scene2d

KTX: Scene2D type-safe builders

Utilities for creating Scene2D widgets using Kotlin type-safe builders.

Why?

Building Scene2D GUIs in Java usually leads to overly verbose, complicated code. It is hard to reason about how it will actually look on the screen. As opposed to HTML, XML or other easily readable markup languages, Java rarely forms visually appealing GUI templates. Kotlin type-safe builders DSL is a great compromise between the two: readability of a markup language meets flexibility and expressiveness of a programming language.

Guide

ktx-scene2d provides a number of factory and extension methods that mimic the original Scene2D API, allowing you to use Kotlin type-safe builder DSL. Methods were designed to match the original API as closely as possible.

All ktx-scene2d DSL methods that create Scene2D widgets match names of widget classes converted to lower camel case, and contain similar parameters to widgets constructors. For example, you can use label method to construct a com.badlogic.gdx.scenes.scene2d.ui.Label, passing a CharSequence (text of the label) and optional Skin and LabelStyle name as parameters. If you are familiar with Scene2D API, ktx-scene2d should feel natural.

To access the Scene2D DSL, you can use the scene2d object:

import ktx.scene2d.*

val myFirstActor = scene2d.label(text = "Hello World!")

To avoid passing a custom Skin instance to create the actors, make sure to set the default Skin before building UIs:

import com.badlogic.gdx.scenes.scene2d.ui.Skin
import ktx.scene2d.*

fun setDefaultSkin() {
  Scene2DSkin.defaultSkin = Skin()
}

scene2d provides factory methods for all official Scene2D actors. These can be divided into the following groups:

• Root actors: these actors are usually added directly to a Stage:
  • dialog
  • window
• Parent actors - widgets that can have nested children:
  • Parental widgets that were designed to store actors and control their layout:
    • buttonTable
    • container
    • horizontalGroup
    • scrollPane
    • splitPane
    • stack
    • table
    • tree
    • verticalGroup
  • Parental widgets that can store actors due to their type hierarchy, but do not have to:
    • button
    • checkBox
    • imageButton
    • imageTextButton
    • textButton
• Child actors - basic Scene2D widgets that cannot have any children:
  • image
  • label
  • listWidget, listWidgetOf
  • particleEffect
  • progressBar
  • selectBox, selectBoxOf
  • slider
  • textArea
  • textField
  • touchpad

Additionally, tooltip and textTooltip extension methods were added to all actors to ease creation of tooltips.

When it comes to building user interfaces, you usually start with a single parent or root actor from the list above and fill it with widgets:

import ktx.scene2d.*

scene2d.table {
  label("Hello world!")
}

The example above would create a Table instance with a single Label child.

All factory methods return instances of the created actors. If you need a direct reference to an actor, you can always assign the result of the factory method:

import ktx.scene2d.*

val myRoot = scene2d.table {
  val myLabel = label("Hello world!")
  // myLabel is Label
}
// myRoot is Table

Actors also can be nested:

import ktx.scene2d.*

val myTable = scene2d.table {
  button {
    label("Click me.")
  }
}

The snippet above would create a Table instance with a single Button child containing a nested Label. There are no practical nesting limits: as long as you do not manually set up circular references (actors being parents of themselves), your widget hierarchies should just work.

Thanks to Kotlin's @DslMarker, you cannot implicitly access parents from within children blocks, which resolves most scoping issues. For example, Label is not a Group and it cannot have children, so this code would not compile:

import ktx.scene2d.*

val myTable = scene2d.table {
  label("Not a real parent.") {
    label("Invalid.") // !!! Does not compile: outside the `table` scope.
  }
}

This ensures that the layout of your code will translate to the actual widgets hierarchy in the GUI.

While certainly less verbose than Java, this DSL would not be very useful without the possibility to customize the widgets. Let's consider this example of widget customization written in Java:

public Table createTableWithLabel(Skin skin) {
    Table table = new Table(skin);
    table.pad(4f);

    Label label = new Label("Example.", skin);
    label.setColor(Color.RED);

    table.add(label);

    return table;
}

Notice how you have to pass the Skin instance every time you want to create a new widget with a predefined style. The widgets hierarchy is not clear at first glance, which only gets worse as more widgets are added. You have to read through the code and find add calls to understand how the UI is built.

