ReactiveCommand CanExecute 对集合中的变化做出反应 [英] ReactiveCommand CanExecute reacting to changes in a collection

问题描述

我有一个充满项目的 ReactiveCollection(也是 ReactiveObjects).

I have a ReactiveCollection filled with Items (that are ReactiveObjects as well).

我想创建一个只有当集合中的任何项的某些属性设置为 true 时才应启用的 ReactiveCommand，例如:

I want to create a ReactiveCommand that should be enabled only when any of the items in the collection has some property set to true, something like:

MyCommand = ReactiveCommand.Create( watch items in collection to see if item.MyProp == true ) 

因此，只要其中一项属性设置为 true，就应该启用该命令.

So anytime there is one of the items with the property set to true, the command should be enabled.

谢谢，保罗.生成的代码是这样的:

Thanks, Paul. The resulting code is this:

public MainViewModel()
{
    Items = new ReactiveList<ItemViewModel>
                {
                    new ItemViewModel("Engine"),
                    new ItemViewModel("Turbine"),
                    new ItemViewModel("Landing gear"),
                    new ItemViewModel("Wings"),
                };

    Items.ChangeTrackingEnabled = true;

    var shouldBeEnabled = Items.CreateDerivedCollection(x => x.IsAdded);

    var shouldRecheck = Observable.Merge(
        // When items are added / removed / whatever
        shouldBeEnabled.Changed.Select(_ => Unit.Default),
        // When any of the bools in the coll change
        shouldBeEnabled.ItemChanged.Select(_ => Unit.Default));

    // Kick off a check on startup
    shouldRecheck = shouldRecheck.StartWith(Unit.Default);

    ClearCommand = ReactiveCommand.Create(shouldRecheck.Select(_ => shouldBeEnabled.Any(x => x)));
}

编辑 2:我发现了一个陷阱！如果你修改这一行:

EDIT 2: I've discovered a trap! If you modify this line:

new ItemViewModel("Engine");

并像这样设置 IsAdded = true

and set the IsAdded = true like this

new ItemViewModel("Engine") { IsAdded = true };

...当您运行时，应用程序启动时按钮被禁用，应该启用.似乎在发生某些更改后不会对表达式进行评估.我该如何解决?

… when you run the button is disabled when the application starts and it should be enabled. It seems like the expression doesn't evaluate after some change occurs. How can I solve it?

编辑 3: 解决了！正如@paul-betts 所说，可以通过添加以下行来解决:

EDIT 3: Solved! As @paul-betts says, it can be solved adding this line:

// Kick off a check on startup
shouldRecheck = shouldRecheck.StartWith(Unit.Default);

上面的代码示例也更新了(在他的回答中也是如此).

The code sample is also update above (and in his answer, too).

推荐答案

这个怎么样

mySourceCollection.ChangeTrackingEnabled = true;
shouldBeEnabled = mySourceCollection.CreateDerivedCollection(x => x.MyProp);

var shouldRecheck = Observable.Merge(
    // When items are added / removed / whatever
    shouldBeEnabled.Changed.Select(_ => Unit.Default),

    // When any of the bools in the coll change
    shouldBeEnabled.ItemChanged.Select(_ => Unit.Default));

// Kick off a check on startup
shouldRecheck = shouldRecheck.StartWith(Unit.Default);

myCmd = ReactiveCommand.Create(shouldRecheck.Select(_ => shouldBeEnabled.All(x => x));

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