如何从函数中读取IObservable中最近的项目

Question

我在写一个用户界面。在某些操作中，我想知道来自特定IObservable的最新值。我怎么知道呢?

这是一个模拟我想要达到的效果的测试。你能帮忙修吗?

var selectedItemOut = new Subject<int?>();
// idea: define some other IObservable or subject here
var getLatest = new Func<int?>(() =>
{
    // fix me
    return 0;
});
Assert.AreEqual(null, getLatest());
selectedItemOut.OnNext(4);
Assert.AreEqual(4, getLatest());
selectedItemOut.OnNext(5);
Assert.AreEqual(5, getLatest());
selectedItemOut.OnNext(6);
selectedItemOut.OnNext(7);
Assert.AreEqual(7, getLatest());

Answer 1

如果你可以改变你的选择的主题是行为主题像Slugart有，那将是最好的(我会改变First()采取(1)，但这只是我)。如果不能，则需要在函数作用域之外创建一个变量。

var selectedItemOut = new Subject<int?>();
// Hold the last result from the OnNext
int? results = null;
// idea: define some other IObservable or subject here
// update var each time OnNext is called
var lastResult = selectedItemOut.Subscribe(i => results = i);
// just a wrapper around the results var.
var getLatest = new Func<int?>(() => results);
Assert.AreEqual(null, getLatest());
selectedItemOut.OnNext(4);
Assert.AreEqual(4, getLatest());
selectedItemOut.OnNext(5);
Assert.AreEqual(5, getLatest());
selectedItemOut.OnNext(6);
selectedItemOut.OnNext(7);
Assert.AreEqual(7, getLatest());
//Need to tell the observable to stop updating the results var.
lastResultDispose.Dispose();

现在你的函数只是引用结果变量，你需要告诉它何时停止跟踪它。这不是很Rx，因为Rx是关于推入值的，你想从中获取一个值。

Answer 2

如果selectedItemOut可以是BehaviourSubject<int?>，你可以使用它的Value属性:

var selectedItemOut = new BehaviorSubject<int?>(null);
var getLatest = new Func<int?>(() =>
{
    return selectedItemOut.Value;
});

否则CharlesNRice的答案看起来很好，或者你可以使用MostRecent:

var mostRecent = selectedItemOut.MostRecent(null).GetEnumerator();    
var getLatest = new Func<int?>(() =>
{
    mostRecent.MoveNext();
    return mostRecent.Current;
});
// need to clean up this up
mostRecent.Dispose() 

Answer 3

如果您想要一个初始值(即使它是null)，那么最近您可以使用BehaviorSubject与First()。关于上下文的更多信息将有助于找到更好的解决方案，因为不建议使用First()。

var selectedItemOut = new BehaviorSubject<int?>(null);
// idea: define some other IObservable or subject here
Assert.AreEqual(null, selectedItemOut.First());
selectedItemOut.OnNext(4);
Assert.AreEqual(4, selectedItemOut.First());
selectedItemOut.OnNext(5);
Assert.AreEqual(5, selectedItemOut.First());
selectedItemOut.OnNext(6);
selectedItemOut.OnNext(7);
Assert.AreEqual(7, selectedItemOut.First());