How to use the TabNavigator component (Starling version)

The TabNavigator class supports navigation between screens or pages using a TabBar sub-component.

The Basics

First, let's create a TabNavigator component and add it to the display list:

var navigator:TabNavigator = new TabNavigator();
this.addChild( navigator );

You may set its width and height, but that's optional because the TabNavigator will automatically resize itself to fill the entire stage, if you don't provide explicit dimensions.

To add a new screen that the navigator can show, call addScreen() and pass in an ID string to associate with the screen along with a TabNavigatorItem:

var newsFeedItem:TabNavigatorItem = new TabNavigatorItem( NewsFeedTab, "News" );
navigator.addScreen( "newsFeed", newsFeedItem );

This screen's ID is "newsFeed". We can use use this ID later to programatically show the screen.

The first argument required by the TabNavigatorItem constructor may be one of three types. We can pass in a Class to instantiate, a display object that has already been instantiated, or a Function that returns a display object. In most cases, a Class is recommended. For more details, see the screen property.

In the example above, NewsFeedTab is another class that we create in our project that extends the Screen class. In general, it's best to extend a class that implements the IScreen interface, like Screen, PanelScreen, or ScrollScreen. Each offers different features. For instance, Screen is the most basic with optional support for layouts, while PanelScreen offers layouts, scrolling, and a customizable header and footer.

The second argument required by the TabNavigatorItem is the label to display on the tab associated with the screen. In this case, we've set it to "News".

Finally, we can also pass in an icon to be displayed on the tab. However, this is optional, and we've skipped it in the example above.

The active screen changes when the user selects a tab. When the first tab is added to a TabNavigator, it is automatically selected.

To show a specific screen programatically, we can set the selectedIndex:

navigator.selectedIndex = 1;

Alternatively, call showScreen(), and pass in the screen's ID. For this example, we'll use the "newsFeed" string that we registered with addScreen() earlier:

navigator.showScreen( "newsFeed" );

The showScreen() method supports an additional arugment that allows a custom animated transition between screens. We'll look at transitions in a moment.

To access the currently visible screen, use the activeScreen property.

var screen:NewsFeedTab = NewsFeedTab( navigator.activeScreen );

You can also use activeScreenID to get the ID of the active screen. In this case, again, it would be "newsFeed".

Animated Transitions

As we learned above, we can change to a new screen in a couple of different ways. The transition between two screens can also be animated, improving the user experience and adding a little bit of life to our games and apps. This animation during navigation is called a transition.

We can find a number of useful transition classes in the feathers.motion package. One example is the Fade class, which fades a screen by animating its alpha property.

Each of the built-in transition classes has one or more static methods that you can call to create a transition function that tab navigator calls when navigating to a different screen. In this case, let's call Fade.createFadeInTransition().

We can pass the result to the tab navigator's transition property:

navigator.transition = Fade.createFadeInTransition();

In the code above, we didn't pass any arguments to Fade.createFadeInTransition(). However, this function exposes some optional parameters that we can customize, if desired. For instance, we might want to customize the duration of the animation (in seconds) and the easing function:

navigator.transition = Fade.createFadeInTransition( 0.75, Transitions.EASE_IN_OUT );

Now, the animation will last a little longer while easing in and out.

See Transitions for Feathers screen navigators for a more detailed look at the available transitions, including instructions for creating custom transitions.

Events when transitions start and complete

A TabNavigator dispatches FeathersEventType.TRANSITION_START when a new screen is being shown and the transition animation begins. Similarly, it dispatches FeathersEventType.TRANSITION_COMPLETE when the transition animation has ended.

If a specific screen needs to know when its transition in (or out) starts or completes, we can listen for different events that provide a little more control. See How to use the Screen component (or ScrollScreen or PanelScreen) for details.

Let's listen for FeathersEventType.TRANSITION_COMPLETE:

navigator.addEventListener( FeathersEventType.TRANSITION_COMPLETE, navigator_transitionCompleteHandler );

The event listener might look like this:

private function navigator_transitionCompleteHandler( event:Event ):void
    // do something after the transition animation

Property Injection

Optionally, we can pass properties to the screen before it is shown. If we have multiple screens that need to share some data, this is a useful way to ensure that each screen has access to it. For instance, we might have an OptionsData class that stores things like audio volume and other common options. We'd want to pass that to the OptionsScreen to let the user change the volume, obviously. We'd also want to pass it to other screens that play audio so that it plays at the correct volume.

When we create the TabNavigator, let's create an OptionsData instance too. In a moment, we'll pass it to each screen that needs it.

var optionsData:OptionsData = new OptionsData();

Now, when we add our OptionsScreen to the TabNavigator, we pass it the OptionsData instance in using the properties property on the TabNavigatorItem:

var optionsItem:TabNavigatorItem = new TabNavigatorItem( OptionsScreen ); = optionsData;

In OptionsScreen, we need to add a variable or a getter and setter named options to match up with

protected var _options:OptionsData;
public function get options():OptionsData
    return this._options;
public function set options( value:OptionsData ):void
    this._options = value;

We want to update the screen when the options property changes, so we should invalidate the screen, and the draw() function will be called again:

public function set options( value:OptionsData ):void
    if(this._options == value)
    this._options = value;
    this.invalidate( INVALIDATION_FLAG_DATA );

Objects that are passed by value (like Number, Boolean, and int) should not be used directly with property injection. Each screen will get a copy instead of a reference, so if one screen changes the value, another won't see the change. Always combine simple values like this together into a custom class that can be passed by reference.