OC和JS交互 JS注入

 //网页加载完毕的回调，OCCSwift调用JS
    func webViewDidFinishLoad(_ webView: UIWebView) {
        //调用JS的DOM文档对象
       let title =  self.webview.stringByEvaluatingJavaScript(from: "document.title");
        //调用JS的方法
         self.webview.stringByEvaluatingJavaScript(from: "clickme()");
        print(title ?? "")
    }
    //网页即将加载的回调，JS调用OC Swift
    func webView(_ webView: UIWebView, shouldStartLoadWith request: URLRequest, navigationType: UIWebViewNavigationType) -> Bool {
        var urlString = request.url?.absoluteString

  unsigned int count = 0;
    Method *methods = class_copyMethodList([UITextField class], &count);
    for (int i = 0; i < count; i++ ) {
        Method method = methods[i];
       NSString *string =  NSStringFromSelector(method_getName(method));
        NSLog(@"%@",string);
    }
    //修改 ： _placeholderLabel
    [self.textfield setValue:[UIColor redColor] forKeyPath:@"placeholderLabel.textColor"];
    //修改 ： placeholder
    [self.textfield setValue:@"FUCK" forKey:@"placeholder"];