问题
This might be a very simple question, but I searched and found no other way to do it. It doesn't make sense to redraw the background on every Draw. Is there a way to draw some things and leave them on the screen?
I've tried to comment-out the
GraphicsDevice.Clear(Color.CornflowerBlue);
But that doesn't help. (What is its purpose?)
回答1:
The dark purple colour you are seeing is used by XNA and DirectX to indicate an uninitialised buffer. XNA will also clear buffers to this colour to emulate the behaviour of the Xbox 360 or Windows Phone, so that if you build a game on Windows, it "just works" on those other platforms (or, rather, so it fails in the same way, so you can debug it).
XNA is double-buffered. You don't draw directly to the screen, but to a "backbuffer". The screen only displays the "front buffer". Every time GraphicsDevice.Present gets called (Game calls it for you in EndDraw), those two buffers get swapped, and what you were drawing gets displayed (and you get a fresh buffer to draw on).
I'm not sure why XNA marks the buffer as uninitialised when it gets swapped. I haven't come across this behaviour before - mostly because it's very unusual to want to swap buffers and preserve their contents.
Usually what you want to do is call Game.SupressDraw, when you know you're not going to modify the contents of the screen (saving both a call to Draw and a swap). See also answers here and here.
Keep in mind that clearing the screen with GraphicsDevice.Clear is extremely fast. And that XNA has no concept of "background" or "foreground" (you're always drawing on top of whatever is already in the buffer).
If you do have some expensive-to-render content that you want to re-use between frames, generally you would draw it into to a render target once, and then draw that to the screen each frame. But, as always, avoid premature optimisation! Graphics cards are designed specifically to redraw scenes every frame - they're pretty damn fast!
回答2:
See this, if you want to just prevent it clearing the image you can do:
GraphicsDevice.GetType().GetField("lazyClearFlags", BindingFlags.NonPublic | BindingFlags.Instance).SetValue(GraphicsDevice, ClearOptions.DepthBuffer);
来源：https://stackoverflow.com/questions/16904829/redraw-unchanging-background-on-every-draw