得到的IObservable前一个元素，而无需重新评估顺序 [英] Get previous element in IObservable without re-evaluating the sequence

问题描述

在的IObservable 序列（在无扩展为.NET），我想获得以前和当前元素的值，这样我可以比较它们。我在网上找到类似下面这完成任务的一个例子：

In an IObservable sequence (in Reactive Extensions for .NET), I'd like to get the value of the previous and current elements so that I can compare them. I found an example online similar to below which accomplishes the task:

sequence.Zip(sequence.Skip(1), (prev, cur) => new { Previous = prev, Current = cur })

它工作正常，除了它评估序列两次，我想避免的。你可以看到，它正在与此代码两次评估：

It works fine except that it evaluates the sequence twice, which I would like to avoid. You can see that it is being evaluated twice with this code:

var debugSequence = sequence.Do(item => Debug.WriteLine("Retrieved an element from sequence"));
debugSequence.Zip(debugSequence.Skip(1), (prev, cur) => new { Previous = prev, Current = cur }).Subscribe();

输出显示两倍的调试线路，因为有序列中的元素。

The output shows twice as many of the debug lines as there are elements in the sequence.

我明白为什么会这样，但到目前为止，我还没有发现，不计算两次序列的替代品。我怎么可以结合以前和当前只有一个序列的评价？

I understand why this happens, but so far I haven't found an alternative that doesn't evaluate the sequence twice. How can I combine the previous and current with only one sequence evaluation?

推荐答案

有对这个更好的解决方案，我认为，使用Observable.Scan和避免双重订阅：

There's a better solution to this I think, that uses Observable.Scan and avoids the double subscription:

public static IObservable<Tuple<TSource, TSource>>
    PairWithPrevious<TSource>(this IObservable<TSource> source)
{
    return source.Scan(
        Tuple.Create(default(TSource), default(TSource)),
        (acc, current) => Tuple.Create(acc.Item2, current));
}

我写这件事在我的博客在这里：的http://www.zerobugbuild.com/?p=213

进一步的修改，您可以使用任意类型的更清洁利用结果选择工作：

A further modification allows you to work with arbitrary types more cleanly by using a result selector:

public static IObservable<TResult> CombineWithPrevious<TSource,TResult>(
    this IObservable<TSource> source,
    Func<TSource, TSource, TResult> resultSelector)
{
    return source.Scan(
        Tuple.Create(default(TSource), default(TSource)),
        (previous, current) => Tuple.Create(previous.Item2, current))
        .Select(t => resultSelector(t.Item1, t.Item2));
}

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