emrul · February 26, 2021 23:25 · Feb 26, 2021
diff --git a/ComponentBag.kt b/ComponentBag.kt
@@ -0,0 +1,46 @@
+package nl.avwie
+
+interface Tag<T>
+
+interface Tagged<T> {
+    val tag: Tag<T>
+}
+
+class ComponentBag {
+    private val components = mutableMapOf<Tag<*>, Any>()
+
+    fun <T : Tagged<T>> set(component: T) {
+        components[component.tag] = component
+    }
+
+    @Suppress("UNCHECKED_CAST") // but we are bloody sure the tag is correct
+    operator fun <T> get(tag: Tag<T>): T? = components[tag] as? T
+}
+
+data class Name(val value: String): Tagged<Name> {
+    override val tag: Tag<Name> = Name // <- this is the companion object
+    companion object : Tag<Name>
+}
+
+data class Age(val value: Int): Tagged<Age> {
+    override val tag: Tag<Age> = Age
+    companion object : Tag<Age>
+}
+
+data class Health(val value: Int): Tagged<Health> {
+    override val tag: Tag<Health> = Health
+    companion object : Tag<Health>
+}
+
+fun main() {
+    val player = ComponentBag()
+    player.set(Name("Beeblebrox"))
+    player.set(Age(42))
+    player.set(Health(1000))
+
+    // typesafe fetch
+    val age = player[Age] // this is automatically if type Age?
+    val health = player[Health] // this is automatically of type Health?
+    require(age?.value == 42)
+    require(health?.value == 1000)
+}
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