iPhone Dev - Lazy loading a Tab Bar app

Question

How can I make it so when a tab is selected, the current one is unloaded, and the next one is loaded so only one loaded at a time? Or should I not even do this? I know how to do it with a normal UIViewController as the root VC, but not sure with a UITabBarController. Also, is there a way to animate the transition from one tab to the next? Any help? Thanks!!

EDIT: ... If I unload the view controllers, then their icons on the tab bar are gone... maybe I'll just unload their views..

Answer 1

You cant really manage the UITabBarController unfortunaly so you cant do lazy loading. You can by managining your own TabBar but you said u knew that already,

to manage your own tab bar though all you gotta do is setup a UITabBar with its TabBarItems in a ViewController, then implement the TabBar Delegate protocol, mainly the – tabBar:didSelectItem: method which is called whenever the tabbarItem selection is changed, then based on the item id you can load your new ViewController and release any others so: Edit: this code goes in your UIViewController

  -(void)addTabBar{
    NSMutableArray* items=[[NSMutableArray alloc] init];
    UITabBarItem *eventsItem= [[UITabBarItem alloc] initWithTitle:@"Events" image:nil   tag:0];
    UITabBarItem *albumItems=[[UITabBarItem alloc] initWithTitle:@"Album" image:nil tag:1]; //the tag is how you tell what was clicked
    [items addObject:homeItem];
    [items addObject:albumItems];
      //MyTabBar is of type UITabBar
    myTabBar=[[UITabBar alloc] initWithFrame:CGRectMake(0,411,320,49)];
    [myTabBar setItems:items];
    myTabBar.delegate=self; //you gotta implement the UITabBar delegate protocol
    [myTabBar setSelectedItem:eventItem]; //set the selected item
    [homeItem release];
    [eventsItem release];
    [albumItems release];
    [items release];
   [self.view addSubview:myTabBar]
}

then the protocol method would look something like below - (void)tabBar:(UITabBar *)tabBar didSelectItem:(UITabBarItem *)item { if(item.tag == 0 ) { //load the ViewController that pertains to this item and release others } ...etc

}

Answer 2

Not sure why you'd want to do this, the current tab will get unloaded anyway if there's a memory issue involved. That's what -viewWillAppear, -viewDidUnload, etc. are for.

Answer 3

I can answer both questions in one...

You just need a class that acts as the UITabBarController delegate, then implement a method like so:

// Animate tab selections so they fade in and fade out
-(void)tabBarController:(UITabBarController*)tbc didSelectViewController:(UIViewController*)newSelection
{
    [UIView beginAnimations:@"TabFadeIn" context:nil];
    [UIView setAnimationDuration:0.6];
    for( UIViewController* vc in tbc.viewControllers )
        vc.view.alpha = (vc==newSelection) ? 1 : 0;
    [UIView commitAnimations];  
}

Now my code simply makes the tab bars fade in and out, but you could also do work here to unload non-used tabs. Sometimes that is a good idea if some of the tabs will be using a ton of memory.

Answer 4

UITabBarController does lazy load all of its view controllers. When a tab is switched out, then it's view is subject to being deallocated in a memory tight situation. It is then recreated when it is chosen the second time. Furthermore, most of your memory hits are in your views and not the view controllers. Hence, don't worry about the memory hit from the view controller. The view is the proze.

If you are running on v3 of the OS, then you can use the -viewDidUnload method to ensure the maximal amount of memory reduction.

Andrew