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CHAPTER 11 BUILDING GRAPHICAL USER INTERFACES
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Let s introduce three menus to configure the application. They re used to check features to be drawn: let let let let newMenu (s:string) = new ToolStripMenuItem(s,Checked=true,CheckOnClick=true) menuBezier = newMenu "Show &B zier" menuCanonical = newMenu "Show &Canonical spline" menuControlPoints = newMenu "Show control &points" You use a scrollbar to define different values for the tension parameter of the canonical spline curve: let scrollbar = new VScrollBar(Dock=DockStyle.Right, LargeChange=2, Maximum=10) Control points are drawn if required, and an ellipse is used to mark them. The function receives the device context in the form of a Graphics object; you draw the ellipse by invoking the DrawEllipse primitive on it. You use a Pen to draw the ellipse in this case, a red pen: let drawPoint (g:Graphics) (p:Point) = g.DrawEllipse(Pens.Red, p.X - 2, p.Y - 2, 4, 4)
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Brushes and Pens
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Windows Forms uses two kinds of objects to define colored primitives: brushes and pens. A brush is used to fill an area with a given pattern. A number of different patterns are available; solid colors are provided by the SolidBrush class, hatched patterns are provided by HatchBrush , gradients are provided by LinearGradientBrush and PathGradientBrush , and textured gradients are provided by TextureBrush . The Brushes class provides a number of static brush objects describing solid colors. Pens are brushes with a contour. The line drawn by a pen has a filling (the brush part) but also a width and different styles (dashed or not, with different caps at the beginning and at the end). The Pens class provides a number of static pen objects with the basic solid colors. Both pen and brush objects contain resources of the graphical system; it s important to dispose of them as soon as they aren t required anymore. A use binding or the using function discussed in s 4 and 8 helps ensure that you don t forget to call the Dispose method that all these objects provide from the IDisposable interface that otherwise should be called explicitly.
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You re now ready to define the function responsible for drawing in your window. You can t assume anything about the current state of the window; thus, the paint function always draws the visible primitives1 depending on the state of menu entries: let paint (g:Graphics) = if (menuBezier.Checked) then g.DrawLine(Pens.Red, cpt.[0], cpt.[1]) g.DrawLine(Pens.Red, cpt.[2], cpt.[3]) g.DrawBeziers(Pens.Black, cpt) if (menuCanonical.Checked) then
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If primitives fall out of the area allowed for drawing, they re clipped in part or entirely.
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CHAPTER 11 BUILDING GRAPHICAL USER INTERFACES
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g.DrawCurve(Pens.Blue, cpt, float32 scrollbar.Value) if (menuControlPoints.Checked) then for i = 0 to cpt.Length - 1 do drawPoint g cpt.[i] Figure 11-3 shows the result of the drawing all the elements. The B zier curve, widely used in image-processing and vector applications, uses the four control points to define the start and end points of the curve and the two segments tangent to the curve at its ends. The cubic parametric curve is calculated from these points and produces the lines shown in Figure 11-3. The canonical spline, on the other hand, is a curve that traverses all the control points; the tension parameter controls how curvy the curve is.
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Figure 11-3. Output of the Curves application You now want to allow users to move control points by dragging and dropping. You re interested in mouse events, in particular when the mouse button is pressed, when it moves, and when the button is released. Thanks to the well-defined model for rendering the application, you can update the state of your variables and ask the graphical system to issue a paint message that causes the window to receive a paint message and update the current frame.
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CHAPTER 11 BUILDING GRAPHICAL USER INTERFACES
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Background Painting
In the Curves application, you draw assuming that the window is clean but who is responsible for clearing the previous drawing in the window Windows Forms calls the OnPaintBackground method before calling the paint handler, and this method by default clears the area with the color returned by the BackColor property. When the function responsible for painting draws the area entirely, painting the background could be useless and even problematic: the quick repaint may flicker because the eye can perceive the background clear and then the drawing of the current frame. You can use the SetStyle method of the Form class to configure the application to do all the paint operations in the handler of the paint event, and you can override OnPaintBackground to an empty method to avoid this effect.
You define a helper function to define a circular area around a point that is sensible to your interaction. This is required in order to not require the user to pick the exact pixel corresponding to the control point: let isClose (p:Point) (l:Point) = let dx = p.X - l.X let dy = p.Y - l.Y (dx * dx + dy * dy) < 6 When the mouse button is pressed, you check whether the click is over any control point. In this case, you store its index in the movingPoint variable; otherwise, the event is ignored: let mouseDown (p:Point) = try let idx = cpt |> Array.findIndex (isClose p) movingPoint <- idx with _ -> () When the mouse moves over the client area of the window, the mouse move event is generated. If the movingPoint member has a value other than 1, you have to update the corresponding control point with the current position of the mouse defined by the variable p: let mouseMove (p:Point) = if (movingPoint <> -1) then cpt.[movingPoint] <- p form.Invalidate() You next define for the window a File menu and a Settings submenu. The first features the classic Exit option, and the second shows the three checked menu items that control what the paint method should draw. You define menus by composing objects that correspond to the various menu entries. You also define the event handlers associated with each menu item. When Exit is clicked, the form is disposed. In all the other cases, you rely on the menu item s ability to change its checked state, and you invalidate the form content to force the redraw of the window: let setupMenu () = let menu = new MenuStrip() let fileMenuItem = new ToolStripMenuItem("&File") let settMenuItem = new ToolStripMenuItem("&Settings") let exitMenuItem = new ToolStripMenuItem("&Exit")
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