ModifierNodeElement

abstract class ModifierNodeElement<N : Modifier.Node> : Modifier.Element, InspectableValue

A Modifier.Element which manages an instance of a particular Modifier.Node implementation. A given Modifier.Node implementation can only be used when a ModifierNodeElement which creates and updates that implementation is applied to a Layout.

A ModifierNodeElement should be very lightweight, and do little more than hold the information necessary to create and maintain an instance of the associated Modifier.Node type.

Functions

abstract fun create(): N

This will be called the first time the modifier is applied to the Layout and it should construct and return the corresponding Modifier.Node instance.

abstract fun update(node: N)

Called when a modifier is applied to a Layout whose inputs have changed from the previous application. This function will have the current node instance passed in as a parameter, and it is expected that the node will be brought up to date.

open fun InspectorInfo.inspectableProperties()

Populates an InspectorInfo object with attributes to display in the layout inspector. This is called by tooling to resolve the properties of this modifier. By convention, implementors should set the name to the function name of the modifier.

The default implementation will attempt to reflectively populate the inspector info with the properties declared on the subclass. It will also set the name property to the name of this instance's class by default (not the name of the modifier function). Modifier property population depends on the kotlin-reflect library. If it is not in the classpath at runtime, the default implementation of this function will populate the properties with an error message.

If you override this function and provide the properties you wish to display, you do not need to call super. Doing so may result in duplicate properties appearing in the layout inspector.

Code Examples

ModifierNodeElementSample

@Composable
fun ModifierNodeElementSample() {
    class Circle(var color: Color) : DrawModifierNode, Modifier.Node() {
        override fun ContentDrawScope.draw() {
            drawCircle(color)
        }
    }
    data class CircleElement(val color: Color) : ModifierNodeElement<Circle>() {
        override fun create() = Circle(color)
        override fun update(node: Circle) {
            node.color = color
        }
        override fun InspectorInfo.inspectableProperties() {
            name = "circle"
            properties["color"] = color
        }
    }
    fun Modifier.circle(color: Color) = this then CircleElement(color)
}

SemanticsModifierNodeSample

@Suppress("LocalVariableName")
@Composable
fun SemanticsModifierNodeSample() {
    class HeadingNode : SemanticsModifierNode, Modifier.Node() {
        override fun SemanticsPropertyReceiver.applySemantics() {
            heading()
        }
    }
    val HeadingElement =
        object : ModifierNodeElement<HeadingNode>() {
            override fun create() = HeadingNode()
            override fun update(node: HeadingNode) {
                // Nothing to update.
            }
            override fun InspectorInfo.inspectableProperties() {
                name = "heading"
            }
            override fun hashCode(): Int = "heading".hashCode()
            override fun equals(other: Any?) = (other === this)
        }
    fun Modifier.heading() = this then HeadingElement
}