When to it comes to ktx-scene2d, widgets can be customized in the following ways:

• Method parameters: values necessary to construct the widgets can be passed as parameters.
• Building blocks: lambdas can be used along with the factory methods to customize the widgets. Lambdas receive the widget as this, which makes it very simple to access widget properties.

For example:

val label = scene2d.label("Label text is a necessary parameter!") {
  this is Label // == true
  // You can customize your label here! Let's change the color of the label:
  color = Color.RED
}

Now, let's rewrite the previous Java customization example in Kotlin with ktx-scene2D DSL:

import ktx.scene2d.*
import com.badlogic.gdx.graphics.Color

val myTable = scene2d.table {
  pad(4f) // Setting table padding.

  label("Example.") {
    color = Color.RED // Setting label color.
  }
}

Not only is the hierarchy clearer - the label is clearly a child of the table - but also the settings are less verbose to customize.

Building your code with Kotlin DSL is also safer. Actors hierarchy in your code is preserved and reflected in the UI. You do not have to remember to add the Label to the Table explicitly, as the structure of your code already dictates how and when the actors will be connected.

Also, you do not have to worry to accidentally changing properties of nested actors - Kotlin DSL capabilities prevent that during compilation:

scene2d.table {
  label("This would not compile") {
    pad(4f) // !!! Compilation error: `pad` belongs to the `table`, not `label`.
  }
  pad(4f) // This is OK, as we are still in the `table` scope.
}

In the building blocks you have full access to the Scene2D widgets API - type-safe Kotlin builders are just providing some syntax sugar. Keep in mind that Scene2D was written in Java, so while some properties are Kotlin-compatible (color = Color.RED), most are not (setWrap(true)). This is because we wanted to create a thin wrapper over Scene2D, and we opted against duplicating its entire API. It is still pretty straightforward to configure most actors.

All building blocks are inlined during compilation, which means there is little to no runtime overhead when using ktx-scene2d. This code will be pretty much as fast as your good old Java, while remaining cleaner and safer.

Note that the scene2d. prefix is only necessary for top-level actors to start using the DSL. In effect, it's only necessary if you'd like to assign the widget to a variable and add it to a Stage or another widget group later on. When inside a DSL block, scene2d. prefix is not only no longer necessary to define actors, but it will actually prevent them from being added to their immediate parent. Consider the following example:

import ktx.scene2d.*
import com.badlogic.gdx.scenes.scene2d.Stage

fun addActors(stage: Stage) {
  // This actor is manually added to a Stage:
  val table = scene2d.table {
    label("This label is a child of the table!")
  }
  stage.addActor(table)

  // Using `stage.actors` - scene2d. prefix is no longer necessary:
  stage.actors {
    table {
      label("This table is automatically added to the stage!")
    }
  }

  // Invalid usage - nested scene2d. prefix:
  scene2d.table {
    // WARNING: This label will not be added to the table!  
    scene2d.label("This label is NOT a child of the table.")
    // DO NOT DO THIS, unless you deliberately do NOT want
    // the actor to be immediately added to its parent.
  }
}

Working with Skin

Skin instances store styles of the widgets and other GUI assets - like fonts and drawables.

Additionally to standard constructor parameters, most factory methods also include skin and style parameters that default to Scene2DSkin.defaultSkin and "default" respectively. To globally set the Skin instance that will be used by default to create your widgets, modify defaultSkin field of Scene2DSkin container:

import com.badlogic.gdx.scenes.scene2d.ui.Skin
import ktx.scene2d.*

fun createSkin() {
  val mySkin: Skin = TODO("Create your skin.")
  Scene2DSkin.defaultSkin = mySkin
}

If the default Skin is not set, any attempt to create a widget that requires a style will end with an exception unless a custom Skin instance is passed to the factory method.

Scene2DSkin class includes support for listeners that will be invoked each time you reload your skin. For example, you can introduce different GUI skins in your application and add a listener that recreates the GUI each time the theme is changed:

import com.badlogic.gdx.scenes.scene2d.ui.Skin
import ktx.scene2d.*

fun setupListener(myViewsManager: MyClass) {
  Scene2DSkin.addListener {
    // Invoked each time defaultSkin is changed.
    myViewsManager.reloadViews()
  }

  Scene2DSkin.defaultSkin = Skin() // Invokes the listener.  
}

This makes it easy to access Skin resources for your widgets, as the Skin will be provided by default.

style name parameter allows you to choose the look of your widgets by fetching the widget style defined in your Skin. For example, you can easily change styles of labels by using their style and skin parameters:

import ktx.scene2d.*

val labelWithCustomStyle = scene2d.label(text = "Custom style!", style = "custom")

// You can also omit Scene2DSkin.defaultSkin altogether and pass a custom Skin:
val labelWithCustomSkin = scene2d.label(text = "Custom skin!", skin = mySkin, style = "custom")

When constructing Skin instances and defining styles for your UI, ktx-style module might prove useful.

