iris: share resource handle bookkeeping

core/src/primitive/color.rs

@@ -1,10 +1,5 @@
-use crate::util::{Dirty, RefCounter};
-use std::{
-    cell::RefCell,
-    fmt,
-    rc::Rc,
-    sync::mpsc::{Receiver, Sender, channel},
-};
+use crate::util::{Dirty, Resources, StrongRscId};
+use std::{cell::RefCell, fmt, rc::Rc};
 
 /// Encoded, straight-alpha sRGB at an input boundary.
 ///
@@ -167,30 +162,7 @@ mod private {
     impl Sealed for super::PaintId {}
 }
 
-#[derive(Debug)]
-struct PaintLease {
-    slot: u32,
-    counter: RefCounter,
-    send: Sender<u32>,
-}
-
-impl Clone for PaintLease {
-    fn clone(&self) -> Self {
-        Self {
-            slot: self.slot,
-            counter: self.counter.clone(),
-            send: self.send.clone(),
-        }
-    }
-}
-
-impl Drop for PaintLease {
-    fn drop(&mut self) {
-        if self.counter.drop() {
-            let _ = self.send.send(self.slot);
-        }
-    }
-}
+struct PaintRsc;
 
 /// A stable reference to one entry in a UI's paint table.
 ///
@@ -200,7 +172,7 @@ impl Drop for PaintLease {
 #[derive(Clone, Debug)]
 pub struct PaintId {
     slot: u32,
-    lease: Option<PaintLease>,
+    strong: Option<StrongRscId<PaintRsc>>,
 }
 
 impl PaintId {
@@ -221,7 +193,7 @@ impl PaintId {
     pub const NONE: Self = Self::builtin(14);
 
     const fn builtin(slot: u32) -> Self {
-        Self { slot, lease: None }
+        Self { slot, strong: None }
     }
 
     pub(crate) fn slot(&self) -> u32 {
@@ -229,7 +201,7 @@ impl PaintId {
     }
 
     fn is_managed(&self) -> bool {
-        self.lease.is_some()
+        self.strong.is_some()
     }
 }
 
@@ -346,22 +318,22 @@ const BUILTIN_PAINTS: [Srgba8; 15] = [
 
 /// CPU-side paint table and the dirty set for its GPU mirror.
 pub struct Paints {
+    resources: Resources<PaintRsc>,
     entries: Vec<LinearRgba>,
-    free: Vec<u32>,
     dirty: Dirty,
-    send: Sender<u32>,
-    recv: Receiver<u32>,
 }
 
 impl Paints {
     pub fn new() -> Self {
-        let (send, recv) = channel();
+        let mut resources = Resources::new();
+        for (slot, _) in BUILTIN_PAINTS.iter().enumerate() {
+            let id = resources.add_static(PaintRsc);
+            assert_eq!(id.slot(), slot as u32);
+        }
         Self {
+            resources,
             entries: BUILTIN_PAINTS.map(Srgba8::to_linear).to_vec(),
-            free: Vec::new(),
             dirty: Dirty::new_all(),
-            send,
-            recv,
         }
     }
 
@@ -371,23 +343,19 @@ impl Paints {
 
     fn add_linear(&mut self, value: LinearRgba) -> PaintId {
         self.free_released();
-        let slot = if let Some(slot) = self.free.pop() {
+        let old_capacity = self.resources.capacity();
+        let strong = self.resources.add(PaintRsc);
+        let slot = strong.slot();
+        if (slot as usize) < old_capacity {
             self.entries[slot as usize] = value;
             self.dirty.mark(slot as usize);
-            slot
         } else {
-            let slot = self.entries.len() as u32;
             self.entries.push(value);
             self.dirty.mark(slot as usize);
-            slot
-        };
+        }
         PaintId {
             slot,
-            lease: Some(PaintLease {
-                slot,
-                counter: RefCounter::new(),
-                send: self.send.clone(),
-            }),
+            strong: Some(strong),
         }
     }
 
@@ -411,11 +379,13 @@ impl Paints {
     }
 
     pub fn free_released(&mut self) {
-        for slot in self.recv.try_iter() {
-            self.entries[slot as usize] = LinearRgba::NONE;
-            self.dirty.mark(slot as usize);
-            self.free.push(slot);
-        }
+        let entries = &mut self.entries;
+        let dirty = &mut self.dirty;
+        self.resources.apply(|id, _| {
+            let slot = id.slot();
+            entries[slot as usize] = LinearRgba::NONE;
+            dirty.mark(slot as usize);
+        });
     }
 
     /// A new GPU device has no copy of this table even when the CPU-side UI

core/src/primitive/text.rs

@@ -1,6 +1,6 @@
 use crate::{
     Align, GlyphAtlas, GlyphKey, PaintId, PlacedGlyph, RegionAlign, Textures, WidgetId,
-    util::{SlotId, SlotVec, Vec2},
+    util::{Resources, RscHandle, StrongRscId, Vec2, WeakRscId},
 };
 use parley::{
     Alignment, AlignmentOptions, FontContext, FontFamily, FontStyle, FontWeight, GenericFamily,
@@ -8,15 +8,12 @@ use parley::{
     fontique::{Blob, FontInfoOverride},
 };
 use std::{
-    cell::{Ref, RefCell},
+    cell::{Ref, RefCell, RefMut},
     collections::HashMap,
     fmt,
     ops::{Deref, DerefMut, Range},
     rc::Rc,
-    sync::{
-        Arc,
-        mpsc::{Receiver, Sender, channel},
-    },
+    sync::Arc,
 };
 use swash::{
     FontRef,
@@ -610,7 +607,7 @@ pub struct RenderedText {
     pub generation: u64,
 }
 
