Introduction to UIViewController

Normally, every app has at least one view controller (an instance of UIViewController class). Each view controller has one root view (an instance of UIView class). Commonly, one view controller manages one page.

View Controller's responsibilities:
- Updating the contents of the views
- Responding to user interactions with the views
- Resizing the views and managing the overall interface
- Coordinating with other objects, including other view controllers, in your app.

When creating Single View Application project, Xcode adds a view controller by default. The page or view that the view controller manages can be designed in the Storyboard (Main.storyboard file). For any additional setups for or actions to do responding to the user's interactions with the views in the page can be programmatically done in the corresponding ViewController.swift file. The ViewController class is connected to the view controller in the Storyboard through Class setting in Identity Inspector.

View Controller's Lifecycle

There are a bunch of methods, for example viewWillAppear(), are called according to its state. Usually, the property initializations are done in viewDidLoad() method. Note that any operations related to the view's or the subview's bounds property should be done in viewDidAppear() method because that's when the value of the bounds property is finalized. However, if there is a delay in the view rending and it does not look good to the user, the operations can be done in viewWillAppear() method instead but the layoutIfNeeded() method must be called first so that the system will layout any pending views and then you get the correct value of the bounds property.

Embedding A View Controller's Root View in Another View Controller's View

This post shows how to do it.