Stage

Stage is a crucial part of the Scene2D API. It contains the root actor that is the parent of all the displayed widgets. It handles input processing, rendering and resizing. If you are unfamiliar with Stage API, try reading the resources listed at the bottom of the page.

Initiating Stage instances:

import com.badlogic.gdx.scenes.scene2d.Stage

// Default settings:
val stage = Stage()

// Custom viewport:
val stage = Stage(ExtendViewport(800, 600))

// Custom viewport and batch:
val stage = Stage(ExtendViewport(800, 600), SpriteBatch())

Using ktx-actors:

import ktx.actors.stage

val stage = stage(batch = SpriteBatch())

If you are already using a SpriteBatch to render your textures, reusing the same instance for Stage is recommended.

Make sure to set the Stage as the input processor before it is rendered to listen for input events:

Gdx.input.inputProcessor = stage

ktx-scene2d provides a Stage.actors extension method that allows to add actors directly to a Stage:

stage.actors {
  label("Will be added to the stage!")
}

You can define multiple widgets in the actors block. All top-level widgets will be added to the stage:

stage.actors {
  label("This stage will have 3 direct children:")
  textButton("A label, a text button and a table.")
  table {
    label("Nested actors still work as expected.")
  }
}

Remember that if you do not use the actors extension, you have to add your actors to the Stage with addActor. For example:

val myTable = scene2d.table {
  label("This table will be added to a stage manually.")
}
stage.addActor(myTable)

This is a minimal example application that creates and renders a very basic UI:

import com.badlogic.gdx.ApplicationAdapter
import com.badlogic.gdx.Gdx
import com.badlogic.gdx.scenes.scene2d.Stage
import com.kotcrab.vis.ui.VisUI
import ktx.scene2d.*

class Example: ApplicationAdapter() {
  lateinit var stage: Stage

  override fun create() {
    stage = Stage()
    Scene2DSkin.defaultSkin = TODO("Load your skin here.")

    stage.actors {
      // Root actor added directly to the stage - a table:
      table {
        // Table settings:
        setFillParent(true)
        background("white")
        // Table children:
        label("Hello world!")
      }
    }

    Gdx.input.inputProcessor = stage
  }

  override fun render() {
    // Clear screen here. ktx-app might be useful.
    stage.act()
    stage.draw()
  }
}

Note that ktx-app module makes it easier to work with input processors and multiple application screens, while ktx-actors module has some general Scene2D utilities including Stage factory function.

KWidgets

This section describes the internal implementation of the DSL.

To avoid method collisions and ease the implementation of type-safe builders, most so-called "parental" widgets (extending Group class) were extended with K prefix. Table-extending widgets in ktx-scene2d now implement the KTable interface, which gives access to inlined builder methods that allows to customize the added widgets, as well as their Cell instances. Regular WidgetGroup-extending widgets now implement KGroup interface, which allows to build their children with type-safe builder syntax.

The only exceptions from the naming scheme are the KTableWidget, KTreeWidget and KListWidget, which extend Table, Tree and List respectively. Also, KListWidget factory methods are named listWidget and listWidgetOf instead of list or listOf to avoid collisions with Kotlin standard library.

You generally do not have to worry about the KWidgets: you can still reference these by their original extended class. This is basically just an internal representation: we chose not to create entirely new class hierarchy for the builders (as opposed to simply extending existing Scene2D classes) as it would basically require to rewrite the entirety of Scene2D module.

Usage examples

Loading and setting up the default Skin:

import ktx.scene2d.*
import com.badlogic.gdx.Gdx
import com.badlogic.gdx.scenes.scene2d.ui.Skin

fun loadSkin() {
  Scene2DSkin.defaultSkin = Skin(Gdx.files.internal("ui.json"))
}
// Note: ktx-assets and ktx-asset-async might help with asset loading.

