i_slint_core/

partial_renderer.rs

// Copyright © SixtyFPS GmbH <info@slint.dev>
// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0

//! Module for a renderer proxy that tries to render only the parts of the tree that have changed.
//!
//! This is the way the partial renderer work:
//!
//! 1. [`PartialRenderer::compute_dirty_regions`] will go over the items and try to compute the region that needs to be repainted.
//!    If either the bounding box has changed, or the PropertyTracker that tracks the rendering properties is dirty, then the
//!    region is marked dirty.
//!    That pass also register dependencies on every geometry, and on the non-dirty property trackers.
//! 2. The Renderer calls [`PartialRenderer::filter_item`] For most items.
//!    This assume that the cached geometry was requested in the previous step. So it will not register new dependencies.
//! 3. Then the renderer calls the rendering function for each item that needs to be rendered.
//!    This register dependencies only on the rendering tracker.
//!

use crate::Coord;
use crate::item_rendering::{
    ItemRenderer, ItemRendererFeatures, RenderBorderRectangle, RenderImage, RenderRectangle,
    RenderText,
};
use crate::item_tree::{ItemTreeRc, ItemTreeWeak, ItemVisitorResult};
#[cfg(feature = "path")]
use crate::items::Path;
use crate::items::{BoxShadow, Clip, ItemRc, ItemRef, Layer, Opacity, RenderingResult, TextInput};
use crate::lengths::{
    ItemTransform, LogicalBorderRadius, LogicalLength, LogicalPoint, LogicalPx, LogicalRect,
    LogicalSize, LogicalVector,
};
use crate::properties::PropertyTracker;
use crate::window::WindowAdapter;
use alloc::boxed::Box;
use alloc::rc::Rc;
use core::cell::{Cell, RefCell};
use core::pin::Pin;

/// This structure must be present in items that are Rendered and contains information.
/// Used by the backend.
#[derive(Default, Debug)]
#[repr(C)]
pub struct CachedRenderingData {
    /// Used and modified by the backend, should be initialized to 0 by the user code
    pub(crate) cache_index: Cell<usize>,
    /// Used and modified by the backend, should be initialized to 0 by the user code.
    /// The backend compares this generation against the one of the cache to verify
    /// the validity of the cache_index field.
    pub(crate) cache_generation: Cell<usize>,
}

impl CachedRenderingData {
    /// This function can be used to remove an entry from the rendering cache for a given item, if it
    /// exists, i.e. if any data was ever cached. This is typically called by the graphics backend's
    /// implementation of the release_item_graphics_cache function.
    fn release(
        &self,
        cache: &mut PartialRendererCache,
    ) -> Option<CachedItemBoundingBoxAndTransform> {
        if self.cache_generation.get() == cache.generation() {
            let index = self.cache_index.get();
            self.cache_generation.set(0);
            Some(cache.remove(index).data)
        } else {
            None
        }
    }

    /// Return the value if it is in the cache
    fn get_entry<'a>(
        &self,
        cache: &'a mut PartialRendererCache,
    ) -> Option<&'a mut PartialRenderingCachedData> {
        let index = self.cache_index.get();
        if self.cache_generation.get() == cache.generation() { cache.get_mut(index) } else { None }
    }
}

/// After rendering an item, we cache the geometry and the transform it applies to
/// children.
#[derive(Clone, PartialEq)]
pub enum CachedItemBoundingBoxAndTransform {
    /// A regular item with a translation
    RegularItem {
        /// The item's bounding rect relative to its parent.
        bounding_rect: LogicalRect,
        /// The item's offset relative to its parent.
        offset: LogicalVector,
    },
    /// An item such as Rotate that defines an additional transformation
    ItemWithTransform {
        /// The item's bounding rect relative to its parent.
        bounding_rect: LogicalRect,
        /// The item's transform to apply to children.
        transform: Box<ItemTransform>,
    },
    /// A clip item.
    ClipItem {
        /// The item's geometry relative to its parent.
        geometry: LogicalRect,
    },
}

impl CachedItemBoundingBoxAndTransform {
    fn bounding_rect(&self) -> &LogicalRect {
        match self {
            CachedItemBoundingBoxAndTransform::RegularItem { bounding_rect, .. } => bounding_rect,
            CachedItemBoundingBoxAndTransform::ItemWithTransform { bounding_rect, .. } => {
                bounding_rect
            }
            CachedItemBoundingBoxAndTransform::ClipItem { geometry } => geometry,
        }
    }

