使用 GCD 连续显示多个视图

Question

我有 4 个选项卡，每个选项卡都有 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 class 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选项卡加载。
我用 GCD 和 dispatch_async

试过了

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];
                });
            });
        });
    });
});

但这也不管用。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: 并不像你想的那样工作。上面的所有代码都非常等于：

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。思路是：

第一个视图控制器的 webView 开始加载请求异步；
当请求加载（或失败）时，webView 会通知您代表；
用 webView 通知所有其他视图控制器，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，'subscribe' 所有其他（仅选项卡）视图控制器用于 FirstViewControllerDidLoadWebView 通知并在通知到达后加载它们的 WebView。

有关详细信息，请查看 NSNotificationCenter and UIWebViewDelegate

使用 GCD 连续显示多个视图

Display multiple views serially using GCD

iphone

objective-c

uiwebview

grand-central-dispatch

ios