-pub struct TextResource {
+pub struct TextRsc {
     buffer: TextBuffer,
     attrs: TextAttrs,
     rendered: Option<RenderedText>,
@@ -619,7 +616,7 @@ pub struct TextResource {
     owner: Option<WidgetId>,
 }
 
-impl TextResource {
+impl TextRsc {
     fn new(buffer: TextBuffer, attrs: TextAttrs) -> Self {
         Self {
             buffer,
@@ -641,57 +638,54 @@ impl TextResource {
 /// compact [`TextHandle`] into this arena rather than owning Parley layouts.
 pub struct TextResources {
     data: TextData,
-    entries: SlotVec<TextResource>,
-    send: Sender<SlotId>,
-    recv: Receiver<SlotId>,
+    entries: Rc<RefCell<Resources<TextRsc>>>,
 }
 
 impl TextResources {
     pub fn new() -> Self {
-        let (send, recv) = channel();
         Self {
             data: TextData::default(),
-            entries: SlotVec::new(),
-            send,
-            recv,
+            entries: Rc::new(RefCell::new(Resources::new())),
         }
     }
 
     pub fn add(resources: Rc<RefCell<Self>>, buffer: TextBuffer, attrs: TextAttrs) -> TextHandle {
-        let (id, send) = {
+        let entries = {
             let mut resources = resources.borrow_mut();
             resources.free_released();
-            let id = resources.entries.add(TextResource::new(buffer, attrs));
-            (id, resources.send.clone())
+            resources.entries.clone()
         };
         TextHandle {
-            id,
+            rsc: RscHandle::add(entries, TextRsc::new(buffer, attrs)),
             resources,
-            send,
         }
     }
 
-    fn entry(&self, id: SlotId) -> &TextResource {
-        self.entries
-            .get(id)
-            .expect("text resource handle points at a released slot")
+    pub fn handle(resources: Rc<RefCell<Self>>, id: StrongRscId<TextRsc>) -> TextHandle {
+        let entries = resources.borrow().entries.clone();
+        TextHandle {
+            rsc: RscHandle::new(id, entries),
+            resources,
+        }
     }
 
-    fn entry_mut(&mut self, id: SlotId) -> &mut TextResource {
-        self.entries
-            .get_mut(id)
-            .expect("text resource handle points at a released slot")
+    pub fn upgrade(resources: Rc<RefCell<Self>>, id: WeakRscId<TextRsc>) -> Option<TextHandle> {
+        let entries = {
+            let mut resources = resources.borrow_mut();
+            resources.free_released();
+            resources.entries.clone()
+        };
+        let id = entries.borrow_mut().upgrade(id)?;
+        Some(Self::handle(resources, id))
     }
 
     pub fn free_released(&mut self) {
-        for id in self.recv.try_iter() {
-            self.entries.free(id);
-        }
+        self.entries.borrow_mut().apply(|_, _| {});
     }
 
     pub fn invalidate_all(&mut self) -> Vec<WidgetId> {
         let mut owners = Vec::new();
-        for resource in self.entries.values_mut() {
+        for resource in self.entries.borrow_mut().values_mut() {
             resource.invalidate();
             if let Some(owner) = resource.owner
                 && !owners.contains(&owner)
@@ -702,27 +696,22 @@ impl TextResources {
         owners
     }
 
-    fn shape(&mut self, id: SlotId) {
+    fn shape(&mut self, resource: &mut TextRsc) {
         let density = self.data.density;
-        let Self { data, entries, .. } = self;
-        let resource = entries
-            .get_mut(id)
-            .expect("text resource handle points at a released slot");
         resource
             .buffer
-            .shape(data, &resource.attrs, resource.width, density);
+            .shape(&mut self.data, &resource.attrs, resource.width, density);
     }
 
     fn render(
         &mut self,
-        id: SlotId,
+        resource: &mut TextRsc,
         width: Option<f32>,
         owner: WidgetId,
         textures: &mut Textures,
         density: f32,
     ) -> (RenderedText, bool) {
         let atlas_generation = self.data.atlas.generation();
-        let resource = self.entry_mut(id);
         resource.owner = Some(owner);
         if resource.width == width
             && resource.density == density
@@ -733,11 +722,7 @@ impl TextResources {
         }
         resource.width = width;
         resource.density = density;
-        let Self { data, entries, .. } = self;
-        let resource = entries
-            .get_mut(id)
-            .expect("text resource handle points at a released slot");
-        let rendered = data.render(
+        let rendered = self.data.render(
             &mut resource.buffer,
             &resource.attrs,
             width,
@@ -773,28 +758,30 @@ impl DerefMut for TextResources {
 /// shared once per UI; constructing a handle does not allocate a resource of
 /// its own.
 pub struct TextHandle {
-    id: SlotId,
+    rsc: RscHandle<TextRsc>,
     resources: Rc<RefCell<TextResources>>,
-    send: Sender<SlotId>,
 }
 
