Pass mutable scene builder reference to paint

[raphlinus]

Instead of the paint method returning a Rendered which was just a newtype around SceneFragment, pass in &mut SceneBuilder. Also store a scene fragment in each Pod, which is the storage for these builders.

This change has a number of consequences. Less boilerplate in the paint method, and also less allocation, as the storage can get reused. More importantly, though we don't have the logic for this yet, it sets the stage for retaining scene fragments when widgets don't change.

[xarvic]

Could we make the scene_builder argument a member of PaintCx like it was in druid?

And i am trying to understand how widgets render their children. The paint method of Pod has no scene_builder parameter and there is no method to retrieve the SceneFragment of the Pod. I would be in favor of adding the scene_builder parameter and let the Pod apply transform and write its SceneFragment into that of its parent.

[raphlinus]

I'm not sure moving it into PaintCx is a win. In Druid it's only ergonomic because the Cx derefs into a piet RenderContext. Also, in Druid that RenderContext is basically constant for the scope of the paint cycle, where in Xilem each builder is local to the widget's scene fragment.

I haven't implemented containers yet (soon), so it's not clear from the code. Pod will have a method to return a scene fragment reference, and the parent will do append() with that fragment (also apply a transform based on layout / scrolling / etc). The motivation for this is retaining the rendered scene fragments, so you only call into paint when they're invalidated. The idea is that append is cheap - currently it is a handful of memcpy operations, but shouldn't allocate in the steady case, and if the memcpy operations do turn out to be expensive, we could go to non-contiguous storage.

[raphlinus]

Hopefully that commit makes what I'm trying to do a little clearer.

[xarvic]

Yes, thanks :)

From what i understand painting a child widget will look like this most of the time:

child.paint(ctx);
builder.push_layer(..., Affine::translate(child.origin().to_vec2()), ...);
builder.append(child.fragment());
builder.pop_layer();

What i wanted to say is that is could be done by the Pod of child if its paint method receives the parent SceneBuilder. This was also my rationale for proposing to include the scene_builder argument into the PaintCx

[raphlinus]

Ah, now I understand. I was confusing the paint method of Pod with the paint method on the Widget trait.

In the general case we will not need push_layer(), only when doing clips and blends. Also note that the affine transform has changed from stateful (PostScript/canvas-like) to stateless, so it needs to be given to the append method. Thus, though we could certainly add a convenience method to Pod to append its fragment, I'm not sure it helps that much.

[xarvic]

Ah! I over read the transform parameter in append. Now it makes sense. Thanks for the explanation :)

[dfrg]

This looks good! I spent some time with it but wanted to give others a chance to offer approval.