Created Presenter.

Author: mvarnagiris

build.gradle

@@ -11,16 +11,18 @@ buildscript {
     }
 }
 
+repositories {
+    mavenCentral()
+    jcenter()
+}
+
 apply plugin: 'kotlin'
 
 dependencies {
     compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
-}
+    compile "io.reactivex:rxkotlin:0.55.0"
 
-repositories {
-    mavenCentral()
-}
-
-sourceSets {
-    main.java.srcDirs += 'src/main/kotlin'
-}
+    testCompile "junit:junit:4.12"
+    testCompile "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
+    testCompile "com.nhaarman:mockito-kotlin:0.5.0"
+}

src/main/kotlin/com/mvcoding/mvp/Presenter.kt

@@ -1,4 +1,47 @@
 package com.mvcoding.mvp
 
-class Presenter {
+import rx.Observable
+import rx.Subscription
+import rx.subscriptions.CompositeSubscription
+
+abstract class Presenter<VIEW : View> {
+    private lateinit var viewSubscriptions: CompositeSubscription
+    private var view: View? = null
+
+    fun attach(view: VIEW) {
+        ensureViewIsNotAttached(view)
+        this.view = view
+        this.viewSubscriptions = CompositeSubscription()
+        onViewAttached(view)
+    }
+
+    fun detach(view: VIEW) {
+        ensureGivenViewIsAttached(view)
+        this.view = null
+        this.viewSubscriptions.unsubscribe()
+        onViewDetached(view)
+    }
+
+    open protected fun onViewAttached(view: VIEW) {
+    }
+
+    open protected fun onViewDetached(view: VIEW) {
+    }
+
+    protected fun unsubscribeOnDetach(subscription: Subscription) {
+        viewSubscriptions.add(subscription)
+    }
+
+    protected fun <T> Observable<T>.subscribeUntilDetached(onNext: (T) -> Unit) = subscribe(onNext).apply { unsubscribeOnDetach(this) }
+
+    private fun ensureViewIsNotAttached(view: VIEW) {
+        if (this.view != null) throw IllegalStateException("Cannot attach $view, because ${this.view} is already attached")
+    }
+
+    private fun ensureGivenViewIsAttached(view: VIEW) {
+        if (this.view == null)
+            throw IllegalStateException("View is already detached.")
+        else if (this.view != view)
+            throw IllegalStateException("Trying to detach different view. We have view: ${this.view}. Trying to detach view: $view")
+    }
 }

src/main/kotlin/com/mvcoding/mvp/View.kt

@@ -0,0 +1,3 @@
+package com.mvcoding.mvp
+
+interface View

src/test/kotlin/com/mvcoding/mvp/PresenterTest.kt

@@ -0,0 +1,63 @@
+package com.mvcoding.mvp
+
+import com.nhaarman.mockito_kotlin.mock
+import com.nhaarman.mockito_kotlin.whenever
+import org.junit.Test
+import rx.Observable
+import rx.Observable.just
+import kotlin.test.assertFalse
+
+class PresenterTest {
+    val view = mock<View>()
+    val presenter: Presenter<View> = object : Presenter<View>() {
+    }
+
+    @Test(expected = IllegalStateException::class)
+    fun throwsIllegalStateExceptionWhenViewIsAlreadyAttached() {
+        presenter.attach(view)
+        presenter.attach(view)
+    }
+
+    @Test(expected = IllegalStateException::class)
+    fun throwsIllegalStateExceptionWhenViewWasNotAttached() {
+        presenter.detach(view)
+    }
+
+    @Test(expected = IllegalStateException::class)
+    fun throwsIllegalStateExceptionWhenViewWasAlreadyDetached() {
+        presenter.attach(view)
+        presenter.detach(view)
+        presenter.detach(view)
+    }
+
+    @Test(expected = IllegalStateException::class)
+    fun throwsIllegalStateExceptionWhenTryingToDetachDifferentView() {
+        presenter.attach(view)
+        presenter.detach(mock<View>())
+    }
+
+    @Test
+    fun subscribeUntilDetachUnsubscribesAfterDetach() {
+        val presenter = PresenterForTest()
+        val view = mock<ViewForTest>()
+        var isSubscribed = false
+        val events = just(Unit).doOnSubscribe { isSubscribed = true }.doOnUnsubscribe { isSubscribed = false }
+        whenever(view.events()).thenReturn(events)
+
+        presenter.attach(view)
+        presenter.detach(view)
+
+        assertFalse { isSubscribed }
+    }
+
+    interface ViewForTest : View {
+        fun events(): Observable<Unit>
+    }
+
+    class PresenterForTest : Presenter<ViewForTest>() {
+        override fun onViewAttached(view: ViewForTest) {
+            super.onViewAttached(view)
+            view.events().subscribeUntilDetached { }
+        }
+    }
+}