.method isEqualToString:@"openappurl"]) {
        
        NSString *url = [message.params objectForKey:@"url"];
        if (url.length) {
            XYWKWebViewController *webViewController = [[XYWKWebViewController alloc] init];
            webViewController.url = url;
            
            [self.navigationController pushViewController:webViewController animated:YES];
        }
    }
}