    fn transform(&self) -> ItemTransform {
        match self {
            CachedItemBoundingBoxAndTransform::RegularItem { offset, .. } => {
                ItemTransform::translation(offset.x as f32, offset.y as f32)
            }
            CachedItemBoundingBoxAndTransform::ItemWithTransform { transform, .. } => **transform,
            CachedItemBoundingBoxAndTransform::ClipItem { geometry } => {
                ItemTransform::translation(geometry.origin.x as f32, geometry.origin.y as f32)
            }
        }
    }

    fn new<T: ItemRendererFeatures>(
        item_rc: &ItemRc,
        window_adapter: &Rc<dyn WindowAdapter>,
    ) -> Self {
        let geometry = item_rc.geometry();

        if item_rc.borrow().as_ref().clips_children() {
            return Self::ClipItem { geometry };
        }

        // Evaluate the bounding rect untracked, as properties that affect the bounding rect are already tracked
        // at rendering time.
        let bounding_rect = crate::properties::evaluate_no_tracking(|| {
            item_rc.bounding_rect(&geometry, window_adapter)
        });

        if let Some(complex_child_transform) = (T::SUPPORTS_TRANSFORMATIONS
            && window_adapter.renderer().supports_transformations())
        .then(|| item_rc.children_transform())
        .flatten()
        {
            Self::ItemWithTransform {
                bounding_rect,
                transform: complex_child_transform
                    .then_translate(geometry.origin.to_vector().cast())
                    .into(),
            }
        } else {
            Self::RegularItem { bounding_rect, offset: geometry.origin.to_vector() }
        }
    }
}

struct PartialRenderingCachedData {
    /// The geometry of the item as it was previously rendered.
    pub data: CachedItemBoundingBoxAndTransform,
    /// The property tracker that should be used to evaluate whether the item needs to be re-rendered
    pub tracker: Option<core::pin::Pin<Box<PropertyTracker>>>,
}
impl PartialRenderingCachedData {
    fn new(data: CachedItemBoundingBoxAndTransform) -> Self {
        Self { data, tracker: None }
    }
}

/// The cache that needs to be held by the Window for the partial rendering
struct PartialRendererCache {
    slab: slab::Slab<PartialRenderingCachedData>,
    generation: usize,
}

impl Default for PartialRendererCache {
    fn default() -> Self {
        Self { slab: Default::default(), generation: 1 }
    }
}

impl PartialRendererCache {
    /// Returns the generation of the cache. The generation starts at 1 and is increased
    /// whenever the cache is cleared, for example when the GL context is lost.
    pub fn generation(&self) -> usize {
        self.generation
    }

    /// Retrieves a mutable reference to the cached graphics data at index.
    pub fn get_mut(&mut self, index: usize) -> Option<&mut PartialRenderingCachedData> {
        self.slab.get_mut(index)
    }

    /// Inserts data into the cache and returns the index for retrieval later.
    pub fn insert(&mut self, data: PartialRenderingCachedData) -> usize {
        self.slab.insert(data)
    }

    /// Removes the cached graphics data at the given index.
    pub fn remove(&mut self, index: usize) -> PartialRenderingCachedData {
        self.slab.remove(index)
    }

    /// Removes all entries from the cache and increases the cache's generation count, so
    /// that stale index access can be avoided.
    pub fn clear(&mut self) {
        self.slab.clear();
        self.generation += 1;
    }
}

/// A region composed of a few rectangles that need to be redrawn.
#[derive(Default, Clone)]
pub struct DirtyRegion {
    rectangles: [euclid::Box2D<Coord, LogicalPx>; Self::MAX_COUNT],
    count: usize,
}

impl core::fmt::Debug for DirtyRegion {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(f, "{:?}", &self.rectangles[..self.count])
    }
}

impl DirtyRegion {
    /// The maximum number of rectangles that can be stored in a DirtyRegion
    pub const MAX_COUNT: usize = 3;

    /// An iterator over the part of the region (they can overlap)
    pub fn iter(&self) -> impl Iterator<Item = euclid::Box2D<Coord, LogicalPx>> + '_ {
        (0..self.count).map(|x| self.rectangles[x])
    }

