ObjectiveC modal dialog. There is something I am missing with the class interaction

In the following code (from Apple example pages)

- (void)showCustomDialog: (NSWindow *)window

// User has asked to see the dialog. Display it.
{
    if (!myCustomDial开发者_运维知识库og)
        [NSBundle loadNibNamed: @"MyCustomDialog" owner: self];



    [NSApp beginSheet: myCustomDialog
        modalForWindow: window
        modalDelegate: nil
        didEndSelector: nil
        contextInfo: nil];

    [NSApp runModalForWindow: myCustomDialog];

    // Dialog is up here.

    [NSApp endSheet: myCustomDialog];

    [myCustomDialog orderOut: self];
}

How is the myCustomDialog variable ever set to anything, and when?

It will be an outlet in your class, in the nib of the dialogue, file's owner is set to be whatever class you are calling this from, and the outlet is connected to it. When loading the nib with owner self, the outlet is populated.