ios - 当iOS应用程序进入后台时读取本地存储

Question

Objective-c 对我来说非常陌生:

当我的 iPhone 应用程序移至后台时，将触发以下方法。在那一刻，我想抓取 UIWebView 本地存储中的项目并将其存储在应用程序中。

我收到错误“选择器‘stringByEvaluatingJavaScriptFromString:jsString’没有已知的类”。如何在保留 UIApplication 的同时将此依赖项注入(inject)到该函数中？

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later. 
    // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.

    //Persist local storage data
    NSLog(@"Persisting local storage since app entering background");

    //Reading local storage item
    NSString *jsString = @"localStorage.getItem('mpo.subdomain');";
    NSString *someKeyValue = [UIWebView stringByEvaluatingJavaScriptFromString:jsString];

    // Store your subdomain in iPhone persistence variable and use it later in the application
    NSUserDefaults *userdefault = [NSUserDefaults standardUserDefaults];
    [userdefault setObject:someKeyValue forKey:@"subdomain"];
    [userdefault synchronize];

    //use of User default
    NSLog(@"User Subdomain %@",[userdefault valueForKey:@"subdomain"]);
}

Answer 1

代码[UIWebView stringByEvaluatingJavaScriptFromString:]正在尝试调用UIWebview类上名为stringByEvaluatingJavaScriptFromString的CLASS方法。 stringByEvaluatingJavaScriptFromString 方法是一个实例方法。您需要将该消息发送到 UIWebView 的实例，而不是类。

如果你有一个属性 myWebView 那么你会使用代码:

[self.myWebView stringByEvaluatingJavaScriptFromString:]

它将 stringByEvaluatingJavaScriptFromString 消息发送到 self.myWebView 属性指向的 UIWebview 实例。那就是你想要的。