    /// Add a rectangle to the region.
    ///
    /// Note that if the region becomes too complex, it might be simplified by being bigger than the actual union.
    pub fn add_rect(&mut self, rect: LogicalRect) {
        self.add_box(rect.to_box2d());
    }

    /// Add a box to the region
    ///
    /// Note that if the region becomes too complex, it might be simplified by being bigger than the actual union.
    pub fn add_box(&mut self, b: euclid::Box2D<Coord, LogicalPx>) {
        if b.is_empty() {
            return;
        }
        let mut i = 0;
        while i < self.count {
            let r = &self.rectangles[i];
            if r.contains_box(&b) {
                // the rectangle is already in the union
                return;
            } else if b.contains_box(r) {
                self.rectangles.swap(i, self.count - 1);
                self.count -= 1;
                continue;
            }
            i += 1;
        }

        if self.count < Self::MAX_COUNT {
            self.rectangles[self.count] = b;
            self.count += 1;
        } else {
            let best_merge = (0..self.count)
                .map(|i| (i, self.rectangles[i].union(&b).area() - self.rectangles[i].area()))
                .min_by(|a, b| PartialOrd::partial_cmp(&a.1, &b.1).unwrap())
                .expect("There should always be rectangles")
                .0;
            self.rectangles[best_merge] = self.rectangles[best_merge].union(&b);
        }
    }

    /// Make an union of two regions.
    ///
    /// Note that if the region becomes too complex, it might be simplified by being bigger than the actual union
    #[must_use]
    pub fn union(&self, other: &Self) -> Self {
        let mut s = self.clone();
        for o in other.iter() {
            s.add_box(o)
        }
        s
    }

    /// Bounding rectangle of the region.
    #[must_use]
    pub fn bounding_rect(&self) -> LogicalRect {
        if self.count == 0 {
            return Default::default();
        }
        let mut r = self.rectangles[0];
        for i in 1..self.count {
            r = r.union(&self.rectangles[i]);
        }
        r.to_rect()
    }

    /// Intersection of a region and a rectangle.
    #[must_use]
    pub fn intersection(&self, other: LogicalRect) -> DirtyRegion {
        let mut ret = self.clone();
        let other = other.to_box2d();
        let mut i = 0;
        while i < ret.count {
            if let Some(x) = ret.rectangles[i].intersection(&other) {
                ret.rectangles[i] = x;
            } else {
                ret.count -= 1;
                ret.rectangles.swap(i, ret.count);
                continue;
            }
            i += 1;
        }
        ret
    }

    fn draw_intersects(&self, clipped_geom: LogicalRect) -> bool {
        let b = clipped_geom.to_box2d();
        self.iter().any(|r| r.intersects(&b))
    }
}

impl From<LogicalRect> for DirtyRegion {
    fn from(value: LogicalRect) -> Self {
        let mut s = Self::default();
        s.add_rect(value);
        s
    }
}

/// This enum describes which parts of the buffer passed to the `SoftwareRenderer` may be re-used to speed up painting.
// FIXME: #[non_exhaustive] #3023
#[derive(PartialEq, Eq, Debug, Clone, Default, Copy)]
pub enum RepaintBufferType {
    #[default]
    /// The full window is always redrawn. No attempt at partial rendering will be made.
    NewBuffer,
    /// Only redraw the parts that have changed since the previous call to render().
    ///
    /// This variant assumes that the same buffer is passed on every call to render() and
    /// that it still contains the previously rendered frame.
    ReusedBuffer,

    /// Redraw the part that have changed since the last two frames were drawn.
    ///
    /// This is used when using double buffering and swapping of the buffers.
    SwappedBuffers,
}

/// Put this structure in the renderer to help with partial rendering
///
/// This is constructed from a [`PartialRenderingState`]
pub struct PartialRenderer<'a, T> {
    cache: &'a RefCell<PartialRendererCache>,
    /// The region of the screen which is considered dirty and that should be repainted
    pub dirty_region: DirtyRegion,
    /// The actual renderer which the drawing call will be forwarded to
    pub actual_renderer: T,
    /// The window adapter the renderer is rendering into.
    pub window_adapter: Rc<dyn WindowAdapter>,
}

impl<'a, T: ItemRenderer + ItemRendererFeatures> PartialRenderer<'a, T> {
    /// Create a new PartialRenderer
    fn new(
        cache: &'a RefCell<PartialRendererCache>,
        initial_dirty_region: DirtyRegion,
        actual_renderer: T,
    ) -> Self {
        let window_adapter = actual_renderer.window().window_adapter();
        Self { cache, dirty_region: initial_dirty_region, actual_renderer, window_adapter }
    }

