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Declarative Navigation Bar Appearance Configuration

The last couple of month I’ve been working on an app that required implementation of color changing behavior.

In this app, all users can choose a color for their profile, and the color of the “User Details” navigation bar depends on the avatar. This created a lot of issues in the app development including the need to make a status bar readable on the navigation bar’s background. What’s more, a tool bar in the app can only be visible for users’ friends.

How do you implement color changing features?

naive approach was to perform the navigation controller appearance configuration in the UIViewController.viewWillAppear.

But with this implementation I shortly found my view controllers full of unnecessary and even odd knowledge, such as navigation bar’s background image names, toolbar’s tint color alpha, and so on. Moreover, this logic was duplicated in several independent classes.

Given this nuicance I decided to refactor UIViewController.viewWillAppear and turn it into a small and handy tool that could free view controllers from repeated imperative appearance configurations. I wanted my implementation to have a UIKit-like declarative style as UIViewController.preferredStatusBarStyle. Under this logic, view controllers will be asked the number of questions or requirements that tell them how to behave.

The principle of work of AppearanceNavigationController

To achieve a declarative behaviour we need to become a UINavigationControllerDelegate to handle push and pop between the view controllers.

In the navigationController(_:willShowViewController:animated:) we need to ask view controller that is being shown the following questions:

  • Do we need to display a navigation bar?
  • What color should it be?
  • Do we need to display a toolbar?
  • What color should it be?
  • What style for the status bar is preferred?

Let’s turn it into a Swift’s struct:

public struct Appearance {

   public struct Bar {  

       var style: UIBarStyle = .Default       

       var backgroundColor = UIColor(red: 234 / 255, green: 46 / 255, blue: 73 / 255, alpha: 1)

       var tintColor = UIColor.whiteColor()

       var barTintColor: UIColor?

   }

   var statusBarStyle: UIStatusBarStyle = .Default

   var navigationBar = Bar()

   var toolbar = Bar()

}

You may have noticed that the flags of the navigation bar and toolbar visibility are missing, and here is why: most of the time I didn’t care about the bars appearance. All I needed is to hide them. Therefore, I decided to keep them separate.

As you remember, we’re going to “ask” our view controller about the preferred apperance. Let’s implement this as follows:

public protocol NavigationControllerAppearanceContext: class {

   func prefersNavigationControllerBarHidden(navigationController: UINavigationController) -> Bool

   func prefersNavigationControllerToolbarHidden(navigationController: UINavigationController) -> Bool

   func preferredNavigationControllerAppearance(navigationController: UINavigationController) -> Appearance?

}

Since not every UIViewController will configure appearance, we’re not going to extend UIViewController with AppearanceNavigationControllerContext. Instead, let’s provide a default implementation using protocol extension introduced in Swift 2.0 so that anyone that confroms to the NavigationControllerAppearanceContext can implement only the methods they are interested in:

extension NavigationControllerAppearanceContext {

   func prefersNavigationControllerBarHidden(navigationController: UINavigationController) -> Bool {

       return false

   }

   func prefersNavigationControllerToolbarHidden(navigationController: UINavigationController) -> Bool {

       return true

   }

   func preferredNavigationControllerAppearance(navigationController: UINavigationController) -> Appearance? {

       return nil
   }
}

As you may have noticed preferredNavigationControllerAppearance allows us to return`nil` which is useful to interpret as “Ok, this controller doesn’t want to affect the current appearance.”

Now let’s implement the basics of our Navigation Controller:

​public class AppearanceNavigationController: UINavigationController, UINavigationControllerDelegate {

   public required init?(coder decoder: NSCoder) {

       super.init(coder: decoder)
       
       delegate = self

   }

   override public init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: NSBundle?) {

       super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)

       delegate = self

   }

   override public init(rootViewController: UIViewController) {

       super.init(rootViewController: rootViewController)

       delegate = self

   }
   
   // MARK: - UINavigationControllerDelegate 

   public func navigationController(

       navigationController: UINavigationController,

       willShowViewController viewController: UIViewController, animated: Bool

   ) {

       guard let appearanceContext = viewController as? NavigationControllerAppearanceContext else {

           return

       }

       setNavigationBarHidden(appearanceContext.prefersNavigationControllerBarHidden(self), animated: animated)

       setToolbarHidden(appearanceContext.prefersNavigationControllerToolbarHidden(self), animated: animated)

       applyAppearance(appearanceContext.preferredNavigationControllerAppearance(self), animated: animated)

   }

   // mark: - Appearance Applying    

   private func applyAppearance(appearance: Appearance?, animated: Bool) {        

       // apply

   }

}

Appearance Configuration

Now it’s time to implement the appearance applying the details: 

private func applyAppearance(appearance: Appearance?, animated: Bool) {

   if let appearance = appearance {

       if !navigationBarHidden {

           let background = ImageRenderer.renderImageOfColor(appearance.navigationBarColor)

           navigationBar.setBackgroundImage(background, forBarMetrics: .Default)

           navigationBar.tintColor = appearance.navigationBarTintColor

           navigationBar.shadowImage = appearance.navigationBarDropsShadow ? nil : UIImage()

           navigationBar.titleTextAttributes = [

               NSForegroundColorAttributeName: appearance.navigationBarTintColor

           ]

       }

       if !toolbarHidden {

           toolbar.setBackgroundImage(

               ImageRenderer.renderImageOfColor(appearance.toolbarColor),

               forToolbarPosition: .Any,

               barMetrics: .Default

           )

           toolbar.tintColor = appearance.toolbarTintColor

       }

   }

}

If the View Controller’s appearance isn’t nil, we need to apply the apprearance differently – just ignore it. Code, that applies the appearance is fairly simple, except ImageRenderer.renderImageOfColor(color) which returns a colored image with 1x1 pixel size.

