LouisCAD · January 21, 2024 07:45 · May 1, 2020 · May 1, 2020
diff --git a/companion object Powers example.kt → Companion object Powers example.kt b/companion object Powers example.kt → Companion object Powers example.kt
diff --git a/SomInterfaceOtherImpl.kt b/SomInterfaceOtherImpl.kt
@@ -0,0 +1,30 @@
+package i_love_companion_objects
+
+import kotlinx.coroutines.*
+
+// Extension function on the companion object, easy to discover. Can be suspending.
+suspend fun SomeInterface.Companion.createWithDefaults(): SomeInterface {
+    return SomeInterfaceOtherImpl(
+        someParameter = pretendToDoSomeWorkAndReturnIt()
+    )
+}
+
+// Looks like a constructor, but suspends. Could take any parameters, can have overloads.
+suspend operator fun SomeInterface.Companion.invoke(): SomeInterface {
+    return createWithDefaults()
+}
+
+
+private suspend fun pretendToDoSomeWorkAndReturnIt(): Any {
+    return delay(timeMillis = 10)
+}
+
+// Implementation is hidden, no need to guess its name.
+private class SomeInterfaceOtherImpl(
+    private val someParameter: Any
+) : SomeInterface {
+
+    override suspend fun someFunction() {
+        println("FYI: someFunction called in $javaClass having someParameter set to $someParameter")
+    }
+}
diff --git a/SomeInterface.kt b/SomeInterface.kt
@@ -0,0 +1,10 @@
+package i_love_companion_objects
+
+interface SomeInterface {
+    companion object; // Semicolon needed to avoid next word ("suspend" in this case)
+    // being picked up as the companion object name.
+    // BTW, here, we keep the default one, which is "Companion".
+
+    // This is a random function, we don't really care about it, the focus is companion objects.
+    suspend fun someFunction()
+}
diff --git a/SomeInterfaceImpl.kt b/SomeInterfaceImpl.kt
@@ -0,0 +1,22 @@
+package i_love_companion_objects
+
+// Extension function on the companion object, easy to discover. Can use any name.
+fun SomeInterface.Companion.create(
+    parameterOne: Any,
+    parameterTwo: Any
+): SomeInterface {
+    return SomeInterfaceImpl(
+        parameterOne = parameterOne,
+        parameterTwo = parameterTwo
+    )
+}
+
+private class SomeInterfaceImpl(
+    private val parameterOne: Any,
+    private val parameterTwo: Any
+) : SomeInterface {
+
+    override suspend fun someFunction() {
+        println("FYI: someFunction called in $javaClass with some parameters :)")
+    }
+}
diff --git a/companion object Powers example.kt b/companion object Powers example.kt
@@ -0,0 +1,23 @@
+package i_love_companion_objects
+
+suspend fun main() {
+    // Discoverability is easy with the companion object
+    // (although autocomplete could use some performance improvements...)
+    doStuff(SomeInterface.createWithDefaults())
+    val someInstance: SomeInterface = SomeInterface.create(
+        parameterOne = 0,
+        parameterTwo = 1
+    )
+    doStuff(someInstance)
+    doStuff(SomeInterface.appInstance)
+}
+
+// This is a random function that would need an instance of SomeInterface.
+private suspend fun doStuff(someInterface: SomeInterface) {
+    someInterface.someFunction()
+}
+
+// Extension property on the companion object, easy to discover.
+val SomeInterface.Companion.appInstance: SomeInterface get() = someInterfaceAppInstance
+
+private val someInterfaceAppInstance = SomeInterface.create(0, 0)