Struct Renderer 

pub struct Renderer<B: Toolkit> {
    pub kind: PieceKind,
    pub make: fn(&mut B, &dyn Any, NodeId) -> B::Handle,
    pub update: fn(&mut B, &B::Handle, &dyn Any),
    pub measure: Option<MeasureFn<B>>,
    pub release: Option<fn(&mut B, &B::Handle)>,
}

A third-party piece’s per-toolkit implementation. make receives the concrete backend (public helper surface) and returns a native handle the backend then owns like any built-in.

Fields

kind: PieceKind

make: fn(&mut B, &dyn Any, NodeId) -> B::Handle

update: fn(&mut B, &B::Handle, &dyn Any)

measure: Option<MeasureFn<B>>

release: Option<fn(&mut B, &B::Handle)>

Teardown, run from the release queue just before Toolkit::release frees the handle.

A piece that keeps per-view state (a retained delegate, a session map, a native observer) has no other place to drop it: make/update are the only other hooks, and a disposed node never calls them again. Without this a piece leaks one entry per realized view, and any map keyed by the handle’s address can go on to answer for a later view that the allocator hands the same address.

None for the pieces that own nothing (most of them).