change ViewController with SegmentedControl without destroy it - ios

Question

I have a few controllers. for some reason, I'm using UISegmentedControl instead of tab bar.
each few controllers download data from the servers. my problem is, if I move to next view controller and go back to the previous view controllers, I need to redownload again.
how to change view controller with UISegmentedControl without destroy the previous controller, so I don't need to redownload again. each time I move to different viewcontroller
here's my code
class ContentViewController: UIViewController {
private let homeViewController: HomeViewController!
private let aboutViewController: AboutViewController!
private let liveTVViewController: LiveTVViewController!
private let programsViewController: ProgramsViewController!
private var currentViewController: UIViewController!
var userDeviceType: Int!
override func viewDidLoad() {
super.viewDidLoad()
let viewController = viewControllerForSegmentIndex(0)
self.addChildViewController(viewController)
viewController.view.frame = self.view.bounds
self.view.addSubview(viewController.view)
currentViewController = viewController
NSNotificationCenter.defaultCenter().addObserver(self, selector: "segmentChanged:", name: "SegmentChangedNotification", object: nil)
}
deinit {
NSNotificationCenter.defaultCenter().removeObserver(self)
}
func segmentChanged(notification: NSNotification) {
let userInfo = notification.userInfo as [String: AnyObject]
let selectedIndex = userInfo["selectedIndex"] as Int
let viewController = viewControllerForSegmentIndex(selectedIndex)
self.addChildViewController(viewController)
self.transitionFromViewController(currentViewController,
toViewController: viewController,
duration: 0.0,
options: UIViewAnimationOptions.CurveEaseIn,
animations: { () -> Void in
self.currentViewController.view.removeFromSuperview()
viewController.view.frame = self.view.bounds
self.view.addSubview(viewController.view)
}) { (finished: Bool) -> Void in
viewController.didMoveToParentViewController(self)
self.currentViewController.removeFromParentViewController()
self.currentViewController = viewController
}
}
func viewControllerForSegmentIndex(index: Int) -> UIViewController {
var viewController: UIViewController
if index == 0 {
viewController = self.storyboard!.instantiateViewControllerWithIdentifier("HomePage") as HomeViewController
(viewController as HomeViewController).userDeviceType = userDeviceType
} else if index == 1 {
viewController = self.storyboard!.instantiateViewControllerWithIdentifier("ProgramsPage") as ProgramsViewController
} else if index == 2 {
viewController = self.storyboard!.instantiateViewControllerWithIdentifier("LiveTVPage") as LiveTVViewController
} else if index == 3 {
viewController = self.storyboard!.instantiateViewControllerWithIdentifier("AboutPage") as AboutViewController
} else {
viewController = self.storyboard!.instantiateViewControllerWithIdentifier("HomePage") as HomeViewController
}
return viewController
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
}
thank you very much and sorry for my bad English

Answer 1

You should adopt the MVC design pattern. This will allow you to store the data you need downloaded in the Model. Then, when a certain view controller is loaded, you simply ask the model if that data exists. If it does, you'll get it back. Otherwise, you can download it as normal.
To further explain:
The Model-View-Controller (MVC) design pattern assigns objects in an application one of three roles: model, view, or controller. The pattern defines not only the roles objects play in the application, it defines the way objects communicate with each other.
Model objects encapsulate the data specific to an application and define the logic and computation that manipulate and process that data. For example, a model object might represent a character in a game or a contact in an address book. A model object can have to-one and to-many relationships with other model objects, and so sometimes the model layer of an application effectively is one or more object graphs. Much of the data that is part of the persistent state of the application (whether that persistent state is stored in files or databases) should reside in the model objects after the data is loaded into the application. Because model objects represent knowledge and expertise related to a specific problem domain, they can be reused in similar problem domains. Ideally, a model object should have no explicit connection to the view objects that present its data and allow users to edit that data—it should not be concerned with user-interface and presentation issues.
User actions in the view layer that create or modify data are communicated through a controller object and result in the creation or updating of a model object. When a model object changes (for example, new data is received over a network connection), it notifies a controller object, which updates the appropriate view objects
The above quotes are from the link I mentioned in the first paragraph.

Answer 2

You are instantiate new view controllers in viewControllerForSegmentIndex(index: Int) by mistake.
What you should do to avoid instantiate each view controller every time you switch back to it is to modify the properties as:
private let homeViewController: HomeViewController = self.storyboard!.instantiateViewControllerWithIdentifier("HomePage") as HomeViewController
private let aboutViewController: AboutViewController = self.storyboard!.instantiateViewControllerWithIdentifier("AboutPage") as AboutViewController
private let liveTVViewController: LiveTVViewController = self.storyboard!.instantiateViewControllerWithIdentifier("LiveTVPage") as LiveTVViewController
private let programsViewController: ProgramsViewController = self.storyboard!.instantiateViewControllerWithIdentifier("ProgramsPage") as ProgramsViewController
And change viewControllerForSegmentIndex(index: Int) to:
func viewControllerForSegmentIndex(index: Int) -> UIViewController {
var viewController: UIViewController
switch index {
case 0:
return homeViewController
case 1:
return programsViewController
case 2:
return liveTVViewController
case 3:
return aboutViewController
default:
return homeViewController
}
}