Display multiple views serially using GCD - ios

Question

I have 4 tabs and all have UIWebView. I want that the first tab should load and displayed immediately without waiting for others to load. For which I did this in UITabBarController class:
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];
}
}
}
But the main thread waits for all tabs to load.
I tried this with GCD using dispatch_async
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];
});
});
});
});
});
But this doesn't work either. It takes the same time to display the first tab.
How can this be fixed?

Answer 1

Ehm.. GCD and UIWebView's -loadRequest: doesn't work as you think. All the code above is pretty equal to:
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];
});
});
To solve your problem you need to implement queue depends on - (void)webViewDidFinishLoad:(UIWebView *)webView. The idea is:
First view controller's webView starts loading request
asynchronously;
When request is loaded (or failed), webView will notify your
delegate;
Notify all other view controllers with webView, to let them start
loading their requests;
If I were you, I would implement it in next way:
// 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];
}
After that, 'subscribe' all other(only tabs) view controllers for FirstViewControllerDidLoadWebView notification and load their WebView after it arrives.
For more details check NSNotificationCenter and UIWebViewDelegate