[macOS] Implement IsEnabled property in MenuFlyoutItems

[jsuarezruiz]

Description of Change

Implement IsEnabled property in MenuFlyoutItems on Catalyst.

(Work in progress)

Menu

Flyout

Issues Fixed

Fixes #9583
Fixes #15896

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[hartez]

Also, what happens with Command.CanExecute?

I just tested this, and when the Command's CanExecute changes, the state of the menu item changes.

[hartez]

Also, is the IsEnabled not propagating to all the children in the xplat code?

It doesn't appear to propagate; setting a parent Menu Item to IsEnabled=False does not modify the child item in any way. On Windows, it doesn't matter - disabling a menu item with sub items disables it completely, to the point that you can't even reveal the sub items. So their state becomes moot.

So we need to answer the question of what should happen here - should disabling a menu with sub items propagate the xplat property? Or should we just handle that on the platform side (which is doable in Catalyst, it just takes a little extra code).

@PureWeen @mattleibow thoughts?

[mattleibow]

We probably should use the same method that I did for VisualElement. We do propagate that enabled value, but not actually - otherwise we end up not knowing if we should re-enable.

On the base menu element, you have a protected property:

maui/src/Controls/src/Core/VisualElement/VisualElement.cs

Lines 574 to 591 in 34b481d

	protected virtual bool IsEnabledCore
	{
	get
	{
	if (_isEnabledExplicit == false)
	{
	// If the explicitly set value is false, then nothing else matters
	// And we can save the effort of a Parent check
	return false;
	}
	var parent = Parent as VisualElement;
	if (parent is not null && !parent.IsEnabled)
	return false;
	return _isEnabledExplicit;
	}
	}

The real IsEnabled property will coerce the value and make sure to do the math but preserve the original value:

maui/src/Controls/src/Core/VisualElement/VisualElement.cs

Lines 1294 to 1303 in 34b481d

	static object CoerceIsEnabledProperty(BindableObject bindable, object value)
	{
	if (bindable is VisualElement visualElement)
	{
	visualElement._isEnabledExplicit = (bool)value;
	return visualElement.IsEnabledCore;
	}
	return false;
	}

The coersion will fire for all controls and disable if something is disabled, but caches the real value in a field.

And it will propagate the enabled value:

maui/src/Controls/src/Core/VisualElement/VisualElement.cs

Lines 1387 to 1390 in 34b481d

	void IPropertyPropagationController.PropagatePropertyChanged(string propertyName)
	{
	if (propertyName == null || propertyName == IsEnabledProperty.PropertyName)
	this.RefreshPropertyValue(IsEnabledProperty, _isEnabledExplicit);

In the derived menu items, it just overrides the IsEnabledCore with any additional things - like the command:

maui/src/Controls/src/Core/Button/Button.cs

Lines 462 to 466 in 34b481d

	void ICommandElement.CanExecuteChanged(object sender, EventArgs e) =>
	RefreshIsEnabledProperty();
	protected override bool IsEnabledCore =>
	base.IsEnabledCore && CommandElement.GetCanExecute(this);

[mattleibow]

Maybe separate blocks?

[mattleibow]

woohoo!