Creating a Table with a Label child:

import ktx.scene2d.*

val table = scene2d.table {
  label("Hello world!")
}

Creating a Table with a customized background image and a Label with custom color:

import ktx.scene2d.*
import com.badlogic.gdx.graphics.Color

val table = scene2d.table {
  background("button")
  label("Hello world!") {
    color = Color.CORAL
  }
}

Adding actors to a Stage:

import com.badlogic.gdx.scenes.scene2d.Stage
import ktx.scene2d.*

fun addActorsToStage(stage: Stage) {
  stage.actors {
    table {
      label("This table will be added to the stage.")
    }
    textButton("This button will also be added to the stage.")
  }
}

Accessing nested actors:

import com.badlogic.gdx.scenes.scene2d.ui.Label
import ktx.scene2d.*

// Thanks to Kotlin contracts, our widget builders are ensured
// to be executed exactly once, which makes assignment of nested
// actors possible:

val myLabel: Label
val myTable = scene2d.table {
  myLabel = label("Kotlin contracts make it possible!")
}

Customizing layout of a Window using Cell properties:

import ktx.scene2d.*
import com.badlogic.gdx.graphics.Color

val window = scene2d.window(title = "Hello world!") {
  button { cell ->
    // Changing button properties - button is "this":
    color = Color.CORAL
    // Changing cell properties:
    cell.fillX().row()
  }
  table {
    // Changing nested table properties:
    defaults().pad(2f)
    // Adding table children:
    label("Nested")
    label("table")
    label("of")
    label("labels.")
    // Cell of the nested actor is also available through "it":
    it.spaceBottom(10f).row()
  }
  textButton(text = "Click me!")
  // Packing the root window:
  pack()
}

Accessing Cell instances of Table children:

import ktx.scene2d.*
import com.badlogic.gdx.scenes.scene2d.ui.Cell
import com.badlogic.gdx.scenes.scene2d.ui.Label

val table = scene2d.table {
  // Actors within tables (and other widgets that extend table like windows)
  // have access to their cells which allow to customize the layout.
  val label: Label = label("Cell properties modified.").cell(expand = true)
  val cell: Cell<Label> = label("Wrapped in cell.").inCell
  val combined: Cell<Label> = label("Modified and wrapped.").cell(expand = true).inCell
  val afterBuildingBlock: Cell<Label> = label("Not limited to no init block actors.") {
    setWrap(true)
  }.cell(expand = true).inCell
  val inBuildingBlock = label("Also available as building block parameter.") { cell ->
    cell.expand()
  }

  // Cells are available only for direct children of tables (or its extensions).
  stack {
    // These would not compile:
    label("Invalid.").cell(expand = true) // Not in a table!
    label("Invalid").inCell // Not in a cell!
  }
}

Creating Tree of Label instances:

import ktx.scene2d.*

val tree = scene2d.tree {
  label("Node")
  label("Node")
  label("Nest") { node ->
    node {
      label("Nested") { node ->
        node.label("Nested")
      }
      label("Nested")
    }
  }
  label("Node")
}

Accessing Node instances of Tree children outside of building blocks (note that KNode is KTX custom wrapper of libGDX Tree.Node with additional building API support):

import ktx.scene2d.*
import com.badlogic.gdx.scenes.scene2d.ui.Label

val tree = scene2d.tree {
  // Children on trees have access to nodes, which allow to customize tree layout:
  val label: Label = label("Node properties modified.").node(expanded = true)
  val cell: KNode<Label> = label("Wrapped in node.").inNode
  val combined: KNode<Label> = label("Modified and wrapped.").node(expanded = true).inNode
  val afterBuildingBlock: KNode<Label> = label("Not limited to no init block actors.") {
    setWrap(true)
  }.node(expanded = true).inNode
  val inBuildingBlock = label("Also available as building block parameter.") { node ->
    node.isExpanded = true
  }
}

Creating List and SelectBox widgets storing strings:

import ktx.scene2d.*
import com.badlogic.gdx.utils.Array as GdxArray

val table = scene2d.table {
  // List and SelectBox generics represent the types of items that they store.
  listWidget<String> {
    -"First"
    -"Second"
    -"Third"
  }
  selectBox<String> {
    -"First"
    -"Second"
    -"Third"
    // Note that these building blocks have no cell or node parameters.
    // You can access table cells and tree nodes outside the lambdas:
  }.cell(row = true)
}

val withoutBuildingBlocks = scene2d.table {
  // You can also define the widgets with lists of items:
  listWidgetOf(GdxArray.with("First", "Second", "Third"))
  selectBoxOf(GdxArray.with("First", "Second", "Third"))

