我有一個 ViewModel：一個列表 ReactiveList<MyObject>單個 MyObjectIsBusy 布爾值當 MyObject 列表或單個 MyObject 中的“Active”屬性都不為真或在任何情況下 IsBusy 為真時，我想禁用該命令。在將 IsBusy 添加到圖片之前，我想出的最佳解決方案是：var canSave = this.WhenAnyObservable( x => x.MyObjectsList.ItemChanged, x => x.MyObject.Changed) .Where(x => x.PropertyName == "Active") .Select(_ => MyObjectsList.Any(x => x.Active) || MyObject.Active);SaveCommand = ReactiveCommand.Create(Save, canSave);這個想法是每次 Active 屬性更改時都會重新評估。不確定這是否是最好的解決方案（因此歡迎任何改進它的建議），但我絕對無法將 IsBusy 添加到圖片中，以便在 IsBusy 時重新評估 Select 子句（包括 IsBusy 狀態）變化。
2 回答
回首憶惘然
TA貢獻1847條經驗 獲得超11個贊
我認為您的答案可以簡化。
var itemChangedObs = this.WhenAnyValue(x => x.MyObject.Active);
var isBusyObs = this.WhenAnyValue(x => x.IsBusy);
var listItemChangedObs = this.WhenAnyObservable(x => x.MyObectsList.ItemChanged).Where(x => x.PropertyName == "Active").Select(_ => MyObjectsList.Any(x => x.Active)).StartsWith(false)
var canRunCommand = itemChangedObs.CombineLatest(listItemChangedObs, isBusyObs, (itemActive, listItemActive, isBusy) => (itemActive || listItemActive) && !isBusy);
這個版本本質上使用了一個 CombineLatest ，它在將兩個 observable 組合在一起后采用您想要的值的 Lambda。
在兩個 Observables 都發出一個值之前，CombineLatest() 不會產生值，因此為什么 listItemChanged 在前面有一個 StartsWith(false)。
默認情況下，WhenAnyValue() 將始終發出值的 default(T) 作為其初始值，因此您不需要這些語句的 StartsWith。
慕妹3242003
TA貢獻1824條經驗 獲得超6個贊
這是另一種替代方法，它避免了對復雜度為 O(n) 的 MyObjectList.Any() 的需要。此解決方案涉及更多，但具有提高效率的潛力。除了計算listItemChangedObs observable的方式外，它與 Glenn 的CombineLatest方法相同。此版本保持列表中活動對象數的運行總數。因此，每次觸發 ItemChanged 時，它只需執行 +1 或 -1 操作。然后它只檢查它是否大于 0。
public MyViewModel()
{
var itemChangedObs = this.WhenAnyValue(x => x.MyObject.Active);
var isBusyObs = this.WhenAnyValue(x => x.IsBusy);
// Recalculate the # of active objects each time ObjectList is reassigned.
var activeListItemCountInitializedObs = this
.WhenAnyValue(x => x.ObjectList)
.Select(
list =>
{
// Return 0 if ObjectList is null.
return list == null ? Observable.Return(0) : list
.ToObservable()
// Otherwise, increment by 1 for each active object.
.Select(x => x.Active ? 1 : 0)
// We use Aggregate, which is a single value sequence, because
// we're only interested in the final result.
.Aggregate((acc, current) => acc + current);
})
// We no longer need the inner observable from the last time active item count
// was initialized. So unsubscribe from that one and subscribe to this most recent one.
.Switch();
var activeListItemCountChangedObs = this
.WhenAnyObservable(x => x.ObjectList.ItemChanged)
.Where(x => x.PropertyName == "Active")
// Increment or decrement the number of active objects in the list.
.Select(x => x.Sender.Active ? 1 : -1);
// An IObservable<bool> that signals if *any* of objects in the list are active.
var anyListItemsActiveObs = activeListItemCountInitializedObs
.Select(
// Use the initialized count as the starting value for the Scan accumulator.
initialActiveCount =>
{
return activeListItemCountChangedObs
.Scan(initialActiveCount, (acc, current) => acc + current)
// Return true if one or more items are active.
.Select(x => x > 0)
.StartWith(initialActiveCount > 0);
})
// ObjectList was completely reassigned, so the previous Scan accumulator is
// no longer valid. So we "reset" it by "switching" to the new one.
.Switch();
var canRunCommand = itemChangedObs
.CombineLatest(
anyListItemsActiveObs,
isBusyObs,
(itemActive, listItemActive, isBusy) => (itemActive || listItemActive) && !isBusy);
Save = ReactiveCommand.CreateFromObservable(() => Observable.Return(Unit.Default), canRunCommand);
}
這是我運行代碼時通過的單元測試。它基本上檢查 ReactiveCommand 的CanExecute更改狀態的次數，以及它是真還是假，每次變量之一發生變化時。
[Fact]
public void TestMethod1()
{
var objectList = new ReactiveList<IMyObject>(
initialContents: new[] { new MyObject(), new MyObject() },
resetChangeThreshold: 0.3,
scheduler: ImmediateScheduler.Instance);
objectList.ChangeTrackingEnabled = true;
IMyViewModel myViewModel = new MyViewModel
{
ObjectList = objectList,
MyObject = new MyObject()
};
var canExecute = myViewModel.Save
.CanExecute
.CreateCollection(scheduler: ImmediateScheduler.Instance);
Assert.Equal(1, canExecute.Count);
Assert.False(canExecute[0]);
myViewModel.ObjectList[0].Active = true;
Assert.Equal(2, canExecute.Count);
Assert.True(canExecute[1]);
myViewModel.MyObject.Active = true;
Assert.Equal(2, canExecute.Count);
myViewModel.IsBusy = true;
Assert.Equal(3, canExecute.Count);
Assert.False(canExecute[2]);
myViewModel.IsBusy = false;
Assert.Equal(4, canExecute.Count);
Assert.True(canExecute[3]);
myViewModel.MyObject.Active = false;
Assert.Equal(4, canExecute.Count);
var object1 = new MyObject { Active = true };
var object2 = new MyObject { Active = true };
myViewModel.ObjectList = new ReactiveList<IMyObject>(
initialContents: new[] { object1, object2 },
resetChangeThreshold: 0.3,
scheduler: ImmediateScheduler.Instance);
Assert.Equal(4, canExecute.Count);
object1 = new MyObject { Active = false };
object2 = new MyObject { Active = false };
myViewModel.ObjectList = new ReactiveList<IMyObject>(
initialContents: new[] { object1, object2 },
resetChangeThreshold: 0.3,
scheduler: ImmediateScheduler.Instance);
Assert.Equal(5, canExecute.Count);
Assert.False(canExecute[4]);
}
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