UI stack partition

[ickshonpe]

Objective

In the UI picking and rendering systems, we first query for all the pickable or renderable UI nodes and then query per node for the details of the camera and its render target. But the typical application using Bevy UI will have hundreds of UI nodes and just one UI camera, so this is extremely inefficient.

Instead, we can partition the UI stack into disjoint slices where all the UI nodes in each slice have the same camera target. Then perform any camera and render target lookups per slice, instead of per node.

Solution

Partition the UI stack into disjoint layers of nodes sharing the same camera target.

• Add a partition: Vec<Range<usize>> field to UiStack.
• Update the partitions in ui_stack_system.
• Query for cameras per slice in ui_focus_system and ui_picking.

Splitting the rendering changes off into their own PR.

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Testing

Some basic checks have been added to the existing test_ui_stack_system and test_with_equal_global_zindex_zindex_decides_order tests.

Examples like ui_target_camera, viewport_node and ui_drag_and_drop can be used to test the changes.

Showcase

yellow this PR, red main:

cargo run --example many_buttons --release --features bevy/trace_tracy

ui_picking

ui_stack_system

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