如何在完成之前取消定时器

Question

我正在开发一款聊天应用.在加载聊天消息并且消息可见5秒后,我想向服务器发送读取确认.这是我到目前为止所提出的:

public async void RefreshLocalData()
{
    // some async code to load the messages

    if (_selectedChat.countNewMessages > 0)
    {
        Device.StartTimer(TimeSpan.FromSeconds(5), SendReadConfirmation);
    }
}

当RefreshLocalData()被叫时,我知道用户选择了另一个聊天,或者当前聊天中有新消息.因此,当RefreshLocalData()被调用时,我必须取消当前的计时器以启动一个新计时器.

另一种情况,当我导航到另一个时,我必须取消计时器Page.这没有问题,因为ViewModel当发生这种情况时整个都被处理掉了.

使用上面的代码,如果RefreshLocalData()再次调用if 但是TimeSpan5秒的声明尚未结束,则该方法仍在执行.

有没有办法取消计时器(如果RefreshLocalData()再次调用)？

Answer 1

我在Xamarin论坛中找到了这个答案:https://forums.xamarin.com/discussion/comment/149877/#Comment_149877

我已经改变了一点以满足我的需求,这个解决方案正在起作用:

public class StoppableTimer
{
    private readonly TimeSpan timespan;
    private readonly Action callback;

    private CancellationTokenSource cancellation;

    public StoppableTimer(TimeSpan timespan, Action callback)
    {
        this.timespan = timespan;
        this.callback = callback;
        this.cancellation = new CancellationTokenSource();
    }

    public void Start()
    {
        CancellationTokenSource cts = this.cancellation; // safe copy
        Device.StartTimer(this.timespan,
            () => {
                if (cts.IsCancellationRequested) return false;
                this.callback.Invoke();
                return false; // or true for periodic behavior
        });
    }

    public void Stop()
    {
        Interlocked.Exchange(ref this.cancellation, new CancellationTokenSource()).Cancel();
    }

    public void Dispose()
    {

    }
}

这就是我在RefreshLocalData()方法中使用它的方式:

private StoppableTimer stoppableTimer;

public async void RefreshLocalData()
{
    if (stoppableTimer != null)
    {
        stoppableTimer.Stop();
    }

    // some async code to load the messages

    if (_selectedChat.countNewMessages > 0)
    {
        if (stoppableTimer == null)
        {
            stoppableTimer = new StoppableTimer(TimeSpan.FromSeconds(5), SendReadConfirmation);
            stoppableTimer.Start();
        }
        else
        {
            stoppableTimer.Start();
        }
    }
}