appDelegate set rootViewController and viewWillAppear is not called? - ios

Question

this is demo ,I set RootViewController in didFinishLaunchingWithOptions,but viewController's viewWillAppear is not called! I guess it's because the NavigationController added the ViewController that the response chain of the message failed?
I know it should be set to
self.window.rootViewController = navigationController
But I want to know what I set to
self.window.rootViewController = mainViewController
Why not call mainViewController's viewWillAppear? Thank you.
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
self.window.backgroundColor = [UIColor whiteColor];
ViewController *mainViewController = [[ViewController alloc] init];
UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:mainViewController];
self.window.rootViewController = mainViewController;
[self.window makeKeyAndVisible];
return YES;
}
viewController.m
#import "ViewController.h"
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.view.backgroundColor = [UIColor whiteColor];
}
- (void)viewWillAppear:(BOOL)animated{
[super viewWillAppear:animated];
NSLog(#"viewWillAppear");
}
#end

Answer 1

Hm... interesting.
Keeping the following:
[[UINavigationController alloc] initWithRootViewController:mainViewController];
even though it's not used, seems to be stealing the ViewController's viewDidAppear.
If you comment that line then it works as you want.
i.e:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
self.window.backgroundColor = [UIColor whiteColor];
ViewController *mainViewController = [[ViewController alloc] init];
//Comment the navigationController instantiation
//UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:mainViewController];
self.window.rootViewController = mainViewController;
[self.window makeKeyAndVisible];
return YES;
}
Maybe the navigationController was set to own the ViewController but since it's not presented on the screen, neither the viewDidLoad nor viewDidAppear occurs until later, but... when you reassign the ViewController to the window, the internal view loading logic feels viewDidAppear is no longer needed to be called.
Maybe a bug or intentional, I don't know.
Note: I don't really have a reason for this behavior. I am just shedding light on what caused it.