 impl TextHandle {
+    pub fn strong(&self) -> StrongRscId<TextRsc> {
+        self.rsc.strong()
+    }
+
+    pub fn weak(&self) -> WeakRscId<TextRsc> {
+        self.rsc.weak()
+    }
+
     pub fn text(&self) -> Ref<'_, str> {
-        Ref::map(self.resources.borrow(), |resources| {
-            resources.entry(self.id).buffer.text()
-        })
+        Ref::map(self.rsc.get(), |resource| resource.buffer.text())
     }
 
     pub fn attrs(&self) -> Ref<'_, TextAttrs> {
-        Ref::map(self.resources.borrow(), |resources| {
-            &resources.entry(self.id).attrs
-        })
+        Ref::map(self.rsc.get(), |resource| &resource.attrs)
     }
 
     pub fn set_text(&mut self, text: impl Into<String>) -> bool {
         let text = text.into();
-        let mut resources = self.resources.borrow_mut();
-        let resource = resources.entry_mut(self.id);
+        let mut resource = self.rsc.get_mut();
         if resource.buffer.text() == text {
             return false;
         }
@@ -804,46 +791,45 @@ impl TextHandle {
     }
 
     pub fn edit_text<R>(&mut self, edit: impl FnOnce(&mut String) -> R) -> R {
-        let mut resources = self.resources.borrow_mut();
-        let resource = resources.entry_mut(self.id);
+        let mut resource = self.rsc.get_mut();
         resource.rendered = None;
         edit(resource.buffer.edit())
     }
 
     pub fn set_spans(&mut self, spans: Vec<SpanStyle>) {
-        let mut resources = self.resources.borrow_mut();
-        let resource = resources.entry_mut(self.id);
+        let mut resource = self.rsc.get_mut();
         resource.buffer.set_spans(spans);
         resource.rendered = None;
     }
 
-    pub fn update_attrs<R>(&mut self, update: impl FnOnce(&mut TextAttrs) -> R) -> R {
-        let mut resources = self.resources.borrow_mut();
-        let resource = resources.entry_mut(self.id);
+    pub fn attrs_mut(&mut self) -> RefMut<'_, TextAttrs> {
+        let mut resource = self.rsc.get_mut();
         resource.invalidate();
-        update(&mut resource.attrs)
+        RefMut::map(resource, |resource| &mut resource.attrs)
     }
 
     pub fn rendered(&self) -> Option<RenderedText> {
-        self.resources.borrow().entry(self.id).rendered.clone()
+        self.rsc.get().rendered.clone()
     }
 
     pub fn width(&self) -> Option<f32> {
-        self.resources.borrow().entry(self.id).width
+        self.rsc.get().width
     }
 
     pub fn with_layout<R>(&self, f: impl FnOnce(&Layout<PaintId>, &str) -> R) -> R {
         let mut resources = self.resources.borrow_mut();
-        resources.shape(self.id);
-        let resource = resources.entry(self.id);
+        let mut resource = self.rsc.get_mut_shared();
+        resources.shape(&mut resource);
         f(resource.buffer.layout(), resource.buffer.text())
     }
 
     pub fn layout(&self) -> Ref<'_, Layout<PaintId>> {
-        self.resources.borrow_mut().shape(self.id);
-        Ref::map(self.resources.borrow(), |resources| {
-            resources.entry(self.id).buffer.layout()
-        })
+        {
+            let mut resources = self.resources.borrow_mut();
+            let mut resource = self.rsc.get_mut_shared();
+            resources.shape(&mut resource);
+        }
+        Ref::map(self.rsc.get(), |resource| resource.buffer.layout())
     }
 
     pub fn render(
@@ -853,15 +839,9 @@ impl TextHandle {
         textures: &mut Textures,
         density: f32,
     ) -> (RenderedText, bool) {
-        self.resources
-            .borrow_mut()
-            .render(self.id, width, owner, textures, density)
-    }
-}
-
-impl Drop for TextHandle {
-    fn drop(&mut self) {
-        let _ = self.send.send(self.id);
+        let mut resources = self.resources.borrow_mut();
+        let mut resource = self.rsc.get_mut_shared();
+        resources.render(&mut resource, width, owner, textures, density)
     }
 }
 
@@ -886,11 +866,11 @@ mod tests {
             TextBuffer::new("temporary"),
             TextAttrs::default(),
         );
-        assert_eq!(resources.borrow().entries.len(), 1);
+        assert_eq!(resources.borrow().entries.borrow().len(), 1);
 
         drop(text);
         resources.borrow_mut().free_released();
 
-        assert!(resources.borrow().entries.is_empty());
+        assert!(resources.borrow().entries.borrow().is_empty());
     }
 }

core/src/primitive/texture.rs

@@ -1,10 +1,6 @@
-use crate::util::{RefCounter, Vec2};
+use crate::util::{Resources, RscHandle, StrongRscId, Vec2, WeakRscId};
 use image::{DynamicImage, GenericImageView};
-use std::{
-    collections::HashMap,
-    ops::Index,
-    sync::mpsc::{Receiver, Sender, channel},
-};
+use std::{cell::RefCell, collections::HashMap, ops::Index, rc::Rc};
 
