AbstractApplier

Kotlin |Java

abstract class AbstractApplier<T> : Applier<T>

kotlin.Any
↳	androidx.compose.runtime.AbstractApplier

Known Direct Subclasses VectorApplier VectorApplier

An abstract Applier implementation.

import androidx.compose.runtime.Composition
import androidx.compose.runtime.ComposeNode
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember

// Provided we have a tree with a node base type like the following
abstract class Node {
    val children = mutableListOf<Node>()
}

// We would implement an Applier class like the following, which would teach compose how to
// manage a tree of Nodes.
class NodeApplier(root: Node) : AbstractApplier<Node>(root) {
    override fun insertTopDown(index: Int, instance: Node) {
        current.children.add(index, instance)
    }

    override fun insertBottomUp(index: Int, instance: Node) {
        // Ignored as the tree is built top-down.
    }

    override fun remove(index: Int, count: Int) {
        current.children.remove(index, count)
    }

    override fun move(from: Int, to: Int, count: Int) {
        current.children.move(from, to, count)
    }

    override fun onClear() {
        root.children.clear()
    }
}

// A function like the following could be created to create a composition provided a root Node.
fun Node.setContent(
    parent: CompositionContext,
    content: @Composable () -> Unit
): Composition {
    return Composition(NodeApplier(this), parent).apply {
        setContent(content)
    }
}

// assuming we have Node sub-classes like "TextNode" and "GroupNode"
class TextNode : Node() {
    var text: String = ""
    var onClick: () -> Unit = {}
}
class GroupNode : Node()

// Composable equivalents could be created
@Composable fun Text(text: String, onClick: () -> Unit = {}) {
    ComposeNode<TextNode, NodeApplier>(::TextNode) {
        set(text) { this.text = it }
        set(onClick) { this.onClick = it }
    }
}

@Composable fun Group(content: @Composable () -> Unit) {
    ComposeNode<GroupNode, NodeApplier>(::GroupNode, {}, content)
}

// and then a sample tree could be composed:
fun runApp(root: GroupNode, parent: CompositionContext) {
    root.setContent(parent) {
        var count by remember { mutableStateOf(0) }
        Group {
            Text("Count: $count")
            Text("Increment") { count++ }
        }
    }
}

Summary

Public constructors
<init>(root: T) An abstract Applier implementation.

Public methods
Unit	clear() Move to the root and remove all nodes from the root, preparing both this Applier and its root to be used as the target of a new composition in the future.
open Unit	down(node: T) Indicates that the applier is getting traversed "down" the tree.
open Unit	up() Indicates that the applier is getting traversed "up" the tree.

Protected methods
Unit	MutableList<T>.move(from: Int, to: Int, count: Int)
abstract Unit	onClear() Called to perform clearing of the root when clear is called.
Unit	MutableList<T>.remove(index: Int, count: Int)

Inherited functions

From class Applier Unit insertBottomUp(index: Int, instance: N) Indicates that instance should be inserted as a child of current at index. An applier should insert the node into the tree either in insertTopDown or insertBottomUp, not both. See the description of insertTopDown to which describes when to implement insertTopDown and when to use insertBottomUp. Unit insertTopDown(index: Int, instance: N) Indicates that instance should be inserted as a child to current at index. An applier should insert the node into the tree either in insertTopDown or insertBottomUp, not both. The insertTopDown method is called before the children of instance have been created and inserted into it. insertBottomUp is called after all children have been created and inserted. Some trees are faster to build top-down, in which case the insertTopDown method should be used to insert the instance. Other tress are faster to build bottom-up in which case insertBottomUp should be used. To give example of building a tree top-down vs. bottom-up consider the following tree, R | B / \ A C where the node B is being inserted into the tree at R. Top-down building of the tree first inserts B into R, then inserts A into B followed by inserting C into B`. For example, 1 2 3 R R R | | | B B B / / \ A A C A bottom-up building of the tree starts with inserting A and C into B then inserts B tree into R. 1 2 3 B B R | / \ | A A C B / \ A C To see how building top-down vs. bottom-up can differ significantly in performance consider a tree where whenever a child is added to the tree all parent nodes, up to the root, are notified of the new child entering the tree. If the tree is built top-down, R is notified of B entering. B is notified of A entering, R is notified of A entering. B is notified of C entering, R is notified of C entering. for a total of 5 notifications. The number of notifications grows exponentially with the number of inserts. For bottom-up, the notifications are, B is notified A entering. B is notified C entering. R is notified B entering. The notifications are linear to the number of nodes inserted. If, on the other hand, all children are notified when the parent enters a tree, then the notifications are, for top-down, B is notified it is entering R. A is notified it is entering B. C is notified it is entering B. which is linear to the number of nodes inserted. For bottom-up, the notifications look like, A is notified it is entering B. C is notified it is entering B. B is notified it is entering R, A is notified it is entering R, C is notified it is entering R. which exponential to the number of nodes inserted. Unit move(from: Int, to: Int,