import android.os.Handler;
import android.os.HandlerThread;

import androidx.annotation.Nullable;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 一个在设计在不同线程中运行，异步通知数据的类
 *
 * @param <Param>  运行参数类型
 * @param <Result> 返回结果类型
 */
public abstract class Promise<Param, Result>
{
    // 初始化
    private static final int STATUS_INIT = 0;
    // 正在请求
    private static final int STATUS_REQUESTING = 1;
    // 请求结束
    private static final int STATUS_FINISH = 2;
    // 当前线程
    private final AtomicInteger status = new AtomicInteger(0);
    // 进行超时返回设计，所使用的线程池
    private final static ExecutorService executor = Executors.newCachedThreadPool();
    // 专门Promise使用的，子线程
    private final static HandlerThread thread = new HandlerThread("Promise-Handler");
    // 使用阻塞获取所用到的BlockingQueue
    private final LinkedBlockingQueue<Result> queue = new LinkedBlockingQueue<>();
    // 代码执行所在的Handler
    private Handler handler;
    // 所需要的参数
    private final Param[] params;
    // 默认的超时返回时间
    private long time_out = 7_000;

    @SafeVarargs
    public Promise(Param... params)
    {
        this(null, params);
    }

    @SafeVarargs
    public Promise(Handler handler, Param... params)
    {
        this.params = params;
        if (handler == null)
        {
            if (!thread.isAlive())
            {
                thread.start();
            }
            this.handler = new Handler(thread.getLooper());
        } else
        {
            this.handler = handler;
        }
    }

    /**
     * 设置超时时间
     *
     * @param time_out 超时时间
     */
    public void setTimeOut(long time_out)
    {
        this.time_out = time_out;
    }

    /**
     * 设置Handler
     *
     * @param handler 设置run函数运行在哪个Handler里
     */
    public void setHandler(Handler handler)
    {
        this.handler = handler;
    }

    /**
     * 在handler中执行的run函数
     *
     * @param params run所需要的参数
     */
    protected abstract void run(Param... params);

    /**
     * 发射
     *
     * @param result 等待那边需要的数据
     */
    public void emit(@Nullable Result result)
    {
        queue.offer(result);
    }

    /**
     * 获取数据，使用默认的超时时间，默认值为 null
     *
     * @return 获取的数据
     */
    public Result get()
    {
        return this.get(this.time_out, null);
    }

    /**
     * 获取数据，超出超时时间，则获取默认值
     *
     * @param time_out 超时时间
     * @param def      默认值
     * @return 获取值
     */
    public Result get(long time_out, Result def)
    {
        if (Thread.currentThread() == handler.getLooper().getThread())
        {
            throw new RuntimeException("当前线程，和handler所在的线程不能是同一个线程，因为会阻塞当前线程，进行死锁。");
        }
        if (status.compareAndSet(STATUS_INIT, STATUS_REQUESTING))
        {
            handler.post(() -> this.run(params));
        }

        if (status.compareAndSet(STATUS_FINISH, STATUS_FINISH))
        {
            return def;
        }

        Callable<Result> callable = queue::take;
        FutureTask<Result> futureTask = new FutureTask<>(callable);
        executor.submit(futureTask);
        try
        {
            return futureTask.get(time_out, TimeUnit.MILLISECONDS);
        } catch (Exception e)
        {
            return def;
        } finally
        {
            status.set(STATUS_FINISH);
        }
    }
}

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