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Karibu-DSL, Kotlin extensions/DSL for Vaadin

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package com.github.mvysny.karibudsl.v8

import com.vaadin.ui.Component
import com.vaadin.ui.HasComponents
import java.util.*

/**
 * Given a tree root and a closure which computes children, iterates recursively over a tree of objects.
 * Iterator works breadth-first: initially the root itself is offered, then its children, then the child's children etc.
 * @property root the root object of the tree
 * @property children for given node, returns a list of its children.
 */
public class TreeIterator(private val root: T, private val children: (T) -> Iterable) : Iterator {
    /**
     * The items to iterate over. Gradually filled with children, until there are no more items to iterate over.
     */
    private val queue: Queue = LinkedList(listOf(root))

    override fun hasNext(): Boolean = !queue.isEmpty()

    override fun next(): T {
        if (!hasNext()) throw NoSuchElementException()
        val result = queue.remove()
        queue.addAll(children(result))
        return result
    }
}

/**
 * Walks over this component and all descendants of this component, breadth-first.
 * @return iterable which iteratively walks over this component and all of its descendants.
 */
public fun (@VaadinDsl Component).walk(): Iterable = Iterable {
    TreeIterator(this) { component -> component as? HasComponents ?: listOf() }
}