 /// Which of the two things a texture slot holds. See TEXTURES.md's
 /// "Recommended shape" for why these are drawn so differently: a page is a
@@ -28,19 +24,20 @@ pub struct SharedTextureKey {
     pub id: u64,
 }
 
-#[derive(Debug, Clone)]
-pub struct TextureHandle {
-    slot: u32,
+pub struct TextureRsc {
     kind: TextureKind,
     size: Vec2,
-    counter: RefCounter,
-    send: Sender<(TextureKind, u32)>,
+}
+
+#[derive(Debug, Clone)]
+pub struct TextureHandle {
+    rsc: RscHandle<TextureRsc>,
 }
 
 /// a texture manager for a ui
 /// note that this is heavily oriented towards wgpu's renderer so the primitives don't need mapped
 pub struct Textures {
-    free: Vec<u32>,
+    resources: Rc<RefCell<Resources<TextureRsc>>>,
     images: Vec<Option<DynamicImage>>,
     kinds: Vec<TextureKind>,
     /// Textures built from a description rather than from a file, one per
@@ -48,12 +45,10 @@ pub struct Textures {
     /// reference of its own, so a shared texture outlives every widget
     /// drawing it and its slot is never recycled underneath one.
     shared: HashMap<SharedTextureKey, TextureHandle>,
-    /// Next layer to hand out to an atlas page. Pages are never freed (no
-    /// atlas eviction), so this only grows and `free` never holds one.
+    /// Next layer to hand out to an atlas page. Page layers and resource slots
+    /// are never reused, even if an explicit atlas clear drops their handles.
     next_page_layer: u32,
     updates: Vec<Update>,
-    send: Sender<(TextureKind, u32)>,
-    recv: Receiver<(TextureKind, u32)>,
 }
 
 pub enum TextureUpdate<'a> {
@@ -86,16 +81,13 @@ enum Update {
 
 impl Textures {
     pub fn new() -> Self {
-        let (send, recv) = channel();
         Self {
-            free: Vec::new(),
+            resources: Rc::new(RefCell::new(Resources::new())),
             images: Vec::new(),
             kinds: Vec::new(),
             shared: HashMap::new(),
             next_page_layer: 0,
             updates: Vec::new(),
-            send,
-            recv,
         }
     }
 
@@ -103,14 +95,7 @@ impl Textures {
         let image = image.into();
         let size = image.dimensions().into();
         let kind = TextureKind::Image;
-        let slot = self.push(kind, image);
-        TextureHandle {
-            slot,
-            kind,
-            size,
-            counter: RefCounter::new(),
-            send: self.send.clone(),
-        }
+        self.push(kind, size, image, true)
     }
 
     pub fn add_page(&mut self, image: impl Into<DynamicImage>) -> TextureHandle {
@@ -119,28 +104,49 @@ impl Textures {
         let layer = self.next_page_layer;
         self.next_page_layer += 1;
         let kind = TextureKind::Page { layer };
-        let slot = self.push(kind, image);
+        self.push(kind, size, image, false)
+    }
+
+    pub fn handle(&self, id: StrongRscId<TextureRsc>) -> TextureHandle {
         TextureHandle {
-            slot,
-            kind,
-            size,
-            counter: RefCounter::new(),
-            send: self.send.clone(),
+            rsc: RscHandle::new(id, self.resources.clone()),
         }
     }
 
-    fn push(&mut self, kind: TextureKind, image: DynamicImage) -> u32 {
-        if let Some(i) = self.free.pop() {
+    pub fn upgrade(&mut self, id: WeakRscId<TextureRsc>) -> Option<TextureHandle> {
+        self.free();
+        let id = self.resources.borrow_mut().upgrade(id)?;
+        Some(self.handle(id))
+    }
+
+    fn push(
+        &mut self,
+        kind: TextureKind,
+        size: Vec2,
+        image: DynamicImage,
+        recycle: bool,
+    ) -> TextureHandle {
+        self.free();
+        let old_capacity = self.resources.borrow().capacity();
+        let id = if recycle {
+            self.resources.borrow_mut().add(TextureRsc { kind, size })
+        } else {
+            self.resources
+                .borrow_mut()
+                .add_unrecycled(TextureRsc { kind, size })
+        };
+        let i = id.slot();
+        if (i as usize) < old_capacity {
             self.images[i as usize] = Some(image);
             self.kinds[i as usize] = kind;
             self.updates.push(Update::Set(kind, i));
-            i
         } else {
-            let i = self.images.len() as u32;
             self.images.push(Some(image));
             self.kinds.push(kind);
             self.updates.push(Update::Push(kind, i));
-            i
+        }
+        TextureHandle {
+            rsc: RscHandle::new(id, self.resources.clone()),
         }
     }
 
