Perspex Binding Priorities Part 3

In the second post of the series I described how binding priorites in Perspex were fixed in the light of problems with local values. However, a problem still remained and it showed itself up in the DropDown control.

DropDown contains in its template, among other things, a ToggleButton and a Popup. On the DropDown itself there is the IsDropDownOpen property which - as the name suggests - is true when the popup is showing.

The IsDropDownOpen property is bound two-way with both the ToggleButton.IsChecked and the Popup.IsOpen properties such that any of the following causes the popup to be shown or hidden:

• Setting DropDown.IsDropDownOpen
• Setting ToggleButton.IsChecked
• Setting Popup.IsOpen

The problem arises because two properties are bound to DropDown.IsDropDownOpen but neither of them should take precedence. In the previously described system, newer added bindings take precedence of later added bindings. This is desired behavior for styles, where styles closer to the control should override those further up the tree, but here it breaks DropDown.IsDropDownOpen.

The problem can also be seen in the following:

var tb1 = new TextBox
{
    Text = "A non-wrapping text box. Lorem ipsum dolor sit amet.",
    TextWrapping = TextWrapping.NoWrap,
};

var tb2 = new TextBox
{
    Text = "A non-wrapping text box. Lorem ipsum dolor sit amet.",
    TextWrapping = TextWrapping.NoWrap,
};

var tb3 = new TextBox
{
    Text = "A non-wrapping text box. Lorem ipsum dolor sit amet.",
    TextWrapping = TextWrapping.NoWrap,
};

tb2.BindTwoWay(TextBox.TextProperty, tb1, TextBox.TextProperty);
tb2.BindTwoWay(TextBox.TextProperty, tb3, TextBox.TextProperty);

Here, tb2 should be bound to the Text property of both tb1 and tb2 and so all three TextBoxes should stay in sync, however the binding from tb3 was overriding that from tb1 causing the TextBoxes to get out-of-sync.

The solution I came up with was:

Binding levels with a an even number priority use the last fired binding for the current value, those with odd number priorities use the latest added.

This way for local values and template bindings the last fired binding takes precedence, whereas for styles and animations, the latest added binding takes precedence. This fitted strangely well into the ordering of the BindingPriority enum - I wonder what could be wrong this time…