developing-wpf-customcontrols

WPF CustomControl Development - Parts and States Model

Workflow for developing WPF CustomControls with appearance customization capability.

Development Flow

1. Control Inheritance Decision

UserControl Selection Criteria:
- Need rapid development
- ControlTemplate customization not required
- Complex theme support not required

Control Inheritance Selection Criteria:
- Need appearance customization via ControlTemplate
- Need various theme support
- Need same extensibility as WPF built-in controls

2. Define Control Contract

Declare TemplatePart and TemplateVisualState attributes on the class:

[TemplatePart(Name = PartUpButton, Type = typeof(RepeatButton))]
[TemplatePart(Name = PartDownButton, Type = typeof(RepeatButton))]
[TemplateVisualState(Name = StatePositive, GroupName = GroupValueStates)]
[TemplateVisualState(Name = StateNegative, GroupName = GroupValueStates)]
[TemplateVisualState(Name = StateFocused, GroupName = GroupFocusStates)]
[TemplateVisualState(Name = StateUnfocused, GroupName = GroupFocusStates)]
public class NumericUpDown : Control
{
    // Define Part/State names as const
    private const string PartUpButton = "PART_UpButton";
    private const string PartDownButton = "PART_DownButton";
    private const string GroupValueStates = "ValueStates";
    private const string GroupFocusStates = "FocusStates";
    private const string StatePositive = "Positive";
    private const string StateNegative = "Negative";
    private const string StateFocused = "Focused";
    private const string StateUnfocused = "Unfocused";
}

3. Template Part Property Pattern

Wrap Part elements as private properties, subscribe/unsubscribe events in setter:

private RepeatButton? _upButton;
private RepeatButton? UpButtonElement
{
    get => _upButton;
    set
    {
        // Unsubscribe from existing element's events
        if (_upButton is not null)
            _upButton.Click -= OnUpButtonClick;

        _upButton = value;

        // Subscribe to new element's events
        if (_upButton is not null)
            _upButton.Click += OnUpButtonClick;
    }
}