崩溃提示

Completion handler passed to 
-[WebViewVC webView:runJavaScriptAlertPanelWithMessage:initiatedByFrame:completionHandler:] 
was not called

原因分析

在WKWebView的WKUIDelegate回调方法中，(void (^)(void))completionHandler和(void (^)(BOOL))completionHandler无论YES or No，必须执行。如果当前WebViewVC划走，completionHandler还未执行，那么就会报这个崩溃提示（我延迟了completionHandler的执行时间，复现了崩溃现象）。

解决方法

直接上代码，不需要额外增加属性，在WKUIDelegate几个用到(void (^)(BOOL))completionHandler及(void (^)(void))completionHandler的回调方法中加上判断就可以解决。

#pragma mark -- WKUIDelegate
- (void)webView:(WKWebView *)webView runJavaScriptAlertPanelWithMessage:(NSString *)message initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(void))completionHandler
{
    if ([self.navigationController visibleViewController] != self)
    {
        completionHandler();
        return;
    }
    
    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"" message:message preferredStyle:UIAlertControllerStyleAlert];
    [alertController addAction:[UIAlertAction actionWithTitle:@"确认" style:UIAlertActionStyleCancel handler:^(UIAlertAction *action) { completionHandler();
    }]];
    if ([self.navigationController visibleViewController] == self)
    {
        [self presentViewController:alertController animated:YES completion:nil];
    }
    else
    {
        completionHandler();
    }
}

// 显示两个按钮，通过completionHandler回调判断用户点击的确定还是取消按钮
- (void)webView:(WKWebView *)webView runJavaScriptConfirmPanelWithMessage:(NSString *)message initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(BOOL))completionHandler
{
    if ([self.navigationController visibleViewController] != self)
    {
        completionHandler(NO);
        return;
    }
    
    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:message message:nil preferredStyle:UIAlertControllerStyleAlert];
    [alertController addAction:[UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
        
        completionHandler(YES);
    }]];
    [alertController addAction:[UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
        
        completionHandler(NO);
    }]];
    
    if ([self.navigationController visibleViewController] == self)
    {
        [self presentViewController:alertController animated:YES completion:nil];
    }
    else
    {
        completionHandler(NO);
    }
}

// 显示一个带有输入框和一个确定按钮的，通过completionHandler回调用户输入的内容
- (void)webView:(WKWebView *)webView runJavaScriptTextInputPanelWithPrompt:(NSString *)prompt defaultText:(NSString *)defaultText initiatedByFrame:(WKFrameInfo *)frame completionHandler:(void (^)(NSString * _Nullable))completionHandler{
    
    if ([self.navigationController visibleViewController] != self)
    {
        completionHandler(@"error");
        return;
    }
    
    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"" message:nil preferredStyle:UIAlertControllerStyleAlert];
    [alertController addTextFieldWithConfigurationHandler:^(UITextField * _Nonnull textField) {
        
    }];
    [alertController addAction:[UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
        
        completionHandler(alertController.textFields.lastObject.text);
    }]];
    
    if ([self.navigationController visibleViewController] == self)
    {
        [self presentViewController:alertController animated:YES completion:nil];
    }
    else
    {
        completionHandler(@"error");
    }
}