CDC DrawText

[gurucplusplus]

Hi, I use CDC* pDC->DrawText(" number ") in OnDraw() to update text to a frame. I then call InvalidateRect(NULL,FALSE) to display the text. What is the better way of display text without calling InvalidateRect() because OnDraw() with InvalidateRect() slow down application quite a lot. Thanks.

[Matthew Faithfull]

If you're doing this right you should not have to call InvalidateRect(). OnDraw only gets called when some Rect is alreay invalid, that's why windows generated the WM_PAINT to get you to call OnDraw in the first place. If your text doesn't appear without this then something else is wrong. I'm not sure what you mean by

gurucplusplus wrote:

update text to a frame

. With most windows you can call SetWindowText to set the text that's displayed. If this is a custom window class or an owner draw control then DrawText might be right:~

Nothing is exactly what it seems but everything with seems can be unpicked.

[Mark Salsbery]

You're causing recursive repeated calls to OnDraw() by calling InvalidateRect() from within OnDraw(). That'll certainly slow things down :)

gurucplusplus wrote:

What is the better way of display text without calling InvalidateRect()

Don't call InvalidateRect()! Mark -- modified at 14:29 Thursday 7th June, 2007

"Posting a VB.NET question in the C++ forum will end in tears." Chris Maunder

[gurucplusplus]

How can I update this text to graphic without calling invalidateRect()?

[Mark Salsbery]

When you call DrawText, you ARE updating the text. You use Invalidate methods to force a repaint from outside of OnPaint/OnDraw. For example, if the text you are drawing changes as the result of some process in the application and you want the window updated to reflect the change, then you'd call Invalidate. This will mark the given area of the window as invalid. On the next WM_PAINT message (which will call OnDraw() for CViews) you will call DrawText() with the updated text. I hope that made sense :) Mark

"Posting a VB.NET question in the C++ forum will end in tears." Chris Maunder

[gurucplusplus]

So, If my text drawn changes I have to call invalidateRect to mark invalid for the next WM_PAINT message to update the text. Where should I call invalidaterect then? Not in ondraw()? thanks

[Mark Salsbery]

gurucplusplus wrote:

If my text drawn changes I have to call invalidateRect to mark invalid for the next WM_PAINT message to update the text. Where should I call invalidaterect then?

Right. If the text characters never change (like pDC->DrawText("Text") then there's no reason to ever call InvalidateRect() because every time the window needs repainting the text will just be redrawn. Now suppose you are drawing the text of a string that changes (pDC->DrawText(str)). The user does something that causes the value of str to change and you want to reflect that change in the window. Any time after you change str, call InvalidateRect() to mark the area as invalid. When the resulting WM_PAINT is received by the window, CView::OnPaint() will call OnDraw(). In your OnDraw() override, you call pDC->DrawText(str), drawing the new text on the window. Also remember, WM_PAINT messages are LOW priority (they can get bumped to the back of the message queue). If you need an instant update, like if the text string is changing as the result of a timer (like in a clock implementation), then you can add a call to UpdateWindow() right after the call to InvalidateRect(). This will cause a WM_PAINT to be sent immediately. Remember that, when responding to WM_PAINT, it is your responsibility to validate the region of the window that is invalid to prevent repeated WM_PAINT messages. Most of the time this is done for us by MFC or the default window message handling. So if you invalidate regions of a window while it's trying to validate it, the WM_PAINT messages keep coming. If you want to see the recursive action/result of doing this in your OnDraw(), try this in your OnDraw(). This consumes all the CPU time of one of my processors and renders dialog controls useless: Invalidate(); UpdateWindow(); return; I'm actually surprised it doesn't crash :) Mark

"Posting a VB.NET question in the C++ forum will end in tears." Chris Maunder

[gurucplusplus]

Thanks.