What is the relationship between AppDelegate, RootViewController, and UIApplication?

I am trying to figure out the relationship between the appdelegate, RootViewControoler, and UIApplication. Here is what I kinda have figured out so far:

When starting your application up, main.m gets loaded.

From here, your MainWindow.xib gets loaded.

In your MainWindow.xib, your File's Owner is of type UIApplication.

You set your UIApplication's delegate to your AppDelegate.

In your AppDelegate's source code, you can set your RootViewController to be the first view shown.

Is this right? What prompts AppDelegate to initially run it's

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)l开发者_如何学JAVAaunchOptions { }

method?

When an Objective-C application starts, it starts by running the function named main(). It doesn't have to be in the file "main.m" but that's how the Xcode wizard sets things up.

Inside the wizard-produced main() function, there is this line:

int retVal = UIApplicationMain(argc, argv, nil, nil);

That is what starts the "UIKit" framework that makes up the entire application. Inside UIApplicationMain, an object of type UIApplication is created. And part of what UIApplication does when the application starts is call the applicationDidFinishLaunchingWithOptions method on the delegate member of the UIApplication class. This delegate is set up in the MainWindow.xib file to be an instance of your ProjectAppDelegate class, a subclass of NSObject that conforms to the UIApplicationDelegate protocol.

What prompts AppDelegate to initially run it's ...

Because in your MainWindow.xib file you have connected (well the project wizard did the connection actually) the File's Owner (which is the UIApplication object)'s "delegate" outlet to the UIApplicationDelegate object in the the .xib file, and the class of the UIApplicationDelegate is set to your app's UIApplicationDelegate subclass.

And there's nothing magic about "MainWindow.xib", it could be called "Foo.xib", what's important is that the property in your Info.plist file called "Main nib file base name" is "MainWindow". Trying renaming MainWindow.xib to Foo.xib and changing the "Main nib file base name" in your Info.plist to "Foo" and you'll see it still works.

EDIT: more about RootController

Again, there's nothing magic about the so-called "RootController". This is just the name of the UIViewController subclass created for you by the Xcode new project wizard.

The wizard places code in the project for two classes: ProjectAppDelegate and ProjectViewController. The ProjectAppDelegate class contains two outlet members:

IBOutlet UIWindow *window;
IBOutlet ProjectViewController *viewController;

in the MainWindow.xib file, instances of both UIWindow and ProjectViewController are placed, and hooked up to the above outlets in ProjectAppDelegate.

What gets your stuff up on the screen is this code in your ProjectAppDelegate class:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after application launch.

    // Add the view controller's view to the window and display.
    [self.window addSubview:viewController.view];
    [self.window makeKeyAndVisible];

    return YES;
}

Again, nothing really magic about this: the project wizard created code that adds your "root" ViewController's view to the window's view, and makes the window visible. Your "root" view controller was create in the .xib file, and hooked up to the ProjectAppDelegate outlet.

It is very instructive to try to create an application entirely by yourself without using any of the files from the wizard. You'll learn a lot about how .xib files work and how they relate to code objects.

The starting point of iOS apps is always the main() function (thanks @bogatyr) which usually contains code similar to,

int main(int argc, char *argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    int retVal = UIApplicationMain(argc, argv, nil, nil);
    [pool release];
    return retVal;
}

The last two parameters of UIApplicationMain are important and specify the principal class name, and the application delegate. If they are nil, then Info.plist will be looked up for the main window xib (usually MainWindow.xib).

// If nil is specified for principalClassName, the value for NSPrincipalClass
// from the Info.plist is used. If there is no NSPrincipalClass key specified, the
// UIApplication class is used. The delegate class will be instantiated 
// using init.
.. UIApplicationMain(int argc, char *argv[], NSString *principalClassName, NSString *delegateClassName);

It is not necessary to set the File Owner through xib, and they can be specified directly in this UIApplicationMain function.

principalClassName can be the string UIApplication or a subclass of UIApplication. Similarly delegateClassName can be directly specified in this method. The delegate class is instantiated using init as the docs say. Suppose we specify our delegate class - MyAppDelegate as a string,

UIApplicationMain(int argc, char *argv[], nil, @"MyAppDelegate");

First an instance of UIApplication is instantiated, which will then create the delegate class from this string using NSClassFromString I suppose.

