Skip to content

Display Plugin Library#

The display plugin library provides base and utility classes that reduce the overall effort needed to create a custom display.


The code for this library can be reviewed at Tutorials/DisplayPluginLibrary

TemplateDisplayViewModelBase View Model base class#

TemplateDisplayViewModelBase (a substitute for DisplayPluginViewModel) provides a template to help follow the Data request guidelines properly

  • Data requests are throttled to 5Hz by default
    • Pass a different throttle interval to the constructor if the default is not suitable
  • Properties are provided for the following common services
    • IDataRequestSignalFactory DataRequestSignalFactory
    • IDisplayParameterService DisplayParameterService
    • ILogger Logger
    • ISignalBus SignalBus
  • Append additional disposables to the Disposables property for automatic cleanup
  • Override OnMakeTimebaseDataRequestsAsync() to issue data requests in response to changes to the timebase
  • Override OnMakeCursorDataRequestsAsync() to issue data requests in response to changes to the cursor
  • Call MakeDataRequests() to initiate data requests for any other reason
  • Call the ExecuteOnUiAsync() method to execute code on the UI thread
  • Observe IsDisplayVisible property to determine when appropriate to display UI (by convention display nothing when no set is associated with the display)


When overriding DisplayPluginViewModel notification methods, do not forget to call the base class implementation (i.e. those of TemplateDisplayViewModelBase)

ParameterSampleDisplayViewModelBase View Model base class#

ParameterSampleDisplayViewModelBase<TParameterViewModel> (a substitute for TemplateDisplayViewModelBase) provides automatic retrieval of display parameter sample values

  • TParameterViewModel must be a class derived from ParameterSampleViewModelBase
  • ParameterSampleViewModelBase base class wraps a display parameter
    • Properties of the display parameter such as Name and current Value
    • OnUpdate() and OnValueChanged() notification methods to override
  • An implementation of the OnCreateParameterViewModel() factory method must be provided to supply a TParameterViewModel instance for a display parameter on demand
  • When display parameters are added/removed from the display, the Parameters collection is updated
    • OnCreateParameterViewModel() is called for newly added display parameters
    • OnUpdateParameters() is called whenever the Parameters collection is modified, and OnUpdate() is called on each TParameterViewModel instance
  • When the cursor timestamp changes, a data request is issued for each member of Parameters and the Value property of each is updated and OnValueChanged() called

Tracking Operations#

The OperationTracker<TOperation> class provides support for flow control and throttling of operations such as data or redraw requests

  • Pass to the Constructor a 'throttle interval' and 'action' to execute an operation
    • The 'throttle interval' specifies the minimum time (as a TimeSpan) between operations being executed
    • The 'action' is passed an instance of TOperation and when finished must call Complete()
  • Up to two operations can be tracked at any one time
    • The current (or active) operation (will be the first one added)
    • Operations added when there is already one active will become pending
    • A pending operation will always be the last one added (replacing any previous pending operation)
    • The current and pending operation will not overlap
  • Call Add() to add an operation of type TOperation
  • Call Complete() to indicate that the current operation has finished executing
    • If there is a pending operation it will automatically become active
  • Call GetCurrent() to get a copy of the current operation (if there is one, the method returns true)
  • Call Abort() to clear any active/pending operations

Drawing Graphics#

WPF provides a number of ways to draw graphics.

The VisualLayer class provides a simple way to add graphics drawn using the low-level and efficient DrawingContext class.

  • Add an IVisual instance to the View Model class

    public IVisual ExampleVisual { get; set; }
  • Add a VisualLayer to the XAML

    <UserControl x:Class="DisplayPlugin.SampleDisplayView"
                d:DesignHeight="450" d:DesignWidth="800">
        <displayPluginLibrary:VisualLayer x:Name="ExampleVisualLayer" />
  • In the code behind, associate the ExampleVisualLayer View property to the ExampleVisual View Model property

    public override void OnApplyTemplate()
        if (this.DataContext is SampleDisplayViewModel vm)
            vm.ExampleVisual = this.ExampleVisualLayer.Visual;
  • Draw graphics as required in the View Model

        dc =>
            dc.DrawLine(examplePen, exampleLine.Start, exampleLine.End);


Drawing must be done on the UI thread.

Make use of SynchronizationContext class or call the TemplateDisplayViewModelBase.ExecuteOnUiAsync() method.


To clear the visual layer of all graphics

this.TraceVisual.Draw(delegate { });


ColorToSolidColorBrushValueConverter is a basic WPF converter that converts a simple colour into a solid colour brush.