    /// Visit the tree of item and compute what are the dirty regions
    pub fn compute_dirty_regions(
        &mut self,
        component: &ItemTreeRc,
        origin: LogicalPoint,
        size: LogicalSize,
    ) {
        #[derive(Clone, Copy)]
        struct ComputeDirtyRegionState {
            transform_to_screen: ItemTransform,
            old_transform_to_screen: ItemTransform,
            clipped: LogicalRect,
            must_refresh_children: bool,
        }

        impl ComputeDirtyRegionState {
            /// Adjust transform_to_screen and old_transform_to_screen to map from item coordinates
            /// to the screen when using it on a child, specified by its children transform.
            fn adjust_transforms_for_child(
                &mut self,
                children_transform: &ItemTransform,
                old_children_transform: &ItemTransform,
            ) {
                self.transform_to_screen = children_transform.then(&self.transform_to_screen);
                self.old_transform_to_screen =
                    old_children_transform.then(&self.old_transform_to_screen);
            }
        }

        crate::item_tree::visit_items(
            component,
            crate::item_tree::TraversalOrder::BackToFront,
            |component, item, index, state| {
                let mut new_state = *state;
                let item_rc = ItemRc::new(component.clone(), index);
                let new_geom =
                    CachedItemBoundingBoxAndTransform::new::<T>(&item_rc, &self.window_adapter);

                let rendering_data = item.cached_rendering_data_offset();
                let mut cache = self.cache.borrow_mut();
                match rendering_data.get_entry(&mut cache) {
                    Some(PartialRenderingCachedData { data: cached_geom, tracker }) => {
                        let rendering_dirty = tracker.as_ref().is_some_and(|tr| tr.is_dirty());
                        let old_geom = cached_geom.clone();

                        let geometry_changed = old_geom != new_geom;
                        if ItemRef::downcast_pin::<Clip>(item).is_some()
                            || ItemRef::downcast_pin::<Opacity>(item).is_some()
                        {
                            // When the opacity or the clip change, this will impact all the children, including
                            // the ones outside the element, regardless if they are themselves dirty or not.
                            new_state.must_refresh_children |= rendering_dirty || geometry_changed;

                            if rendering_dirty {
                                // Destroy the tracker as we we might not re-render this clipped item but it would stay dirty
                                *tracker = None;
                            }
                        }

                        if geometry_changed {
                            self.mark_dirty_rect(
                                old_geom.bounding_rect(),
                                state.old_transform_to_screen,
                                &state.clipped,
                            );
                            self.mark_dirty_rect(
                                new_geom.bounding_rect(),
                                state.transform_to_screen,
                                &state.clipped,
                            );

                            new_state.adjust_transforms_for_child(
                                &new_geom.transform(),
                                &old_geom.transform(),
                            );

                            *cached_geom = new_geom;

                            return ItemVisitorResult::Continue(new_state);
                        }

                        new_state.adjust_transforms_for_child(
                            &cached_geom.transform(),
                            &cached_geom.transform(),
                        );

                        let moved = state.must_refresh_children
                            || new_state.transform_to_screen != new_state.old_transform_to_screen;

                        if rendering_dirty {
                            self.mark_dirty_rect(
                                cached_geom.bounding_rect(),
                                state.transform_to_screen,
                                &state.clipped,
                            );
                            if moved {
                                self.mark_dirty_rect(
                                    cached_geom.bounding_rect(),
                                    state.old_transform_to_screen,
                                    &state.clipped,
                                );
                            }

                            ItemVisitorResult::Continue(new_state)
                        } else {
                            if moved {
                                self.mark_dirty_rect(
                                    cached_geom.bounding_rect(),
                                    state.old_transform_to_screen,
                                    &state.clipped,
                                );
                                self.mark_dirty_rect(
                                    cached_geom.bounding_rect(),
                                    state.transform_to_screen,
                                    &state.clipped,
                                );
                            } else if let Some(tr) = &tracker {
                                tr.as_ref().register_as_dependency_to_current_binding();
                            }