@@ -161,13 +167,14 @@ impl Textures {
     }
 
     pub fn image_mut(&mut self, handle: &TextureHandle) -> &mut DynamicImage {
-        self.images[handle.slot as usize]
+        self.images[handle.rsc.id().slot() as usize]
             .as_mut()
             .expect("texture was freed while still held")
     }
 
     pub fn patch(&mut self, handle: &TextureHandle, rect: PatchRect) {
-        self.updates.push(Update::Patch(handle.slot, rect));
+        self.updates
+            .push(Update::Patch(handle.rsc.id().slot(), rect));
     }
 
     /// A new device starts with no textures, and the renderer-side mirror
@@ -185,24 +192,20 @@ impl Textures {
     /// ones after them still land where they were.
     pub fn reupload(&mut self) {
         self.updates.clear();
-        self.updates
-            .extend((0..self.images.len() as u32).map(|i| Update::Push(self.kinds[i as usize], i)));
+        self.updates.extend(
+            (0..self.resources.borrow().capacity() as u32)
+                .map(|i| Update::Push(self.kinds[i as usize], i)),
+        );
     }
 
     pub fn free(&mut self) {
-        for (kind, idx) in self.recv.try_iter() {
-            self.images[idx as usize] = None;
-            self.updates.push(Update::Free(idx));
-            // A page's slot is never reclaimed: `GlyphAtlas` never drops the
-            // handles it holds, and there is no eviction path for a hole in
-            // the middle of the array's layers. If that ever changes, this
-            // is where a freed page's layer would need to go on a free list
-            // of its own, separate from `free`, which only ever holds
-            // ordinary image slots today.
-            if kind == TextureKind::Image {
-                self.free.push(idx);
-            }
-        }
+        let updates = &mut self.updates;
+        let images = &mut self.images;
+        self.resources.borrow_mut().apply(|id, _| {
+            let idx = id.slot();
+            images[idx as usize] = None;
+            updates.push(Update::Free(idx));
+        });
     }
 
     pub fn updates(&mut self) -> impl Iterator<Item = TextureUpdate<'_>> {
@@ -226,7 +229,7 @@ impl Textures {
 
 impl TextureHandle {
     pub fn size(&self) -> Vec2 {
-        self.size
+        self.rsc.get().size
     }
 
     /// The bind-group index this handle draws with. Only valid for a
@@ -235,25 +238,25 @@ impl TextureHandle {
     /// a caller bug (the wrong kind of handle reached the wrong draw path),
     /// not a recoverable condition, so it panics rather than drawing garbage.
     pub fn image_index(&self) -> u32 {
-        match self.kind {
-            TextureKind::Image => self.slot,
+        match self.rsc.get().kind {
+            TextureKind::Image => self.rsc.id().slot(),
             TextureKind::Page { .. } => panic!("image_index() called on an atlas page handle"),
         }
     }
 
     pub fn layer(&self) -> u32 {
-        match self.kind {
+        match self.rsc.get().kind {
             TextureKind::Page { layer } => layer,
             TextureKind::Image => panic!("layer() called on a standalone image handle"),
         }
     }
-}
 
-impl Drop for TextureHandle {
-    fn drop(&mut self) {
-        if self.counter.drop() {
-            let _ = self.send.send((self.kind, self.slot));
-        }
+    pub fn strong(&self) -> StrongRscId<TextureRsc> {
+        self.rsc.strong()
+    }
+
+    pub fn weak(&self) -> WeakRscId<TextureRsc> {
+        self.rsc.weak()
     }
 }
 
@@ -261,7 +264,9 @@ impl Index<&TextureHandle> for Textures {
     type Output = DynamicImage;
 
     fn index(&self, index: &TextureHandle) -> &Self::Output {
-        self.images[index.slot as usize].as_ref().unwrap()
+        self.images[index.rsc.id().slot() as usize]
+            .as_ref()
+            .unwrap()
     }
 }
 
@@ -315,6 +320,19 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_released_atlas_page_slot_is_not_reused_by_an_image() {
+        let mut textures = Textures::new();
+        let page = textures.add_page(image(4));
+        let page_slot = page.rsc.id().slot();
+        drop(page);
+        textures.free();
+
+        let plain = textures.add(image(4));
+
+        assert_ne!(plain.image_index(), page_slot);
+    }
+
     #[test]
     fn reupload_replays_every_slot_in_order_including_the_empty_ones() {
         let mut textures = Textures::new();

core/src/render/atlas.rs

@@ -81,14 +81,13 @@ impl GlyphAtlas {
         let rgba = img.as_mut_rgba8().expect("atlas page is rgba8");
         write_glyph(rgba, image, x, y);
 
