Implement async/await for Android using Kotlin Coroutines

app/build.gradle

@@ -1,5 +1,21 @@
 apply plugin: 'com.android.application'
 apply plugin: 'kotlin-android'
+apply plugin: 'kotlin-android-extensions'
+apply plugin: 'com.jakewharton.hugo'
+
+buildscript {
+    repositories {
+        jcenter()
+        maven {
+            url  "http://dl.bintray.com/kotlin/kotlin-eap-1.1"
+        }
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:2.1.2'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+        classpath 'com.jakewharton.hugo:hugo-plugin:1.2.1'
+    }
+}
 
 android {
     compileSdkVersion 24

app/src/main/kotlin/coroutineandroid/AsyncUi.kt

@@ -0,0 +1,40 @@
+package coroutineandroid
+
+import android.os.Handler
+import android.os.Looper
+import kotlin.coroutines.Continuation
+
+fun asyncUi(coroutine c: AsyncController.() -> Continuation<Unit>): AsyncController {
+    val controller = AsyncController()
+    // TODO If not in UI thread - force run resume() in UI thread
+    controller.c().resume(Unit)
+    return controller
+}
+
+class AsyncController {
+    private var errorHandler: ErrorHandler? = null
+    private val uiHandler = Handler(Looper.getMainLooper())
+
+    suspend fun <V> await(f: () -> V, machine: Continuation<V>) {
+        Thread {
+            try {
+                val value = f()
+                uiHandler.post {
+                    machine.resume(value)
+                }
+            } catch (e: Exception) {
+                uiHandler.post {
+                    errorHandler?.invoke(e) ?: machine.resumeWithException(e)
+                }
+            }
+
+        }.start()
+
+    }
+
+    fun onError(errorHandler: ErrorHandler) {
+        this.errorHandler = errorHandler
+    }
+}
+
+typealias ErrorHandler = (Exception) -> Unit

app/src/main/kotlin/coroutineandroid/MainActivity.kt

@@ -2,12 +2,72 @@ package coroutineandroid
 
 import android.support.v7.app.AppCompatActivity
 import android.os.Bundle
+import android.view.View
 import com.example.coroutineandroid.R
+import hugo.weaving.DebugLog
+import kotlinx.android.synthetic.main.activity_main.*
 
 class MainActivity : AppCompatActivity() {
 
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         setContentView(R.layout.activity_main)
+        button.setOnClickListener {
+            //startCoroutine()
+            startCoroutineUsingMoreConvenientErrorHandling()
+        }
     }
+
+    @DebugLog
+    private fun startCoroutine() {
+        asyncUi {
+            progress.visibility = View.VISIBLE
+            text.text = "Loading..."
+            try {
+                // Release main thread and wait until text loaded
+                // Progress animation shown during loading
+                val loadedText = await(::loadText)
+                // Loaded successfully, come back in UI thread and show result
+                text.text = loadedText + " (to be processed)"
+                // Oh ah we need to run more processing in background
+                text.text = await { processText(loadedText) }
+            } catch (e: Exception) {
+                // Exception could be thrown in UI or background thread
+                // but handled in UI thread
+                text.text = e.message
+            }
+            progress.visibility = View.INVISIBLE
+        }
+    }
+
+    @DebugLog
+    private fun startCoroutineUsingMoreConvenientErrorHandling() {
+        asyncUi {
+            progress.visibility = View.VISIBLE
+            text.text = "Loading..."
+            // Release main thread and wait until text loaded
+            // Progress animation shown during loading
+            val loadedText = await(::loadText)
+            // Loaded successfully, come back in UI thread and show result
+            text.text = loadedText + " (to be processed)"
+            // Oh ah we need to run more processing in background
+            text.text = await { processText(loadedText) }
+            progress.visibility = View.INVISIBLE
+        }.onError {
+            text.text = it.message
+            progress.visibility = View.INVISIBLE
+        }
+    }
+
+}
+@DebugLog
+private fun loadText(): String {
+    Thread.sleep(1000)
+    //if (1 == 1) throw RuntimeException("You are in the wrong place")
+    return "Loaded Text"
 }
+@DebugLog
+private fun processText(input: String): String {
+    Thread.sleep(2000)
+    return "Processed $input"
+}

app/src/main/res/layout/activity_main.xml

@@ -10,8 +10,30 @@
     android:paddingTop="@dimen/activity_vertical_margin"
     tools:context="coroutineandroid.MainActivity">
 
+    <ProgressBar
+        android:id="@+id/progress"
+        style="@style/Widget.AppCompat.ProgressBar.Horizontal"
+        android:indeterminate="true"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:visibility="invisible"/>
+
     <TextView
+        android:id="@+id/text"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_below="@id/progress"
+        android:layout_centerHorizontal="true"
+        android:layout_marginTop="16dp"
+        android:textSize="18sp"
+        tools:text="Bla Bla"/>
+
+    <Button
+        android:id="@+id/button"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
-        android:text="Hello World!"/>
+        android:layout_below="@id/text"
+        android:layout_centerHorizontal="true"
+        android:layout_marginTop="16dp"
+        android:text="Start"/>
 </RelativeLayout>