Lazily load Dispatchers.Main and provide a stub impl on failure

Also specified explicit public visibility for Dispatchers object.

Fixes #658
Fixes #665

Author: elizarov

common/kotlinx-coroutines-core-common/src/Dispatchers.common.kt

@@ -9,7 +9,7 @@ import kotlin.coroutines.experimental.*
 /**
  * Groups various implementations of [CoroutineDispatcher].
  */
-expect object Dispatchers {
+public expect object Dispatchers {
     /**
      * The default [CoroutineDispatcher] that is used by all standard builders like
      * [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc

core/kotlinx-coroutines-core/src/Dispatchers.kt

@@ -19,15 +19,7 @@ public const val IO_PARALLELISM_PROPERTY_NAME = "kotlinx.coroutines.io.paralleli
 /**
  * Groups various implementations of [CoroutineDispatcher].
  */
-actual object Dispatchers {
-
-    private val mainDispatcher = loadMainDispatcher()
-
-    private fun loadMainDispatcher(): MainCoroutineDispatcher? {
-        return MainDispatcherFactory::class.java.let { clz ->
-            ServiceLoader.load(clz, clz.classLoader).toList()
-        }.maxBy { it.loadPriority }?.createDispatcher()
-    }
+public actual object Dispatchers {
 
     /**
      * The default [CoroutineDispatcher] that is used by all standard builders like
@@ -59,8 +51,7 @@ actual object Dispatchers {
      * Implementation note: [MainCoroutineDispatcher.immediate] is not supported on Native and JS platforms.
      */
     @JvmStatic
-    public actual val Main: MainCoroutineDispatcher get() = mainDispatcher ?: error("Module with Main dispatcher is missing. " +
-            "Add dependency with required Main dispatcher, e.g. 'kotlinx-coroutines-android'")
+    public actual val Main: MainCoroutineDispatcher get() = MainDispatcherLoader.dispatcher
 
     /**
      * A coroutine dispatcher that is not confined to any specific thread.
@@ -97,3 +88,47 @@ actual object Dispatchers {
     @JvmStatic
     public val IO: CoroutineDispatcher = DefaultScheduler.IO
 }
+
+// Lazy loader for the main dispatcher
+private object MainDispatcherLoader {
+    @JvmField
+    val dispatcher: MainCoroutineDispatcher =
+        MainDispatcherFactory::class.java.let { clz ->
+            ServiceLoader.load(clz, clz.classLoader).toList()
+        }.maxBy { it.loadPriority }?.tryCreateDispatcher() ?: MissingMainCoroutineDispatcher(null)
+
+    /**
+     * If anything goes wrong while trying to create main dispatcher (class not found,
+     * initialization failed, etc), then replace the main dispatcher with a special
+     * stub that throws an error message on any attempt to actually use it.
+     */
+    private fun MainDispatcherFactory.tryCreateDispatcher(): MainCoroutineDispatcher =
+        try {
+            createDispatcher()
+        } catch (cause: Throwable) {
+            MissingMainCoroutineDispatcher(cause)
+        }
+}
+
+private class MissingMainCoroutineDispatcher(val cause: Throwable?) : MainCoroutineDispatcher(), Delay {
+    override val immediate: MainCoroutineDispatcher get() = this
+
+    override fun dispatch(context: CoroutineContext, block: Runnable) =
+        missing()
+
+    override fun scheduleResumeAfterDelay(timeMillis: Long, continuation: CancellableContinuation<Unit>) =
+        missing()
+
+    private fun missing() {
+        if  (cause == null) {
+            throw IllegalStateException(
+                "Module with the Main dispatcher is missing. " +
+                    "Add dependency providing the Main dispatcher, e.g. 'kotlinx-coroutines-android'"
+            )
+        } else {
+            throw IllegalStateException("Module with the Main dispatcher had failed to initialize", cause)
+        }
+    }
+
+    override fun toString(): String = "Main[missing${if (cause != null) ", cause=$cause" else ""}]"
+}

js/kotlinx-coroutines-core-js/src/Dispatchers.kt

@@ -6,7 +6,7 @@ package kotlinx.coroutines.experimental
 
 import kotlin.coroutines.experimental.*
 
-actual object Dispatchers {
+public actual object Dispatchers {
 
     public actual val Default: CoroutineDispatcher = createDefaultDispatcher()
 

native/kotlinx-coroutines-core-native/src/Dispatchers.kt

@@ -6,8 +6,7 @@ package kotlinx.coroutines.experimental
 
 import kotlin.coroutines.experimental.*
 
-
-actual object Dispatchers {
+public actual object Dispatchers {
 
     public actual val Default: CoroutineDispatcher = createDefaultDispatcher()