-        let handle = page.handle.clone();
         let rect = PatchRect {
             x,
             y,
             width: w,
             height: h,
         };
-        textures.patch(&handle, rect);
+        textures.patch(&page.handle, rect);
 
         let page = &self.pages[page_idx];
         let scale = 1.0 / PAGE as f32;

core/src/ui/mod.rs

@@ -182,8 +182,8 @@ pub trait UiRsc {
         while let Some(id) = self.widgets_mut().free_next() {
             self.on_remove(id);
         }
+        self.ui_mut().text.borrow_mut().free_released();
         self.ui_mut().textures.free();
         self.ui_mut().paints.free_released();
-        self.ui_mut().text.borrow_mut().free_released();
     }
 }

core/src/util/mod.rs

@@ -5,6 +5,7 @@ mod dirty;
 mod id;
 mod math;
 mod refcount;
+mod resources;
 mod slot;
 mod trust;
 mod typemap;
@@ -17,6 +18,7 @@ pub use dirty::*;
 pub use id::*;
 pub use math::*;
 pub use refcount::*;
+pub use resources::*;
 pub use slot::*;
 pub use trust::*;
 pub use typemap::*;

core/src/util/resources.rs

@@ -0,0 +1,414 @@
+use super::{SlotId, SlotVec};
+use std::{
+    cell::{Ref, RefCell, RefMut},
+    fmt,
+    hash::{Hash, Hasher},
+    marker::PhantomData,
+    rc::Rc,
+    sync::mpsc::{Receiver, Sender, channel},
+};
+
+enum Event {
+    Clone(SlotId),
+    Drop(SlotId),
+}
+
+struct Entry<T> {
+    value: T,
+    strong: Option<u32>,
+    recycle: bool,
+}
+
+/// Generational storage and deferred strong-reference accounting for one kind
+/// of UI resource.
+pub struct Resources<T> {
+    entries: SlotVec<Entry<T>>,
+    send: Sender<Event>,
+    recv: Receiver<Event>,
+}
+
+impl<T> Resources<T> {
+    pub fn new() -> Self {
+        let (send, recv) = channel();
+        Self {
+            entries: SlotVec::new(),
+            send,
+            recv,
+        }
+    }
+
+    pub fn add(&mut self, value: T) -> StrongRscId<T> {
+        self.add_with(value, true)
+    }
+
+    pub fn add_unrecycled(&mut self, value: T) -> StrongRscId<T> {
+        self.add_with(value, false)
+    }
+
+    fn add_with(&mut self, value: T, recycle: bool) -> StrongRscId<T> {
+        let id = self.entries.add(Entry {
+            value,
+            strong: Some(1),
+            recycle,
+        });
+        StrongRscId::new(id, self.send.clone())
+    }
+
+    /// Add an entry whose lifetime is the lifetime of the arena itself.
+    pub fn add_static(&mut self, value: T) -> WeakRscId<T> {
+        WeakRscId::new(self.entries.add(Entry {
+            value,
+            strong: None,
+            recycle: false,
+        }))
+    }
+
+    pub fn apply(&mut self, mut dropped: impl FnMut(SlotId, T)) {
+        for event in self.recv.try_iter() {
+            match event {
+                Event::Clone(id) => {
+                    let entry = self
+                        .entries
+                        .get_mut(id)
+                        .expect("cloned resource id points at a released slot");
+                    let strong = entry
+                        .strong
+                        .as_mut()
+                        .expect("a static resource cannot have a strong id");
+                    *strong = strong.checked_add(1).expect("resource reference overflow");
+                }
+                Event::Drop(id) => {
+                    let remove = {
+                        let entry = self
+                            .entries
+                            .get_mut(id)
+                            .expect("dropped resource id points at a released slot");
+                        let strong = entry
+                            .strong
+                            .as_mut()
+                            .expect("a static resource cannot have a strong id");
+                        *strong = strong.checked_sub(1).expect("resource reference underflow");
+                        *strong == 0
+                    };
+                    if remove {
+                        let recycle = self.entries.get(id).unwrap().recycle;
+                        let entry = if recycle {
+                            self.entries.remove(id)
+                        } else {
+                            self.entries.remove_unrecycled(id)
+                        }
+                        .unwrap();
+                        dropped(id, entry.value);
+                    }
+                }
+            }
+        }
+    }
+
+    /// The owning manager must call [`Self::apply`] first so a queued final
+    /// drop cannot be mistaken for a still-live entry.
+    pub fn upgrade(&mut self, id: WeakRscId<T>) -> Option<StrongRscId<T>> {
+        let entry = self.entries.get_mut(id.id)?;
+        let strong = entry.strong.as_mut()?;
+        *strong = strong.checked_add(1).expect("resource reference overflow");
+        Some(StrongRscId::new(id.id, self.send.clone()))
+    }
+
+    pub fn get(&self, id: impl RscId<T>) -> Option<&T> {
+        Some(&self.entries.get(id.rsc_id())?.value)
+    }
+
+    pub fn get_mut(&mut self, id: impl RscId<T>) -> Option<&mut T> {
+        Some(&mut self.entries.get_mut(id.rsc_id())?.value)
+    }
+
+    pub fn values_mut(&mut self) -> impl Iterator<Item = &mut T> {
+        self.entries.values_mut().map(|entry| &mut entry.value)
+    }
+
+    pub fn capacity(&self) -> usize {
+        self.entries.capacity()
+    }
+
+    pub fn len(&self) -> usize {
+        self.entries.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+}
+
+impl<T> Default for Resources<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+pub trait RscId<T> {
+    fn rsc_id(&self) -> SlotId;
+}
+
+/// A sendable owning ID for one entry in [`Resources`]. It keeps the entry
+/// alive but does not provide access to its value.
+pub struct StrongRscId<T> {
+    id: SlotId,
+    send: Sender<Event>,
+    ty: PhantomData<fn() -> T>,
+}
+
+impl<T> StrongRscId<T> {
+    fn new(id: SlotId, send: Sender<Event>) -> Self {