  // Empty list:
  listWidgetOf<String>()
}

Adding tooltips to actors:

import ktx.scene2d.*

val labelWithTooltips = scene2d {
  label("Text") {
    textTooltip(text = "This is a simple text tooltip!") {
      // You can customize tooltip's Label here.
    }
    tooltip {
      label("This is a complex tooltip using a table!").cell(row = true)
      label("It can have multiple widgets and custom layouts.")
    }
  }
}

Creating images with various asset types:

import ktx.scene2d.*

scene2d.table {
  // Extract a named asset from a Skin:
  image("button")

  // Or use the assets directly:
  image(Drawable(...))
  image(NinePatch(...))
  image(TextureRegion(...))
  image(Texture(...))
}

Using the DSL in a custom actor class:

import com.badlogic.gdx.scenes.scene2d.ui.Table
import ktx.scene2d.KTable

// Note the KTable interface:
class MyTable: Table(), KTable {
  init {
    table {
      label("You can use KTX DSL as long as you implement the interface!")
    }
    row()
    textButton("Click me!")
  }
}

/*
Widget interfaces:
- KTable: for actors that extend Table
- KGroup: for basic group actors such as HorizontalGroup
- KTree: for tree-based actors such as Tree
- KWidget: a generic interface that allows to fully customize actor storage
*/

Advanced usage example - extending the ktx-scene2d DSL with a custom widget:

import com.badlogic.gdx.scenes.scene2d.ui.Skin
import com.badlogic.gdx.scenes.scene2d.ui.Table
import ktx.scene2d.*

// Depending on the class hierarchy, your widget should implement
// the following ktx-scene2d interface:
// - Table: ktx.scene2d.KTable
// - Group: ktx.scene2d.KGroup
// - Tree: ktx.scene2d.KTree
// If the actor is not a group and cannot have any children,
// you do not need to implement any interface.

/** Example of a custom widget that extends libGDX Table. */
@Scene2dDsl
class MyCustomWidget(
  styleName: String, skin: Skin
) : Table(skin), KTable {
  // Implement your custom widget here.
  init {
    // Example of widget customization -
    // setting table defaults with a custom style object:
    val style = skin[styleName, MyCustomWidgetStyle::class.java]
    this.pad(style.pad)
  }
}

/** Example of a custom widget style class . */
data class MyCustomWidgetStyle(val pad: Float = 0f)

// Adding a factory method for the custom widget:

/**
 * Adds DSL that creates [MyCustomWidget].
 * @param skin defines style of the widget.
 * @param style name of a [MyCustomWidgetStyle] stored in [skin].
 * @param init customization code.
 */
@Scene2dDsl
inline fun <S> KWidget<S>.myCustomWidget(
  // This ensures a Skin is supplied by default:
  skin: Skin = Scene2DSkin.defaultSkin,
  // This supplies a default style name, making it optional:
  style: String = defaultStyle,
  // This is the lambda that you can use to customize the widget:
  init: MyCustomWidget.(S) -> Unit = {}
): MyCustomWidget = actor(MyCustomWidget(style, skin), init)


// Creating our custom widget with ktx-scene2d DSL:

fun usageExample() {
  scene2d.myCustomWidget {
    // Customize your widget here!
    setFillParent(true)

    // Since we implemented KTable, this actor can have children:
    label("A child of the custom widget!").cell(grow = true)
  }
}

// Note: you can also use experimental contracts to ensure
// that the `init` lambda is executed exactly once.
// See sources of official builders for contract examples.

The example images were made using VisUI skin.

Synergy

Pair this library with ktx-style for type-safe actor styles building and ktx-actors for useful extension methods for Scene2D API. ktx-assets or ktx-assets-async might help with Skin loading and management.

Alternatives

• Creating layouts with Scene2D directly in Kotlin or Java.
• VisUI greatly extends Scene2D API, but lacks first-class Kotlin support.
• LML allows building Scene2D views using HTML-like syntax. It also features a VisUI extension. However, it lacks first-class Kotlin support and the flexibility of a programming language.
• ktx-vis extends ktx-scene2d with type-safe builders of the VisUI widgets.

Additional documentation

• Scene2D article.
• Scene2D UI article.
• Table article.
• Skin article.