StatefulViewController

A protocol to enable UIViewControllers or UIViews to present placeholder views based on content, loading, error or empty states.

Overview

In a networked application a view controller or custom view typically has the following states that need to be communicated to the user:

• Loading: The content is currently being loaded over the network.
• Content: The content is available and presented to the user.
• Empty: There is currently no content available to display.
• Error: An error occurred whilst downloading content.

As trivial as this flow may sound, there are a lot of cases that result in a rather large decision tree.

StatefulViewController is a concrete implementation of this particular decision tree. (If you want to create your own modified version, you might be interested in the state machine that is used to show and hide views.)

Usage

This guide describes the use of the StatefulViewController protocol on UIViewController. However, you can also adopt the StatefulViewController protocol on any UIViewController subclass, such as UITableViewController or UICollectionViewController, as well as your custom UIView subclasses.

First, make sure your view controller adopts to the StatefulViewController protocol.

class MyViewController: UIViewController, StatefulViewController {
    // ...
}

Then, configure the loadingView, emptyView and errorView properties (provided by the StatefulViewController protocol) in viewDidLoad.

override func viewDidLoad() {
    super.viewDidLoad()

    // Setup placeholder views
    loadingView = // UIView
    emptyView = // UIView
    errorView = // UIView
}

In addition, call the setupInitialViewState() method in viewWillAppear: in order to setup the initial state of the controller.

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated)

    setupInitialViewState()
}

After that, simply tell the view controller whenever content is loading and StatefulViewController will take care of showing and hiding the correct loading, error and empty view for you.

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated)

    setupInitialViewState()
    loadDeliciousWines()
}

func loadDeliciousWines() {
    startLoading()

    let url = NSURL(string: "http://example.com/api")
    let session = NSURLSession.sharedSession()
    session.dataTaskWithURL(url) { (let data, let response, let error) in
        endLoading(error: error)
    }.resume()
}

Set custom delays

To minimize a flickering effect between transitions from and to the loading view, StatefulViewController delays between these transitions. If endLoading is faster than the delay before the loading view is shown, the loading view will be skipped and content is shown. On the other hand if the loading view is shown (the delay has finished), another delay is set before the content is shown. The default values of both delays are 1 second, but they can also be customized, by implementing the following methods:

func toLoadingTransitionDelay() -> Double {
    return 0.7
}

func fromLoadingTransitionDelay() -> Double {
    return 1.2
}

Life cycle

StatefulViewController calls the hasContent method to check if there is any content to display. If you do not override this method in your own class, StatefulViewController will always assume that there is content to display.

func hasContent() -> Bool {
    return datasourceArray.count > 0
}

Optionally, you might also be interested to respond to an error even if content is already shown. StatefulViewController will not show its errorView in this case, because there is already content that can be shown.

To e.g. show a custom alert or other unobtrusive error message, use handleErrorWhenContentAvailable: to manually present the error to the user.

func handleErrorWhenContentAvailable(error: ErrorType) {
    let alertController = UIAlertController(title: "Ooops", message: "Something went wrong.", preferredStyle: .Alert)
    alertController.addAction(UIAlertAction(title: "OK", style: .Default, handler: nil))
    presentViewController(alertController, animated: true, completion: nil)
}

Custom Placeholder View insets

Per default, StatefulViewController presents all configured placeholder views fullscreen (i.e. with 0 insets from top, bottom, left & right from the superview). In case a placeholder view should have custom insets the configured placeholderview may conform to the StatefulPlaceholderView protocol and override the placeholderViewInsets method to return custom edge insets.

class MyPlaceholderView: UIView, StatefulPlaceholderView {
    func placeholderViewInsets() -> UIEdgeInsets {
        return UIEdgeInsets(top: 20, left: 20, bottom: 20, right: 20)
    }
}

View State Machine

Note: The following section is only intended for those, who want to create a stateful controller that differs from the flow described above.

You can also use the underlying view state machine to create a similar implementation for your custom flow of showing/hiding views.

let stateMachine = ViewStateMachine(view: view)

// Add states
stateMachine["loading"] = loadingView
stateMachine["other"] = otherView

// Transition to state
stateMachine.transitionToState(.View("loading"), animated: true) {
    println("finished switching to loading view")
}

// Hide all views
stateMachine.transitionToState(.None, animated: true) {
    println("all views hidden now")
}