Programmatically setting an UIViewController's view and life cycle

Question

I have several views in my app targeted for iOS 7+ that show a MKMapView. I'd like to have a single UIViewController to manage those MKMapView views, so I tried to create an UIViewController subclass conforming MKMapViewDelegate protocol:

@interface MapViewController : UIViewController <MKMapViewDelegate>

This MapViewController class has not an associated nib file. Then, in the view controller that manages the view where I want to show a MKMapView:

self.mapController = [[MapViewController alloc] init];
MKMapView *map = [[MKMapView alloc] initWithFrame:CGRectMake(0, 0, 300, 200)];
map.delegate = self.mapController;
[self.mapController setView:map];
[self.view addSubview:self.mapController.view];

This way, I can see that viewWillAppear: method in MapViewController is called, but viewDidLoad: method isn't.

Is this the correct way to do what I want? Why is viewDidLoad: not called?

Thanks in advance

Answer 1

There are two problems here:

• You shouldn't set the view property of a UIViewController. In fact, you should consider the view hierarchy of a UIViewController as private.
• you should use Apple's UIViewController containment API to ensure proper invocation of viewWillAppear: etc.

So here's what you should do:

The MKMapView must be created in -loadView of MapViewController:

- (void)loadView
{
    MKMapView *map = [[MKMapView alloc] initWithFrame:CGRectMake(0, 0, 300, 200)];
    map.delegate = self;
    self.view = map;
}

When you add the child ViewController to the view hierarchy you should do this:

self.mapController = [[MapViewController alloc] init];
[self addChildViewController:self.mapController];
[self.view addSubview:self.mapController.view];
[self.mapController didMoveToParentViewController:self];

when you remove the child ViewController from the view hierarchy for whatever reason you should do this:

[self.mapController willMoveToParentViewController:nil];
[self.mapController.view removeFromSuperview];
[self.mapController removeFromParentViewController];

For more details about container view controllers please refer to https://developer.apple.com/library/content/featuredarticles/ViewControllerPGforiPhoneOS/ImplementingaContainerViewController.html

How does a view controller load its view?

Apple's view property getter implementation looks a lot like this:

- (UIView *)view
{
    [self loadViewIfNeeded];

    return _view;
}

- (void)loadViewIfNeeded
{
    if (_view == nil) {
        [self loadView];
        [self viewDidLoad];
    }
}

when you access the view property of a view controller, it checks if view is actually nil. If it is nil, it invokes -loadView and then -viewDidLoad before returning view.

Therefore when you set the view property before accessing it the first time, -loadView and -viewDidLoad will never be invoked.