func (cw *CustomWidget) bufferedPaint(canvas *Canvas, updateBounds Rectangle) error {
hdc := win.CreateCompatibleDC(canvas.hdc)
if hdc == 0 {
return newError("CreateCompatibleDC failed")
}
defer win.DeleteDC(hdc)
buffered := Canvas{hdc: hdc, doNotDispose: true}
if _, err := buffered.init(); err != nil {
return err
}
w, h := int32(updateBounds.Width), int32(updateBounds.Height)
if w < 1 {
w = 1
}
if h < 1 {
h = 1
}
hbmp := win.CreateCompatibleBitmap(canvas.hdc, w, h)
if hbmp == 0 {
return lastError("CreateCompatibleBitmap failed")
}
defer win.DeleteObject(win.HGDIOBJ(hbmp))
oldbmp := win.SelectObject(buffered.hdc, win.HGDIOBJ(hbmp))
if oldbmp == 0 {
return newError("SelectObject failed")
}
defer win.SelectObject(buffered.hdc, oldbmp)
win.SetViewportOrgEx(buffered.hdc, -int32(updateBounds.X), -int32(updateBounds.Y), nil)
win.SetBrushOrgEx(buffered.hdc, -int32(updateBounds.X), -int32(updateBounds.Y), nil)
err := cw.paint(&buffered, updateBounds)
if !win.BitBlt(canvas.hdc,
int32(updateBounds.X), int32(updateBounds.Y), w, h,
buffered.hdc,
int32(updateBounds.X), int32(updateBounds.Y), win.SRCCOPY) {
return lastError("buffered BitBlt failed")
}
return err
}
func (c *Canvas) init() (*Canvas, error) {
c.dpix = int(win.GetDeviceCaps(c.hdc, win.LOGPIXELSX))
c.dpiy = int(win.GetDeviceCaps(c.hdc, win.LOGPIXELSY))
if win.SetBkMode(c.hdc, win.TRANSPARENT) == 0 {
return nil, newError("SetBkMode failed")
}
switch win.SetStretchBltMode(c.hdc, win.HALFTONE) {
case 0, win.ERROR_INVALID_PARAMETER:
return nil, newError("SetStretchBltMode failed")
}
if !win.SetBrushOrgEx(c.hdc, 0, 0, nil) {
return nil, newError("SetBrushOrgEx failed")
}
return c, nil
}