C ++：RedrawWindow（）闪烁

Question

As a developer who is building a Custome Splitter Wnd using custom class inherited from CWnd. Initially, the screen has one window (custom class - CTile) which has two buttons(vertical - split, horizontal - split). When user clicks one of two buttons, the red split bar is appeared and two child window (CTile) is appeared. As you know, while user drag the red split bar, the child window must be modified. What I say here is that at this moment, the flickering is appeared. The parent wnd has only three elements(two child windows and one split bar), so I think that it never require content of drawing. I mean WM_PAINT message handler. Here's my code.

this->cDiv = new CDivider(this->wth_tile / 2, 1);
this->cDiv->CreateDivider(this, this->hgt_tile);

//cDiv is split bar I used custom class which is inherited from CWnd.
//CreateDivider() is also my self-defined method.

this->first_child = new CTile();

// As I mentioned above, CTile is divided child window which is also inherited from CWnd.

POINT pt;
pt.x = 0;
pt.y = 0;
this->first_child->CreateTile(this, this->cDiv->sd_pos, this->hgt_tile, pt);

this->second_child = new CTile();


pt.x = this->cDiv->sd_pos + 5;
pt.y = 0;

this->second_child->CreateTile(this, this->cDiv->sd_pos, this->hgt_tile, pt);

This is make movable split bar wnd creation code.

And next is about modified child window size while drag the split bar.

void CDivider::OnMouseMove(UINT nFlags, CPoint point)
{
// TODO: Add your message handler code here and/or call default

POINT pt;
HDC hdc;
RECT rect;

this->parentWnd->GetWindowRect(&rect);

//convert the mouse coordinates relative to the top-left of
//the window
ClientToScreen(&point);
pt = point;
pt.x -= rect.left;
pt.y -= rect.top;

if (this->sd_mode == 1)
{
    ::SetCursor(AfxGetApp()->LoadStandardCursor(IDC_SIZEWE));
    if (GetCapture() == this && this->dragged)
    {
        this->sd_pos = pt.x;
        if (pt.x != oldPos.x && nFlags & MK_LBUTTON)
        {
            this->length = this->parentWnd->hgt_tile;
            this->MoveWindow(this->sd_pos, 0, 4, this->length);
            this->parentWnd->ResizeParent();
            this->parentWnd->Invalidate();
            this->parentWnd->UpdateWindow();
            TRACE("Resize Parent\n");
            /*this->parentWnd->first_child->Invalidate();
            this->parentWnd->second_child->Invalidate();*/
        }
    }

}
else if (this->sd_mode == 2)
{
    ::SetCursor(AfxGetApp()->LoadStandardCursor(IDC_SIZENS));

    if (GetCapture() == this && this->dragged)
    {
        this->sd_pos = pt.y;
        if (pt.y != oldPos.y && nFlags & MK_LBUTTON)
        {
            this->Invalidate();
            this->length = this->parentWnd->wth_tile;
            this->MoveWindow(0, this->sd_pos, this->length, 4);

            this->parentWnd->ResizeParent();
            this->parentWnd->Invalidate();
            this->parentWnd->UpdateWindow();
            TRACE("Resize Parent\n");
            /*this->parentWnd->first_child->Invalidate();
            this->parentWnd->second_child->Invalidate();*/
        }
    }
}

CWnd::OnMouseMove(nFlags, point);

}

Here, parentWnd is split bar's parent window - just parent initial window. first_child and second_child is child divided windows. sd_mode means that split method - vertical & horizontal.

Why doesn't this code work?

Answer 1

The flickering happens because painting is in most normal windows a two phase operation. 1. The background is erased, 2. The window is repainted.

So the problem is the child window that needs redrawing. There are a lot of articles about non flickering painting.

Also you can optimize the redrawing in just invalidating just the parts of windows that are affected. You force a redraw of the complete windows. That isn't ideal. In such a case a large portion of the window might flicker.

BTW: RedrawWindow with the specific flags are better than calling Invalidate/Update sequences and may be a tick faster.

Also it may be better to use a method to draw Bars with an XOR-Paining during dragging. And update the window size when the LButtonUp-Event happens. The way CSplitterWnd is doing it... you have the source. Look into it.