                            if let CachedItemBoundingBoxAndTransform::ClipItem { geometry } =
                                &cached_geom
                            {
                                new_state.clipped = new_state
                                    .clipped
                                    .intersection(
                                        &state
                                            .transform_to_screen
                                            .outer_transformed_rect(&geometry.cast())
                                            .cast()
                                            .union(
                                                &state
                                                    .old_transform_to_screen
                                                    .outer_transformed_rect(&geometry.cast())
                                                    .cast(),
                                            ),
                                    )
                                    .unwrap_or_default();
                                if new_state.clipped.is_empty() {
                                    return ItemVisitorResult::SkipChildren;
                                }
                            }
                            ItemVisitorResult::Continue(new_state)
                        }
                    }
                    None => {
                        let cache_entry = PartialRenderingCachedData::new(new_geom.clone());
                        rendering_data.cache_index.set(cache.insert(cache_entry));
                        rendering_data.cache_generation.set(cache.generation());

                        new_state.adjust_transforms_for_child(
                            &new_geom.transform(),
                            &new_geom.transform(),
                        );

                        if let CachedItemBoundingBoxAndTransform::ClipItem { geometry } = new_geom {
                            new_state.clipped = new_state
                                .clipped
                                .intersection(
                                    &state
                                        .transform_to_screen
                                        .outer_transformed_rect(&geometry.cast())
                                        .cast(),
                                )
                                .unwrap_or_default();
                        }

                        self.mark_dirty_rect(
                            new_geom.bounding_rect(),
                            state.transform_to_screen,
                            &state.clipped,
                        );
                        if new_state.clipped.is_empty() {
                            ItemVisitorResult::SkipChildren
                        } else {
                            ItemVisitorResult::Continue(new_state)
                        }
                    }
                }
            },
            {
                let initial_transform =
                    euclid::Transform2D::translation(origin.x as f32, origin.y as f32);
                ComputeDirtyRegionState {
                    transform_to_screen: initial_transform,
                    old_transform_to_screen: initial_transform,
                    clipped: LogicalRect::from_size(size),
                    must_refresh_children: false,
                }
            },
        );
    }

    fn mark_dirty_rect(
        &mut self,
        rect: &LogicalRect,
        transform: ItemTransform,
        clip_rect: &LogicalRect,
    ) {
        #[cfg(not(slint_int_coord))]
        if !rect.origin.is_finite() {
            // Account for NaN
            return;
        }

        if !rect.is_empty()
            && let Some(rect) =
                transform.outer_transformed_rect(&rect.cast()).cast().intersection(clip_rect)
        {
            self.dirty_region.add_rect(rect);
        }
    }

    fn do_rendering(
        cache: &RefCell<PartialRendererCache>,
        rendering_data: &CachedRenderingData,
        item_rc: &ItemRc,
        render_fn: impl FnOnce(),
    ) {
        let mut cache = cache.borrow_mut();
        if let Some(entry) = rendering_data.get_entry(&mut cache) {
            entry
                .tracker
                .get_or_insert_with(|| Box::pin(PropertyTracker::default()))
                .as_ref()
                .evaluate(render_fn);
        } else {
            // This item was created between the computation of the dirty region and the actual rendering.
            // Register a dependency to the geometry since this wasn't done before
            item_rc.geometry();
            render_fn();
        }
    }

    /// Move the actual renderer
    pub fn into_inner(self) -> T {
        self.actual_renderer
    }
}

macro_rules! forward_rendering_call {
    (fn $fn:ident($Ty:ty) $(-> $Ret:ty)?) => {
        fn $fn(&mut self, obj: Pin<&$Ty>, item_rc: &ItemRc, size: LogicalSize) $(-> $Ret)? {
            let mut ret = None;
            Self::do_rendering(&self.cache, &obj.cached_rendering_data, item_rc, || {
                ret = Some(self.actual_renderer.$fn(obj, item_rc, size));
            });
            ret.unwrap_or_default()
        }
    };
}

macro_rules! forward_rendering_call2 {
    (fn $fn:ident($Ty:ty) $(-> $Ret:ty)?) => {
        fn $fn(&mut self, obj: Pin<&$Ty>, item_rc: &ItemRc, size: LogicalSize, cache: &CachedRenderingData) $(-> $Ret)? {
            let mut ret = None;
            Self::do_rendering(&self.cache, &cache, item_rc, || {
                ret = Some(self.actual_renderer.$fn(obj, item_rc, size, &cache));
            });
            ret.unwrap_or_default()
        }
    };
}

impl<T: ItemRenderer + ItemRendererFeatures> ItemRenderer for PartialRenderer<'_, T> {
    fn filter_item(
        &mut self,
        item_rc: &ItemRc,
        window_adapter: &Rc<dyn WindowAdapter>,
    ) -> (bool, LogicalRect) {
        let item = item_rc.borrow();

