How to use a canvas outside of EventHandler

[nobbele]

Describe the bug
Canvas::finish returns RenderError("starting Canvas outside of a frame") when used inside your state constructor.

Hardware and Software:

• ggez version: 9a7ff9f

[PSteinhaus]

Hmm, do we really want to allow to create renderpasses outside of draw?

This led people down weird paths in the past.

But, it can be useful…

Ok, after a bit of searching I realized that this is already possible, if you respect the red tape:

What you need to do to draw outside of draw is, you have to create a new frame yourself and also end it yourself, by calling gfx.begin_frame and gfx.end_frame.

[p9436]

Here is a small example based on this answer:

impl GameState {
    fn new(ctx: &mut Context) -> ggez::GameResult<GameState> {
        // Create a texture to hold the background
        ctx.gfx.begin_frame()?;
        let mut background_image = Image::new_canvas_image(ctx, ctx.gfx.surface_format(), 100, 100, 1);
        let mut background_canvas= Canvas::from_image(ctx, background_image.clone(), None);
        background_canvas.set_sampler(graphics::Sampler::nearest_clamp());
        draw_background(ctx, &mut background_canvas);
        background_canvas.finish(ctx)?;
        ctx.gfx.end_frame()?;
        let background_image = Some(background_image);

        Ok(GameState {
            background_image,
        })
    }
}