knowledge-kit/Chapter1 - iOS/1.3.md

loadView

1. Purpose: load the controller's view
2. When it's called: it's called the first time the controller's view is used
3. Use case: implement this method when you want to provide a custom view for the controller

Accessing the controller's view is equivalent to calling the controller's view getter:

-(UIView *)view
{
    if(_view == nil){
        [self loadView];
        [self viewDidLoad];
    }
    return _view;
}

Controller view loading flow

• The controller's init method internally calls initWithNibName

  MyViewController *vc = [[MyViewController alloc] init];

Notes:

The system's decision logic assumes: no nibName specified; no custom loadView method; the controller is named ...Controller

Decision rules:

• Check whether a nibName was specified; if specified, load that nib
• Check whether there's an xib with the same name as the controller but without "Controller" in its name; if present, load it
• If the previous step doesn't find one, check whether an xib with the same name as the controller class exists; if present, load it
• If no xib describes the controller's view, do not load any xib

How MyViewController loads its view

• Check whether an xibName was specified; if so, load the specified xib
• Check whether there's an xib with the same name as the controller class but without "Controller" in the name
• Check whether there's an xib with the same name as the controller class; if present, load it
• Otherwise, create an empty view directly

Example

// in AppDelegate
ViewController *vc = [[ViewController alloc] init];
vc.view.backgroundColkor = [UIColor redColor];
self.window.rootViewController = vc;
[pself.window makeKeyAndVisable];

// ViewController
-(UIView *)view{
    if(!_view){
        [self loadView];
        [self viewDidLoad];
    }
}

-(void)loadView{
    UIView*view = [[UIView alloc] initWithFrame:[UIScreen mainScreen].bounds];
    view.backgroundColor = [UIColor greenColor];
    self.view = view;
}

-(void)viewDidload{
    [super viewDidload];
    self.view.backgroundColor = [UIColor brownColor];
}

What color will the interface be at this point?

Many people might answer "green." Actually, the answer is red.

Why? In AppDelegate, vc.view.backgroundColor calls vc's view getter. Inside the getter it checks if _view exists; if not, it creates a new UIView by calling [self loadView]. After creating the view it calls viewDidLoad; if the view already exists it returns it directly. So the sequence is: first green, then brown, and finally red.

An official explanation