        // Query untracked, as the bounding rect calculation already registers a dependency on the geometry.
        let item_geometry = crate::properties::evaluate_no_tracking(|| item_rc.geometry());

        let rendering_data = item.cached_rendering_data_offset();
        let mut cache = self.cache.borrow_mut();
        let item_bounding_rect = match rendering_data.get_entry(&mut cache) {
            Some(PartialRenderingCachedData { data, tracker: _ }) => *data.bounding_rect(),
            None => {
                // This item was created between the computation of the dirty region and the actual rendering.
                item_rc.bounding_rect(&item_geometry, window_adapter)
            }
        };

        let clipped_geom = self.get_current_clip().intersection(&item_bounding_rect);
        let draw = clipped_geom.is_some_and(|clipped_geom| {
            let screen_geom =
                self.current_transform().outer_transformed_rect(&clipped_geom.cast()).cast();
            self.dirty_region.draw_intersects(screen_geom)
        });

        (draw, item_geometry)
    }

    forward_rendering_call2!(fn draw_rectangle(dyn RenderRectangle));
    forward_rendering_call2!(fn draw_border_rectangle(dyn RenderBorderRectangle));
    forward_rendering_call2!(fn draw_window_background(dyn RenderRectangle));
    forward_rendering_call2!(fn draw_image(dyn RenderImage));
    forward_rendering_call2!(fn draw_text(dyn RenderText));
    forward_rendering_call!(fn draw_text_input(TextInput));
    #[cfg(feature = "path")]
    forward_rendering_call!(fn draw_path(Path));
    forward_rendering_call!(fn draw_box_shadow(BoxShadow));

    forward_rendering_call!(fn visit_clip(Clip) -> RenderingResult);
    forward_rendering_call!(fn visit_opacity(Opacity) -> RenderingResult);
    forward_rendering_call!(fn visit_layer(Layer) -> RenderingResult);

    fn combine_clip(
        &mut self,
        rect: LogicalRect,
        radius: LogicalBorderRadius,
        border_width: LogicalLength,
    ) -> bool {
        self.actual_renderer.combine_clip(rect, radius, border_width)
    }

    fn get_current_clip(&self) -> LogicalRect {
        self.actual_renderer.get_current_clip()
    }

    fn translate(&mut self, distance: LogicalVector) {
        self.actual_renderer.translate(distance)
    }
    fn current_transform(&self) -> ItemTransform {
        self.actual_renderer.current_transform()
    }

    fn rotate(&mut self, angle_in_degrees: f32) {
        self.actual_renderer.rotate(angle_in_degrees)
    }

    fn scale(&mut self, x_factor: f32, y_factor: f32) {
        self.actual_renderer.scale(x_factor, y_factor)
    }

    fn apply_opacity(&mut self, opacity: f32) {
        self.actual_renderer.apply_opacity(opacity)
    }

    fn save_state(&mut self) {
        self.actual_renderer.save_state()
    }

    fn restore_state(&mut self) {
        self.actual_renderer.restore_state()
    }

    fn scale_factor(&self) -> f32 {
        self.actual_renderer.scale_factor()
    }

    fn draw_cached_pixmap(
        &mut self,
        item_rc: &ItemRc,
        update_fn: &dyn Fn(&mut dyn FnMut(u32, u32, &[u8])),
    ) {
        self.actual_renderer.draw_cached_pixmap(item_rc, update_fn)
    }

    fn draw_string(&mut self, string: &str, color: crate::Color) {
        self.actual_renderer.draw_string(string, color)
    }

    fn draw_image_direct(&mut self, image: crate::graphics::image::Image) {
        self.actual_renderer.draw_image_direct(image)
    }

    fn window(&self) -> &crate::window::WindowInner {
        self.actual_renderer.window()
    }

    fn as_any(&mut self) -> Option<&mut dyn core::any::Any> {
        self.actual_renderer.as_any()
    }
}

