ios - 使用 GCD 连续显示多个 View

Question

我有 4 个选项卡，并且都有 UIWebView。我希望第一个选项卡应该立即加载并显示，而无需等待其他选项卡加载。为此，我在 UITabBarController 类中做了这个:

for(UIViewController * viewController in  self.viewControllers){
        if(![[NSUserDefaults standardUserDefaults]boolForKey:@"isSessionExpired"])
        {
            if((int)[self.viewControllers indexOfObject:viewController] != 4)
            {
                viewController.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:viewController];
                [viewController view];
            }
        }
    }

但是主线程等待所有标签页加载。
我使用 dispatch_async

对 GCD 进行了尝试

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ // 1
        dispatch_async(dispatch_get_main_queue(), ^{
        UIViewController *firstContrl = [self.viewControllers objectAtIndex:0];
        firstContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:firstContrl];
        [firstContrl view];
        dispatch_async(dispatch_get_main_queue(), ^{ // 2
            UIViewController *secContrl = [self.viewControllers objectAtIndex:1];
            secContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:secContrl];
            [secContrl view];
            dispatch_async(dispatch_get_main_queue(), ^{ // 3
                UIViewController *thirdContrl = [self.viewControllers objectAtIndex:2];
                thirdContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:thirdContrl];
                [thirdContrl view];
                dispatch_async(dispatch_get_main_queue(), ^{ // 4
                    UIViewController *fourthContrl = [self.viewControllers objectAtIndex:3];
                    fourthContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:fourthContrl];
                    [fourthContrl view];
                });
            });
        });
    });
}); 

但这也行不通。显示第一个标签需要同样的时间。
如何解决？

Answer 1

嗯.. GCD 和UIWebView 的 -loadRequest: 并不像您想象的那样工作。上面的所有代码都非常等同于:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    dispatch_async(dispatch_get_main_queue(), ^{
        UIViewController *firstContrl = [self.viewControllers objectAtIndex:0];
        firstContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:firstContrl];
        [firstContrl view];

        UIViewController *secContrl = [self.viewControllers objectAtIndex:1];
        secContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:secContrl];
        [secContrl view];

        UIViewController *thirdContrl = [self.viewControllers objectAtIndex:2];
        thirdContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:thirdContrl];
        [thirdContrl view];

        UIViewController *fourthContrl = [self.viewControllers objectAtIndex:3];
        fourthContrl.tabBarItem.tag = (int)[[self viewControllers] indexOfObject:fourthContrl];
        [fourthContrl view];

    });
});

要解决您的问题，您需要实现队列取决于 - (void)webViewDidFinishLoad:(UIWebView *)webView。想法是:

1. 第一个 View Controller 的 webView 开始加载请求异步；
2. 当请求加载(或失败)时，webView 会通知您代表；
3. 使用 webView 通知所有其他 View Controller ，让它们开始加载他们的请求；

如果我是你，我会用下面的方式实现它:

// FirstViewController.h
extern NSString * const FirstViewControllerDidLoadWebView;

// FirstViewController.m
NSString * const FirstViewControllerDidLoadWebView=@"FirstViewControllerDidLoadWebView";
- (void)webViewDidFinishLoad:(UIWebView *)webView
{
// notify other controllers, to let them load their web views:
[[NSNotificationCenter defaultCenter] postNotificationName:FirstViewControllerDidLoadWebView
 object:nil];
}
- (void)webView:(UIWebView *)webView didFailLoadWithError:(NSError *)error
{
    // loading failed. Try to load it few more times but anyway notify other controllers after:
[[NSNotificationCenter defaultCenter] postNotificationName:FirstViewControllerDidLoadWebView
 object:nil];
}

之后，“订阅”所有其他(仅选项卡) View Controller 以获取 FirstViewControllerDidLoadWebView 通知，并在通知到达后加载它们的 WebView。

更多详情请查看NSNotificationCenter和 UIWebViewDelegate