+        Self {
+            id,
+            send,
+            ty: PhantomData,
+        }
+    }
+
+    pub fn weak(&self) -> WeakRscId<T> {
+        WeakRscId::new(self.id)
+    }
+
+    pub fn id(&self) -> SlotId {
+        self.id
+    }
+
+    pub(crate) fn slot(&self) -> u32 {
+        self.id.slot()
+    }
+}
+
+impl<T> Clone for StrongRscId<T> {
+    fn clone(&self) -> Self {
+        let _ = self.send.send(Event::Clone(self.id));
+        Self::new(self.id, self.send.clone())
+    }
+}
+
+impl<T> Drop for StrongRscId<T> {
+    fn drop(&mut self) {
+        let _ = self.send.send(Event::Drop(self.id));
+    }
+}
+
+impl<T> RscId<T> for StrongRscId<T> {
+    fn rsc_id(&self) -> SlotId {
+        self.id
+    }
+}
+
+impl<T> RscId<T> for &StrongRscId<T> {
+    fn rsc_id(&self) -> SlotId {
+        self.id
+    }
+}
+
+impl<T> fmt::Debug for StrongRscId<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.id.fmt(f)
+    }
+}
+
+impl<T> PartialEq for StrongRscId<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.id == other.id
+    }
+}
+
+impl<T> Eq for StrongRscId<T> {}
+
+impl<T> Hash for StrongRscId<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.id.hash(state);
+    }
+}
+
+/// A sendable, copyable ID that does not keep its [`Resources`] entry alive.
+pub struct WeakRscId<T> {
+    id: SlotId,
+    ty: PhantomData<fn() -> T>,
+}
+
+impl<T> WeakRscId<T> {
+    fn new(id: SlotId) -> Self {
+        Self {
+            id,
+            ty: PhantomData,
+        }
+    }
+
+    pub fn id(self) -> SlotId {
+        self.id
+    }
+
+    pub(crate) fn slot(self) -> u32 {
+        self.id.slot()
+    }
+}
+
+impl<T> Clone for WeakRscId<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<T> Copy for WeakRscId<T> {}
+
+impl<T> RscId<T> for WeakRscId<T> {
+    fn rsc_id(&self) -> SlotId {
+        self.id
+    }
+}
+
+impl<T> RscId<T> for &WeakRscId<T> {
+    fn rsc_id(&self) -> SlotId {
+        self.id
+    }
+}
+
+impl<T> fmt::Debug for WeakRscId<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.id.fmt(f)
+    }
+}
+
+impl<T> PartialEq for WeakRscId<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.id == other.id
+    }
+}
+
+impl<T> Eq for WeakRscId<T> {}
+
+impl<T> Hash for WeakRscId<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.id.hash(state);
+    }
+}
+
+/// Convenient UI-thread access to a resource through an owning ID. The `Rc`
+/// deliberately makes this local; tasks carry strong or weak IDs instead.
+pub struct RscHandle<T> {
+    id: StrongRscId<T>,
+    resources: Rc<RefCell<Resources<T>>>,
+}
+
+impl<T> RscHandle<T> {
+    pub fn new(id: StrongRscId<T>, resources: Rc<RefCell<Resources<T>>>) -> Self {
+        Self { id, resources }
+    }
+
+    pub fn add(resources: Rc<RefCell<Resources<T>>>, value: T) -> Self {
+        let id = resources.borrow_mut().add(value);
+        Self::new(id, resources)
+    }
+
+    pub fn get(&self) -> Ref<'_, T> {
+        Ref::map(self.resources.borrow(), |resources| {
+            resources
+                .get(&self.id)
+                .expect("resource handle points at a released slot")
+        })
+    }
+
+    pub fn get_mut(&mut self) -> RefMut<'_, T> {
+        RefMut::map(self.resources.borrow_mut(), |resources| {
+            resources
+                .get_mut(&self.id)
+                .expect("resource handle points at a released slot")
+        })
+    }
+
+    pub(crate) fn get_mut_shared(&self) -> RefMut<'_, T> {
+        RefMut::map(self.resources.borrow_mut(), |resources| {
+            resources
+                .get_mut(&self.id)
+                .expect("resource handle points at a released slot")
+        })
+    }
+
+    pub fn strong(&self) -> StrongRscId<T> {
+        self.id.clone()
+    }
+
+    pub fn weak(&self) -> WeakRscId<T> {
+        self.id.weak()
+    }
+
+    pub fn id(&self) -> SlotId {
+        self.id.id()
+    }
+}
+
+impl<T> Clone for RscHandle<T> {
+    fn clone(&self) -> Self {
+        Self::new(self.id.clone(), self.resources.clone())
+    }
+}
+
+impl<T> fmt::Debug for RscHandle<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.id.fmt(f)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_slot_lives_until_every_strong_id_is_dropped() {
+        let mut resources = Resources::new();
+        let first = resources.add("value");
+        let weak = first.weak();
+        let second = first.clone();
+        drop(first);
+        resources.apply(|_, _| {});
+        assert_eq!(resources.get(weak), Some(&"value"));
+
+        drop(second);
+        resources.apply(|_, _| {});
+        assert_eq!(resources.get(weak), None);
+    }
+
+    #[test]
+    fn a_weak_id_can_be_upgraded_while_the_resource_is_alive() {
+        let mut resources = Resources::new();
+        let first = resources.add("value");
+        let weak = first.weak();
+        let second = resources.upgrade(weak).unwrap();
+        drop(first);
+        resources.apply(|_, _| {});
+        assert_eq!(resources.get(&second), Some(&"value"));
+    }
+
+    #[test]
+    fn strong_and_weak_ids_can_cross_threads() {
+        fn assert_send<T: Send>() {}
+        assert_send::<StrongRscId<String>>();
+        assert_send::<WeakRscId<String>>();
+
+        let mut resources = Resources::new();
+        let first = resources.add("value");
+        let second = first.clone();
+        std::thread::spawn(move || drop(second)).join().unwrap();
+        drop(first);
+        resources.apply(|_, _| {});
+        assert!(resources.is_empty());
+    }
+
+    #[test]
+    fn an_unrecycled_slot_stays_a_hole() {
+        let mut resources = Resources::new();
+        let first = resources.add_unrecycled("first");
+        let first_slot = first.slot();
+        drop(first);
+        resources.apply(|_, _| {});
+
+        let second = resources.add("second");
+
+        assert_ne!(second.slot(), first_slot);
+        assert_eq!(resources.len(), 1);
+    }
+}