Status bar configuration

Note, that status bar style comes in pair with the Appearance and not via UIViewController.preferredStatusBarStyle(). This is because a status bar visibility depends on the navigation bar color brightness, so I decided to keep this “knowledge” about colors in a single place instead of putting it in two separate places.

private var appliedAppearance: Appearance?

private func applyAppearance(appearance: Appearance?, animated: Bool) {

   if let appearance = appearance where appliedAppearance != appearance {

       appliedAppearance = appearance


       // rest of the code


       setNeedsStatusBarAppearanceUpdate()

   }

}


// mark: - Status Bar

public override func preferredStatusBarStyle() -> UIStatusBarStyle {

   return appliedAppearance?.statusBarStyle ?? super.preferredStatusBarStyle()

}

public override func preferredStatusBarUpdateAnimation() -> UIStatusBarAnimation {

   return appliedAppearance != nil ? .Fade : super.preferredStatusBarUpdateAnimation()

}

Since we’re going to use UIKit’s preferred way of Status Bar appearance change, the applied Appearance needs to be preserved. Also, if there is no appearance applied we’re switching to the default super’s implementation.

Appearance Update

Obvisouly, the view controller appearance may change during its lifecycle. In order to update the view controller let’s add a UIKit-like method NavigationControllerAppearanceContext:

public protocol NavigationControllerAppearanceContext: class {    

   // rest of the interface

   func setNeedsUpdateNavigationControllerAppearance()

}

extension NavigationControllerAppearanceContext {

   // rest of the defaul implementation

   func setNeedsUpdateNavigationControllerAppearance() {

       if let

           viewController = self as? UIViewController,

           navigationController = viewController.navigationController as? AppearanceNavigationController

       {

          navigationController.updateAppearanceForViewController(viewController)

       }

   }

}

And a corresponding implementation in the AppearanceNavigationController:

func updateAppearanceForViewController(viewController: UIViewController) {

   if let

       context = viewController as? NavigationControllerAppearanceContext

       where

       viewController == topViewController && transitionCoordinator() == nil

   {

       setNavigationBarHidden(context.prefersNavigationControllerBarHidden(self), animated: true)

       setToolbarHidden(context.prefersNavigationControllerToolbarHidden(self), animated: true)

       applyAppearance(context.preferredNavigationControllerAppearance(self), animated: true)

   }

}

public func updateAppearance() {

   if let top = topViewController {

       updateAppearanceForViewController(top)

   }

}

From this point any AppearanceNavigationControllerContext can ask its container to rerun the appearance configuration in case something gets changed (editing mode, for example). By various checks like viewController == topViewController and transitionCoordinator() == nil we’re disallowing appearance change invoked by an invisible view controller or happened during the interactive pop gesture.

Usage

We’re done with the implementation. Now any view controller can define an appearance context, change appearance in the middle of the lifecycle and so on:

class ContentViewController: UIViewController, NavigationControllerAppearanceContext {

   

   var appearance: Appearance? {

       didSet {

           setNeedsUpdateNavigationControllerAppearance()

       }

   }

   

   // mark: - Actions

   

   override func setEditing(editing: Bool, animated: Bool) {

       super.setEditing(editing, animated: animated)

       

       setNeedsUpdateNavigationControllerAppearance()

   }

   

   // mark: - AppearanceNavigationControllerContent



   func prefersNavigationControllerToolbarHidden(navigationController: UINavigationController) -> Bool {

       // hide toolbar during editing

       return editing

   }

   

   func preferredNavigationControllerAppearance(navigationController: UINavigationController) -> Appearance? {

       // inverse navigation bar color and status bar during editing

       return editing ? appearance?.inversedAppearance : appearance

   }

}

Gathering the appearance together

Now we can gather all the appearance configurations as a category with common configurations, thus eliminating code duplication as in the naive solution:

extension Appearance {

   

   static let lightAppearance: Appearance = {

       var value = Appearance()

       

       value.navigationBar.backgroundColor = UIColor.lightGrayColor()

       value.navigationBar.tintColor = UIColor.whiteColor()

       value.statusBarStyle = .LightContent

       

       return value

   }()

}

Customizing the appearance 

To make the animation more customizeable let’s wrap it into the AppearanceApplyingStrategyhence anyone can extend this behaviour by providing a custom strategy:

public class AppearanceApplyingStrategy {

   

   public func apply(appearance: Appearance?, toNavigationController navigationController: UINavigationController, animated: Bool) {

   }

}

And connect the strategy to the AppearanceNavigationController:

private func applyAppearance(appearance: Appearance?, animated: Bool) {

       // we ignore nil appearance

       if appearance != nil && appliedAppearance != appearance {

           appliedAppearance = appearance

           

           appearanceApplyingStrategy.apply(appearance, toNavigationController: self, animated: animated)

           setNeedsStatusBarAppearanceUpdate()

       }

}



public var appearanceApplyingStrategy = AppearanceApplyingStrategy() {

   didSet {

       applyAppearance(appliedAppearance, animated: false)

   }

}

In summary, this solution doesn’t pretend to be a silver bullet, however with this short and simple implementation we have: 

  • simplified navigation controller’s appearance configuration
  • reduced duplication in code by defining extensions to the Appearance
  • made our code more UIKit-like
  • reduced the number of small and annoying bugs. For example, an accidental status bar style change by MailComposer.

You can find the project here.

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