system.reactive - 如何观察一组项目何时都有效？

Question

我正在使用 ReactiveUI 和提供的 ReactiveCollection<>类。

在 ViewModel 中，我有一个对象集合，我希望创建一个可观察对象来监视这些项目的 IsValid 属性。

这是我要解决的场景。在我的 ViewModel 的构造函数中。

this.Items = new ReactiveCollection<object>();

IObservable<bool> someObservable = // ... how do I watch Items so when 
                                   // any items IsValid property changes, 
                                   // this observable changes. There
                                   // is an IValidItem interface.

this.TheCommand = new ReactiveCommand(someObservable);

...

interface IValidItem { bool IsValid { get; } }

编辑 Ana 的回答让我明白了大部分内容。解决方案如下。

this.Items = new ReactiveCollection<object>();
this.Items.ChangeTrackingEnabled = true;

var someObservable = this.Items.Changed
    .Select(_ => this.Items.All(i => i.IsValid));

Answer 1

这取决于您想对 IsValid 的结果做什么。以下是我的做法，尽管它并不完全直观:

// Create a derived collection which are all the IsValid properties. We don't
// really care which ones are valid, rather that they're *all* valid
var isValidList = allOfTheItems.CreateDerivedCollection(x => x.IsValid);

// Whenever the collection changes in any way, check the array to see if all of
// the items are valid. We could probably do this more efficiently but it gets
// Tricky™
IObservable<bool> areAllItemsValid = isValidList.Changed.Select(_ => isValidList.All());

theCommand = new ReactiveCommand(areAllItemsValid);