core/src/util/slot.rs

@@ -5,6 +5,10 @@ pub struct SlotId {
 }
 
 impl SlotId {
+    pub(crate) fn slot(self) -> u32 {
+        self.idx
+    }
+
     /// A stable, collision-free `u64` encoding of this id -- for a caller
     /// (accesskit's `NodeId`, today) that wants a flat integer key rather
     /// than the two `u32`s. `idx` is offset by one so no real id ever
@@ -18,6 +22,7 @@ impl SlotId {
 pub struct SlotVec<T> {
     data: Vec<(u32, Option<T>)>,
     free: Vec<u32>,
+    len: usize,
 }
 
 impl<T> SlotVec<T> {
@@ -25,11 +30,12 @@ impl<T> SlotVec<T> {
         Self {
             data: Default::default(),
             free: Default::default(),
+            len: 0,
         }
     }
 
     pub fn add(&mut self, x: T) -> SlotId {
-        if let Some(idx) = self.free.pop() {
+        let id = if let Some(idx) = self.free.pop() {
             let (genr, data) = &mut self.data[idx as usize];
             *data = Some(x);
             SlotId { idx, genr: *genr }
@@ -38,14 +44,35 @@ impl<T> SlotVec<T> {
             let genr = 0;
             self.data.push((genr, Some(x)));
             SlotId { idx, genr }
-        }
+        };
+        self.len += 1;
+        id
     }
 
     pub fn free(&mut self, id: SlotId) {
+        let _ = self.remove(id);
+    }
+
+    pub fn remove(&mut self, id: SlotId) -> Option<T> {
+        self.remove_inner(id, true)
+    }
+
+    pub fn remove_unrecycled(&mut self, id: SlotId) -> Option<T> {
+        self.remove_inner(id, false)
+    }
+
+    fn remove_inner(&mut self, id: SlotId, recycle: bool) -> Option<T> {
         let (genr, data) = &mut self.data[id.idx as usize];
+        if *genr != id.genr {
+            return None;
+        }
         *genr += 1;
-        *data = None;
-        self.free.push(id.idx);
+        let value = data.take()?;
+        self.len -= 1;
+        if recycle {
+            self.free.push(id.idx);
+        }
+        Some(value)
     }
 
     pub fn get(&self, id: SlotId) -> Option<&T> {
@@ -65,7 +92,7 @@ impl<T> SlotVec<T> {
     }
 
     pub fn len(&self) -> usize {
-        self.data.len() - self.free.len()
+        self.len
     }
 
     pub fn is_empty(&self) -> bool {
@@ -75,6 +102,10 @@ impl<T> SlotVec<T> {
     pub fn values_mut(&mut self) -> impl Iterator<Item = &mut T> {
         self.data.iter_mut().filter_map(|(_, value)| value.as_mut())
     }
+
+    pub fn capacity(&self) -> usize {
+        self.data.len()
+    }
 }
 
 impl<T> Default for SlotVec<T> {

src/widget/text/mod.rs

@@ -117,8 +117,8 @@ impl TextView {
         self.text.text().to_string()
     }
 
-    pub fn update_attrs<R>(&mut self, update: impl FnOnce(&mut TextAttrs) -> R) -> R {
-        self.text.update_attrs(update)
+    pub fn attrs_mut(&mut self) -> std::cell::RefMut<'_, TextAttrs> {
+        self.text.attrs_mut()
     }
 }
 
@@ -147,8 +147,8 @@ impl Text {
         self.view.content()
     }
 
-    pub fn update_attrs<R>(&mut self, update: impl FnOnce(&mut TextAttrs) -> R) -> R {
-        self.view.update_attrs(update)
+    pub fn attrs_mut(&mut self) -> std::cell::RefMut<'_, TextAttrs> {
+        self.view.attrs_mut()
     }
 }