Add observable value task

Author: MarcinMoskala

src/test/java/functional/ObservableValueTest.kt

@@ -0,0 +1,54 @@
+package functional
+
+import org.junit.Test
+import kotlin.test.assertEquals
+
+class ObservableValue<T>(initial: T) {
+    var value: T = initial
+    
+    fun observe(observer: (T) -> Unit) {
+        TODO()
+    }
+}
+
+class ObservableValueTest {
+
+    @Test
+    fun `should behave like regular value`() {
+        val i = ObservableValue<Int?>(null)
+        assertEquals(null, i.value)
+        i.value = 1
+        assertEquals(1, i.value)
+        i.value = 2
+        assertEquals(2, i.value)
+    }
+
+    @Test
+    fun `should call observer when value changes`() {
+        val o = ObservableValue<String>("A")
+        val called = mutableListOf<String>()
+        o.observe { called += it }
+        o.value = "B"
+        o.value = "C"
+        o.value = "D"
+        assertEquals(listOf("B", "C", "D"), called)
+    }
+
+    @Test
+    fun `should call observers when value changes`() {
+        val o1 = ObservableValue<String>("A")
+        val o2 = ObservableValue<String>("A")
+        val history1 = mutableListOf<String>()
+        o1.observe { history1 += it }
+        o1.value = "B"
+        val history2 = mutableListOf<String>()
+        o1.observe { history2 += it }
+        val history3 = mutableListOf<String>()
+        o2.observe { history3 += it }
+        o1.value = "C"
+        o1.value = "D"
+        assertEquals(listOf("B", "C", "D"), history1)
+        assertEquals(listOf("C", "D"), history2)
+        assertEquals(listOf(), history3)
+    }
+}