kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/CoroutineDispatcher.kt

/*
 * Copyright 2016-2017 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package kotlinx.coroutines.experimental

import kotlin.coroutines.experimental.AbstractCoroutineContextElement
import kotlin.coroutines.experimental.Continuation
import kotlin.coroutines.experimental.ContinuationInterceptor
import kotlin.coroutines.experimental.CoroutineContext

/**
 * Base class that shall be extended by all coroutine dispatcher implementations.
 *
 * The following standard implementations are provided by `kotlinx.coroutines`:
 * * [Unconfined] -- starts coroutine execution in the current call-frame until the first suspension.
 *   On first  suspension the coroutine builder function returns.
 *   The coroutine will resume in whatever thread that is used by the
 *   corresponding suspending function, without confining it to any specific thread or pool.
 *   This in an appropriate choice for IO-intensive coroutines that do not consume CPU resources.
 * * [CommonPool] -- immediately returns from the coroutine builder and schedules coroutine execution to
 *   a common pool of shared background threads.
 *   This is an appropriate choice for compute-intensive coroutines that consume a lot of CPU resources.
 * * Private thread pools can be created with [newSingleThreadContext] and [newFixedThreadPoolContext].
 * * An arbitrary [Executor][java.util.concurrent.Executor] can be converted to dispatcher with [toCoroutineDispatcher] extension function.
 *
 * This class ensures that debugging facilities in [newCoroutineContext] function work properly.
 */
public abstract class CoroutineDispatcher :
        AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
    /**
     * Returns `true` if execution shall be dispatched onto another thread.
     * The default behaviour for most dispatchers is to return `true`.
     *
     * UI dispatchers _should not_ override `isDispatchNeeded`, but leave a default implementation that
     * returns `true`. To understand the rationale beyond this recommendation, consider the following code:
     *
     * ```kotlin
     * fun asyncUpdateUI() = async(MainThread) {
     *     // do something here that updates something in UI
     * }
     * ```
     *
     * When you invoke `asyncUpdateUI` in some background thread, it immediately continues to the next
     * line, while UI update happens asynchronously in the UI thread. However, if you invoke
     * it in the UI thread itself, it updates UI _synchronously_ if your `isDispatchNeeded` is
     * overridden with a thread check. Checking if we are already in the UI thread seems more
     * efficient (and it might indeed save a few CPU cycles), but this subtle and context-sensitive
     * difference in behavior makes the resulting async code harder to debug.
     *
     * Basically, the choice here is between "JS-style" asynchronous approach (async actions
     * are always postponed to be executed later in the even dispatch thread) and "C#-style" approach
     * (async actions are executed in the invoker thread until the first suspension point).
     * While, C# approach seems to be more efficient, it ends up with recommendations like
     * "use `yield` if you need to ....". This is error-prone. JS-style approach is more consistent
     * and does not require programmers to think about whether they need to yield or not.
     */
    public open fun isDispatchNeeded(context: CoroutineContext): Boolean = true

    /**
     * Dispatches execution of a runnable [block] onto another thread in the given [context].
     */
    public abstract fun dispatch(context: CoroutineContext, block: Runnable)

    /**
     * Returns continuation that wraps the original [continuation], thus intercepting all resumptions.
     */
    public override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> =
            DispatchedContinuation(this, continuation)

    /**
     * @suppress **Error**: Operator '+' on two CoroutineDispatcher objects is meaningless.
     * CoroutineDispatcher is a coroutine context element and `+` is a set-sum operator for coroutine contexts.
     * The dispatcher to the right of `+` just replaces the dispatcher the left of `+`.
     */
    @Suppress("DeprecatedCallableAddReplaceWith")
    @Deprecated(message = "Operator '+' on two CoroutineDispatcher objects is meaningless. " +
            "CoroutineDispatcher is a coroutine context element and `+` is a set-sum operator for coroutine contexts. " +
            "The dispatcher to the right of `+` just replaces the dispatcher the left of `+`.",
            level = DeprecationLevel.ERROR)
    public operator fun plus(other: CoroutineDispatcher) = other
}

internal class DispatchedContinuation<in T>(
        val dispatcher: CoroutineDispatcher,
        val continuation: Continuation<T>
): Continuation<T> by continuation {
    override fun resume(value: T) {
        val context = continuation.context
        if (dispatcher.isDispatchNeeded(context))
            dispatcher.dispatch(context, Runnable {
                resumeUndispatched(value)
            })
        else
            resumeUndispatched(value)
    }

    @Suppress("NOTHING_TO_INLINE") // we need it inline to save us an entry on the stack
    inline fun resumeUndispatched(value: T) {
        withCoroutineContext(context) {
            continuation.resume(value)
        }
    }

    override fun resumeWithException(exception: Throwable) {
        val context = continuation.context
        if (dispatcher.isDispatchNeeded(context))
            dispatcher.dispatch(context, Runnable {
                resumeUndispatchedWithException(exception)
            })
        else
            resumeUndispatchedWithException(exception)
    }

    @Suppress("NOTHING_TO_INLINE") // we need it inline to save us an entry on the stack
    inline fun resumeUndispatchedWithException(exception: Throwable) {
        withCoroutineContext(context) {
            continuation.resumeWithException(exception)
        }
    }

    // used by "yield" implementation
    fun resumeYield(job: Job?, value: T) {
        val context = continuation.context
        if (dispatcher.isDispatchNeeded(context))
            dispatcher.dispatch(context, Runnable {
                withCoroutineContext(context) {
                    if (job?.isCompleted == true)
                        continuation.resumeWithException(job.getCompletionException())
                    else
                        continuation.resume(value)
                }
            })
        else
            withCoroutineContext(context) {
                continuation.resume(value)
            }
    }
}