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GDI Topics: Flicker-Free Drawing

1. To produce the flicker effect, create an MFC Application named FlickerFree
2. Create it as a Single Document type
3. Using the Resource Symbols dialog box, create an ID named IDT_OCCUR
4. In the view header file, declare the following variables:
    

   // FlickerFreeView.h : interface of the CFlickerFreeView class // #pragma once class CFlickerFreeView : public CView { protected: // create from serialization only CFlickerFreeView(); DECLARE_DYNCREATE(CFlickerFreeView) // Attributes public: CFlickerFreeDoc* GetDocument() const; // Operations public: // Overrides public: virtual void OnDraw(CDC* pDC); // overridden to draw this view virtual BOOL PreCreateWindow(CREATESTRUCT& cs); protected: virtual BOOL OnPreparePrinting(CPrintInfo* pInfo); virtual void OnBeginPrinting(CDC* pDC, CPrintInfo* pInfo); virtual void OnEndPrinting(CDC* pDC, CPrintInfo* pInfo); // Implementation public: virtual ~CFlickerFreeView(); #ifdef _DEBUG virtual void AssertValid() const; virtual void Dump(CDumpContext& dc) const; #endif protected: // Generated message map functions protected: DECLARE_MESSAGE_MAP() private: int x; int y; int w; int h; BOOL MovingRight; }; #ifndef _DEBUG // debug version in FlickerFreeView.cpp inline CFlickerFreeDoc* CFlickerFreeView::GetDocument() const { return reinterpret_cast<CFlickerFreeDoc*>(m_pDocument); } #endif

5. In the constructor of the view class, initialize the variables as follows:
    

   CFlickerFreeView::CFlickerFreeView() { // TODO: add construction code here x = 0; y = 100; w = 100; h = 150; MovingRight = TRUE; }

6. Generate the OnInitialUpdate event for the view class and use it to initialize the timer as follows:
    

   void CFlickerFreeView::OnInitialUpdate() { CView::OnInitialUpdate(); // TODO: Add your specialized code here and/or call the base class SetTimer(IDT_OCCUR, 10, NULL); }

7. Change the OnDraw event as follows:
    

   void CFlickerFreeView::OnDraw(CDC* pDC) { CFlickerFreeDoc* pDoc = GetDocument(); ASSERT_VALID(pDoc); if (!pDoc) return; // TODO: add draw code for native data here CRect rect; CBrush brsBlack, *brsOld; GetClientRect(&rect); brsBlack.CreateSolidBrush(RGB(0, 0, 0)); brsOld = pDC->SelectObject(&brsBlack); pDC->Rectangle(rect.left, rect.top, rect.Width(), rect.Height()); CPen penRed, *penOld; penRed.CreatePen(PS_SOLID, 1, RGB(255, 0, 0)); penOld = pDC->SelectObject(&penRed); pDC->Rectangle(x, y, x + 100, h); pDC->SelectObject(penOld); pDC->SelectObject(brsOld); }

8. Generate the event of the WM_TIMER message for the view class and implement it as follows:
    

   void CFlickerFreeView::OnTimer(UINT_PTR nIDEvent) { // TODO: Add your message handler code here and/or call default CRect rect; GetClientRect(&rect); if( x < 0 ) { x = 0; MovingRight = TRUE; } if( x > (rect.Width() - 100) ) { x = rect.Width() - 100; MovingRight = FALSE; } Invalidate(); if( MovingRight == TRUE ) x++; else x--; CView::OnTimer(nIDEvent); }

9. Execute the application to see the result and notice the flickering effect
10. Close the application

Fortunately, in most cases, getting rid of, or reducing, the flickering effect can be easy. To take care of it, override the OnEraseBkgnd event and simply return TRUE from it.

1. Generate the event of the WM_ERASEBKGND message of the view class and change its return value as follows:
    

   BOOL CFlickerFreeView::OnEraseBkgnd(CDC* pDC) { // TODO: Add your message handler code here and/or call default return TRUE;// CView::OnEraseBkgnd(pDC); }

2. Execute the application again to see the result