Once delegateObject has been instantiated, and application is ready, this delegateObject will be informed using the delegate method, didFinishLaunchingWithOptions.

Class delegateClass = NSClassFromString(@"MyAppDelegate");
id <UIApplicationDelegate> delegateObject = [[delegateClass alloc] init];

// load whatever else is needed, then launch the app
// once everything is done, call the delegate object to
// notify app is launched
[delegateObject application:self didFinishLaunchingWithOptions:...];

This is how UIApplication would handle it programmatically, if no nib is used. Using a nib in the middle is not much different.

Since your AppDelegate is a delegate of UIApplication - it listens to all notifications that UIApplication class posts during it's lifecycle. didFinishLaunching notification is one of them and it causes your AppDelegate to call aforementioned method.

For Universal — iPhone + iPad — apps you can specify that different NIBs load on each platform, either in the target Info panel or by adding NSMainNibFile~ipad and NSMainNibFile~iphone keys to your Info.plist. Alternatively, you can add a MainWindow~ipad.xib NIB to your target, it will be loaded on the iPad instead of MainWindow.xib, based on the NSMainNibFile key in Info.plist.

If you need more control and customization for a Universal app you can load the starting NIB manually. The "Universal" project template has the boilerplate for this method, so the quickest way to get started using this technique is to just create a new iOS project with the Universal profile.

In the above examples the Main NIB File is set in Info.plist (target settings) so that you will already have a NIB loaded when your application delegate is invoked. Usually in this setup a MyAppDelegate object will also be archived in the NIB (with some IBOutlets) and the NIB's File's Owner will be set to UIApplication.

For a universal project to be able to accommodate two alternative layouts, the Main NIB File key is left out of Info.plist. Then it instantiates the application delegate object programmatically in UIApplicationMain:

#import "MYAppDelegate.h"

int main(int argc, char *argv[])
{
  @autoreleasepool {
    return UIApplicationMain(argc, argv, nil, NSStringFromClass([MYAppDelegate class]));
  }
}

Then check your environment and settings and load the appropriate NIB in application:DidFinishLaunchingWithOptions:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  _window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
  // Override point for customization after application launch.
  if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) {
    _viewController = [[[MYViewController alloc] initWithNibName:@"MYViewController_iPhone" bundle:nil] autorelease];
  } else {
    _viewController = [[[MYViewController alloc] initWithNibName:@"MYViewController_iPad" bundle:nil] autorelease];
  }
  _window.rootViewController = _viewController;
  [_window makeKeyAndVisible];
  return YES;
}

- (void)dealloc {
  [_window release];
  [_viewController release];
  [super dealloc];
}

The new step is to create a root MYViewController manually, loading the appropriate NIB. In this setup the File's Owner is your shiny new MYViewController rather than UIApplication. If you want, MYViewController can adopt much of what you may have been using your application delegate for - which is often to encapsulate the core model class of the app, act as a data source and delegate for the views and other things in the NIB.

So you're expected to have some root UIView in the NIB, and it should be hooked up to the view outlet of the File's Owner (MYViewController).

Note that MYViewController's NIB isn't actually loaded until the first time the MYViewController.view property is accessed. Only then will [MyViewController viewDidLoad] be called! The most likely time for this to occur is when you add it to the root window.

In the template code shown above the root UIWindow is instantiated by the app delegate, but there's no reason you couldn't include it in your NIB instead. If you choose to do this, be careful. If you set the rootViewController of the window in the NIB to the File's owner in that case, it will cause the controller's view to be added to the window when the window is activated. Be careful constructing that first NIB in any case.

The app delegate doesn't necessarily need to have a reference to your root UIWindow if you want MYViewController to manage it, but it may be cleaner overall to keep the root window out of your NIBs and manage it in the app delegate.

Outside of that (!) there's not much different from the single-platform approach.

MainWindow.xib is defined in your info.plist as the Main nib file base name. In your MainWindow.xib, you define the first controller that you want to load, in your case, RootViewController.

didFinishLaunchingWithOptions: is part of the UIApplicationDelegate protocol. This method (in iOS4.0+) is always known to be the first to be called when launching an application.