/// This struct holds the state of the partial renderer between different frames, in particular the cache of the bounding rect
/// of each item. This permits a more fine-grained computation of the region that needs to be repainted.
#[derive(Default)]
pub struct PartialRenderingState {
    partial_cache: RefCell<PartialRendererCache>,
    /// This is the area which we are going to redraw in the next frame, no matter if the items are dirty or not
    force_dirty: RefCell<DirtyRegion>,
    /// Force a redraw in the next frame, no matter what's dirty. Use only as a last resort.
    force_screen_refresh: Cell<bool>,
}

impl PartialRenderingState {
    /// Creates a partial renderer that's initialized with the partial rendering caches maintained in this state structure.
    /// Call [`Self::apply_dirty_region`] after this function to compute the correct partial rendering region.
    pub fn create_partial_renderer<T: ItemRenderer + ItemRendererFeatures>(
        &self,
        renderer: T,
    ) -> PartialRenderer<'_, T> {
        PartialRenderer::new(&self.partial_cache, self.force_dirty.take(), renderer)
    }

    /// Compute the correct partial rendering region based on the components to be drawn, the bounding rectangles of
    /// changes items within, and the current repaint buffer type. Returns the computed dirty region just for this frame.
    /// The provided buffer_dirty_region specifies which area of the buffer is known to *additionally* require repainting,
    /// where `None` means that buffer is not known to be dirty beyond what applies to this frame (reused buffer).
    pub fn apply_dirty_region<T: ItemRenderer + ItemRendererFeatures>(
        &self,
        partial_renderer: &mut PartialRenderer<'_, T>,
        components: &[(ItemTreeWeak, LogicalPoint)],
        logical_window_size: LogicalSize,
        dirty_region_of_existing_buffer: Option<DirtyRegion>,
    ) -> DirtyRegion {
        for (component, origin) in components {
            if let Some(component) = crate::item_tree::ItemTreeWeak::upgrade(component) {
                partial_renderer.compute_dirty_regions(&component, *origin, logical_window_size);
            }
        }

        let screen_region = LogicalRect::from_size(logical_window_size);

        if self.force_screen_refresh.take() {
            partial_renderer.dirty_region = screen_region.into();
        }

        let region_to_repaint = partial_renderer.dirty_region.clone();

        partial_renderer.dirty_region = match dirty_region_of_existing_buffer {
            Some(dirty_region) => partial_renderer.dirty_region.union(&dirty_region),
            None => partial_renderer.dirty_region.clone(),
        }
        .intersection(screen_region);

        region_to_repaint
    }

    /// Add the specified region to the list of regions to include in the next rendering.
    pub fn mark_dirty_region(&self, region: DirtyRegion) {
        self.force_dirty.replace_with(|r| r.union(&region));
    }

    /// Call this from your renderer's `free_graphics_resources` function to ensure that the cached item geometries
    /// are cleared for the destroyed items in the item tree.
    pub fn free_graphics_resources(&self, items: &mut dyn Iterator<Item = Pin<ItemRef<'_>>>) {
        for item in items {
            item.cached_rendering_data_offset().release(&mut self.partial_cache.borrow_mut());
        }

        // We don't have a way to determine the screen region of the delete items, what's in the cache is relative. So
        // as a last resort, refresh everything.
        self.force_screen_refresh.set(true)
    }

    /// Clears the partial rendering cache. Use this for example when the entire underlying window surface changes.
    pub fn clear_cache(&self) {
        self.partial_cache.borrow_mut().clear();
    }

    /// Force re-rendering of the entire window region the next time a partial renderer is created.
    pub fn force_screen_refresh(&self) {
        self.force_screen_refresh.set(true);
    }
}

#[test]
fn dirty_region_no_intersection() {
    let mut region = DirtyRegion::default();
    region.add_rect(LogicalRect::new(LogicalPoint::new(10., 10.), LogicalSize::new(16., 16.)));
    region.add_rect(LogicalRect::new(LogicalPoint::new(100., 100.), LogicalSize::new(16., 16.)));
    region.add_rect(LogicalRect::new(LogicalPoint::new(200., 100.), LogicalSize::new(16., 16.)));
    let i = region
        .intersection(LogicalRect::new(LogicalPoint::new(50., 50.), LogicalSize::new(10., 10.)));
    assert_eq!(i.iter().count(), 0);
}