Android支持库中的CompletableFuture？

Question

因此,我正在将Android Studio项目迁移到Java 8,Android API级别24和Jack工具链,以检查新功能,尤其是lambdas和CompletableFuture.

不幸的是,CompletableFuture似乎仅在API级别24上可用(我的该项目的最低API级别为16).

你知道有关带到CompletableFutureAndroid支持库的任何计划吗？它似乎是Promises模式的一个很好的解决方案.

Answer 1

该streamsupport项目提供了一个补丁包,CompletableFuture在其streamsupport-cfuture组件可用于Android开发,支持所有设备.

要么使用

dependencies {
    compile 'net.sourceforge.streamsupport:streamsupport-cfuture:1.7.0'
}

或Android Studio 3.x更现代的分支

dependencies {
    compile 'net.sourceforge.streamsupport:android-retrofuture:1.7.0'
}

如果你想使用Android Studio 3.x desugar工具链.

CompletableFuture JDK-8211010的新Java 12异常处理方法已集成在1.7.0版中

Answer 2

streamsupport斯蒂芬·佐贝尔提到的图书馆的答案特别针对Android 3.0 desugar工具链,请检查android-retrofuture

Answer 3

如果您不需要CompletableFuture（例如结果链接）的所有功能，则可以使用此类（Kotlin）：

/**
 * A backport of Java `CompletableFuture` which works with old Androids.
 */
class CompletableFutureCompat<V> : Future<V> {
    private sealed class Result<out V> {
        abstract val value: V
        class Ok<V>(override val value: V) : Result<V>()
        class Error(val e: Throwable) : Result<Nothing>() {
            override val value: Nothing
                get() = throw e
        }
        object Cancel : Result<Nothing>() {
            override val value: Nothing
                get() = throw CancellationException()
        }
    }

    /**
     * Offers the completion result for [result].
     *
     * If this queue is not empty, the future is completed.
     */
    private val completion = LinkedBlockingQueue<Result<V>>(1)
    /**
     * Holds the result of the computation. Takes the item from [completion] upon running and provides it as a result.
     */
    private val result = FutureTask<V> { completion.peek()!!.value }
    /**
     * If not already completed, causes invocations of [get]
     * and related methods to throw the given exception.
     *
     * @param ex the exception
     * @return `true` if this invocation caused this CompletableFuture
     * to transition to a completed state, else `false`
     */
    fun completeExceptionally(ex: Throwable): Boolean {
        val offered = completion.offer(Result.Error(ex))
        if (offered) {
            result.run()
        }
        return offered
    }

    /**
     * If not already completed, completes this CompletableFuture with
     * a [CancellationException].
     *
     * @param mayInterruptIfRunning this value has no effect in this
     * implementation because interrupts are not used to control
     * processing.
     *
     * @return `true` if this task is now cancelled
     */
    override fun cancel(mayInterruptIfRunning: Boolean): Boolean {
        val offered = completion.offer(Result.Cancel)
        if (offered) {
            result.cancel(mayInterruptIfRunning)
        }
        return offered
    }

    /**
     * If not already completed, sets the value returned by [get] and related methods to the given value.
     *
     * @param value the result value
     * @return `true` if this invocation caused this CompletableFuture
     * to transition to a completed state, else `false`
     */
    fun complete(value: V): Boolean {
        val offered = completion.offer(Result.Ok(value))
        if (offered) {
            result.run()
        }
        return offered
    }

    override fun isDone(): Boolean = completion.isNotEmpty()

    override fun get(): V = result.get()

    override fun get(timeout: Long, unit: TimeUnit): V = result.get(timeout, unit)

    override fun isCancelled(): Boolean = completion.peek() == Result.Cancel
}