Cacao: Add layout guides

src/main/kotlin/net/shadowfacts/cacao/LayoutVariable.kt

@@ -1,5 +1,6 @@
 package net.shadowfacts.cacao
 
+import net.shadowfacts.cacao.util.LayoutGuide
 import net.shadowfacts.cacao.view.View
 import no.birkett.kiwi.Variable
 
@@ -9,10 +10,23 @@ import no.birkett.kiwi.Variable
  *
  * @author shadowfacts
  */
-class LayoutVariable(val owner: View, val property: String): Variable("LayoutVariable") {
+class LayoutVariable(
+	val view: View?,
+	val layoutGuide: LayoutGuide?,
+	val property: String,
+): Variable("LayoutVariable") {
 
-	override fun getName() = "$owner.$property"
+	constructor(view: View, property: String): this(view, null, property)
+	constructor(layoutGuide: LayoutGuide, property: String): this(null, layoutGuide, property)
+
+	init {
+		if ((view == null) == (layoutGuide == null)) {
+			throw RuntimeException("LayoutVariable must be constructed with either a view or layout guide")
+		}
+	}
+
+	override fun getName() = "${view ?: layoutGuide}.$property"
 
 	override fun toString() = "LayoutVariable(name=$name, value=$value)"
 
-}
+}

src/main/kotlin/net/shadowfacts/cacao/util/LayoutGuide.kt

@@ -0,0 +1,33 @@
+package net.shadowfacts.cacao.util
+
+import net.shadowfacts.cacao.LayoutVariable
+import net.shadowfacts.cacao.geometry.Rect
+import net.shadowfacts.cacao.view.View
+
+/**
+ * A layout guide is a non-view object that represents a rectangular area for the purposes of constraint-based layout.
+ * It allows you to define complex layouts without needing empty container views.
+ *
+ * A layout guide is always owned by a view. The owning view's dimensions are not necessarily tied to the layout guide's.
+ *
+ * To create a layout guide, call [View.addLayoutGuide] on the owning view.
+ *
+ * @author shadowfacts
+ */
+class LayoutGuide(
+	val owningView: View,
+) {
+
+	val leftAnchor: LayoutVariable = LayoutVariable(this, "left")
+	val rightAnchor: LayoutVariable = LayoutVariable(this, "right")
+	val topAnchor: LayoutVariable = LayoutVariable(this, "top")
+	val bottomAnchor: LayoutVariable = LayoutVariable(this, "bottom")
+	val widthAnchor: LayoutVariable = LayoutVariable(this, "width")
+	val heightAnchor: LayoutVariable = LayoutVariable(this, "height")
+	val centerXAnchor: LayoutVariable = LayoutVariable(this, "centerX")
+	val centerYAnchor: LayoutVariable = LayoutVariable(this, "centerY")
+
+	val frame: Rect
+		get() = Rect(leftAnchor.value - owningView.leftAnchor.value, topAnchor.value - owningView.topAnchor.value, widthAnchor.value, heightAnchor.value)
+
+}

src/main/kotlin/net/shadowfacts/cacao/view/View.kt

@@ -84,6 +84,17 @@ open class View(): Responder {
 	 */
 	val centerYAnchor = LayoutVariable(this, "centerY")
 
+	private val _layoutGuides = LinkedList<LayoutGuide>()
+
+	/**
+	 * All the layout guides attached to this view.
+	 *
+	 * To add a layout guide, call [addLayoutGuide].
+	 *
+	 * @see LayoutGuide
+	 */
+	val layoutGuides: List<LayoutGuide> = _layoutGuides
+
 	/**
 	 * Whether this view uses constraint-based layout.
 	 * If `false`, the view's `frame` must be set manually and the layout anchors may not be used.
@@ -229,7 +240,7 @@ open class View(): Responder {
 				for (b in a + 1 until variables.size) {
 					// if the variable views have no common ancestor after the removed view's superview is unset,
 					// the constraint crossed the this<->view boundary and should be removed
-					val ancestor = LowestCommonAncestor.find(variables[a].owner, variables[b].owner, View::superview)
+					val ancestor = LowestCommonAncestor.find(variables[a].viewOrLayoutGuideView, variables[b].viewOrLayoutGuideView, View::superview)
 					if (ancestor == null) {
 						return@filter true
 					}
@@ -247,6 +258,18 @@ open class View(): Responder {
 //		view.wasRemoved()
 	}
 
+	/**
+	 * Creates and returns a new layout guide with this view as its owner.
+	 */
+	fun addLayoutGuide(): LayoutGuide {
+		val guide = LayoutGuide(this)
+		_layoutGuides.add(guide)
+		if (hasSolver) {
+			guide.attachTo(solver)
+		}
+		return guide
+	}
+
 	/**
 	 * Removes this view from its superview, if it has one.
 	 */
@@ -283,6 +306,10 @@ open class View(): Responder {
 	open fun wasAdded() {
 		createInternalConstraints()
 		updateIntrinsicContentSizeConstraints(null, intrinsicContentSize)
+
+		layoutGuides.forEach {
+			it.attachTo(solver)
+		}
 	}
 
 	/**
@@ -463,3 +490,15 @@ open class View(): Responder {
 	}
 
 }
+
+private fun LayoutGuide.attachTo(solver: Solver) {
+	solver.dsl {
+		rightAnchor equalTo (leftAnchor + widthAnchor)
+		bottomAnchor equalTo (topAnchor + heightAnchor)
+		centerXAnchor equalTo (leftAnchor + widthAnchor / 2)
+		centerYAnchor equalTo (topAnchor + heightAnchor / 2)
+	}
+}
+
+private val LayoutVariable.viewOrLayoutGuideView: View
+	get() = view ?: layoutGuide!!.owningView