In iOS 6, viewWillUnload
and viewDidUnload
are deprecated and UIViewControllers no longer unload views that are not visible on screen during a memory warning. The View Controller Programming Guide has an example of how to manually restore this behavior.
Here is the code sample:
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Add code to clean up any of your own resources that are no longer necessary.
if ([self.view window] == nil)
{
// Add code to preserve data stored in the views that might be
// needed later.
// Add code to clean up other strong references to the view in
// the view hierarchy.
self.view = nil;
}
}
Below the code sample is the following note:
The next time the view property is accessed, the view is reloaded exactly as it was the first time.
There is an obvious flaw here. If a view controller that has not loaded its view receives a memory warning it will load its view in the line if ([self.view window] == nil)
and then proceed to clean up and release it again. At best, this is inefficient. At worst, it makes the memory conditions worse if a complex view hierarchy and supporting data are loaded. I verified this behavior in the iOS simulator.
I can certainly code around this but seems odd for Apple docs to have such an error. Am I missing something?
The correct check in a view controller for the view being loaded and on screen is:
if ([self isViewLoaded] && [self.view window] == nil)
My full solution in iOS 6 to have a view controller unload views and cleanup similar to iOS 5 is the following:
// will not be called in iOS 6, see iOS docs
- (void)viewWillUnload
{
[super viewWillUnload];
[self my_viewWillUnload];
}
// will not be called in iOS 6, see iOS docs
- (void)viewDidUnload
{
[super viewDidUnload];
[self my_viewDidUnload];
}
// in iOS 6, view is no longer unloaded so do it manually
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
if ([self isViewLoaded] && [self.view window] == nil) {
[self my_viewWillUnload];
self.view = nil;
[self my_viewDidUnload];
}
}
- (void)my_viewWillUnload
{
// prepare to unload view
}
- (void)my_viewDidUnload
{
// the view is unloaded, clean up as normal
}