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CHAPTER 11 BUILDING GRAPHICAL USER INTERFACES
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[<Category("Graph Style"); Browsable(true)>] member x.VisibleSamples with get() = visibleSamples and set(v:int) = visibleSamples <- v; initView <- startTime - int64(visibleSamples); x.Invalidate() [<Category("Graph Style"); Browsable(true)>] member x.TimeScale with get() = timeScale and set(v:int) = timeScale <- v; x.Invalidate() [<Category("Graph Style"); Browsable(true)>] member x.TimeFormat with get() = timeFormat and set(v:string) = timeFormat <- v; x.Invalidate() // ... Further portions of this class shown further below Listing 11-3 includes the remaining portions of the GraphControl class corresponding to the controller part of the Model-View-Controller paradigm. Samples are added through the AddSample method (AddSampleData generates random samples to display inside the control). This method adds the sample to the inner DataSamples object and updates the values of two fields meant to store the minimum and maximum values recorded for samples; both of these values are used in the display process. Because the model of the control changes, you need to update the view, and you invalidate the control as you did for properties.
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Overriding vs. Delegation
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Event handling can be performed by both subscribing delegates and overriding methods. The former approach is more typical of applications; the latter is more common in control development. Method overriding guarantees more control over event handling because it allows a complete redefinition of the inherited behavior, whereas with delegation you can only add behavior. When an overridden method starts with a call to the method to be overridden in the base class, it s functionally equivalent to using delegation rather than method overriding. It s traditional, however, to use method overriding in the case of control development, to have a uniform notation for event handling. Be careful during method overriding, because if the call to the overridden method is omitted, the corresponding delegate event isn t fired: delegate events are invoked by the event handlers of the base classes.
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Let s look at how you handle the mouse-move events and the use of the mouse wheel. When the wheel of the mouse is scrolled, the control adjusts the scale factor to zoom in or out of the current view. To understand how this method works, you need to know how to decide which portion of the data is made available through the view of the control. You use two fields called initView and visibleSamples. Because you can t assume that all the samples fit in the control s display, the former indicates the time (in the time scale of the samples) corresponding to the leftmost visible value, and the latter indicates the number of time units in the unit scale of samples that should be visible. Zooming is performed by changing the density of time units to be displayed inside the viewport.
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CHAPTER 11 BUILDING GRAPHICAL USER INTERFACES
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The last method of Listing 11-3 is GetTime: it converts the time unit of samples in microseconds using a scale factor that is one of the configuration properties made available by the control. Listing 11-3. Extract of the Controller of the GraphControl Class override x.OnMouseWheel (e:MouseEventArgs) = base.OnMouseWheel(e) x.Zoom(e.Delta) override x.OnSizeChanged (e:EventArgs) = base.OnSizeChanged(e) x.Invalidate() member x.Zoom (amount:int) = let newVisibleSamples = max 5 (visibleSamples + amount) if (initView - startTime < 0L) then let e = initView + int64(visibleSamples) initView <- startTime - int64(newVisibleSamples) + e visibleSamples <- newVisibleSamples x.Invalidate() member if if if x.AddSample (time:int64, value:float32) = (value < absMin) then absMin <- value (value > absMax) then absMax <- value (data.Count > 0) then initView <- initView + time - data.Last.Time data.AddSample(time, value) x.Invalidate()
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member x.GetTime (time:int64) = DateTime(max 0L time * int64(timeScale))
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Building the GraphControl: The View
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The view of the GraphControl is entirely contained within the OnPaint method, which is invoked when the GUI needs to repaint the content of the control or when an invocation of the Invalidate method occurs. Listing 11-4 shows the full code for this method. Programming graphical controls can get complicated, and often the code is factorized further using functions. The OnPaint method begins computing some information such as the rectangles containing the string with the values to be displayed. The dimension of a string depends on the font used for display and the particular device context used to render it. You rely on the MeasureString method of the Graphics object you received from the GUI. You compute the plotBox rectangle, which represents the area where you draw the data; it s obtained by removing from the dimension of the control the margins specified in the configuration and the space required by the labels if visible. You later set an appropriate coordinate system on the device context so that the drawing primitives render in this new system: g.TranslateTransform(float32(plotBox.Left), float32(x.Height - plotBox.Top)) g.ScaleTransform(1.0f, -1.0f) You translate the origin of the coordinate system in the lower-left vertex of the margins rectangle. You also flip the y axis by setting a scale transform that inverts the direction, multiplying y coordinates by 1.0f; in this way, you obtain a coordinate system oriented as in mathematics. Coordinate transformation
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