Add Zoom type and ChangeObserver, also improve performance by tessellating less

src/coords.rs

@@ -2,9 +2,10 @@
 
 use std::fmt;
 use std::fmt::Formatter;
+use std::ops::Add;
 
 use cgmath::num_traits::Pow;
-use cgmath::{Matrix4, Point3, Vector3};
+use cgmath::{AbsDiffEq, Matrix4, Point3, Vector3};
 
 use style_spec::source::TileAddressingScheme;
 
@@ -40,6 +41,68 @@ impl fmt::Debug for Quadkey {
     }
 }
 
+#[derive(Copy, Clone, Debug)]
+pub struct Zoom(f64);
+
+impl Zoom {
+    pub fn new(zoom: f64) -> Self {
+        Zoom(zoom)
+    }
+}
+
+impl Default for Zoom {
+    fn default() -> Self {
+        Zoom(0.0)
+    }
+}
+
+impl fmt::Display for Zoom {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", (self.0 * 100.0).round() / 100.0)
+    }
+}
+
+impl std::ops::Add for Zoom {
+    type Output = Zoom;
+
+    fn add(self, rhs: Self) -> Self::Output {
+        Zoom(self.0 + rhs.0)
+    }
+}
+
+impl std::ops::Sub for Zoom {
+    type Output = Zoom;
+
+    fn sub(self, rhs: Self) -> Self::Output {
+        Zoom(self.0 - rhs.0)
+    }
+}
+
+impl Zoom {
+    pub fn scale_to_tile(&self, coords: &WorldTileCoords) -> f64 {
+        2.0_f64.powf(coords.z as f64 - self.0)
+    }
+
+    pub fn scale_to_zoom_level(&self, z: u8) -> f64 {
+        2.0_f64.powf(z as f64 - self.0)
+    }
+
+    pub fn scale_delta(&self, zoom: &Zoom) -> f64 {
+        2.0_f64.powf(zoom.0 - self.0)
+    }
+
+    pub fn level(&self) -> u8 {
+        self.0.floor() as u8
+    }
+}
+
+impl Eq for Zoom {}
+impl PartialEq for Zoom {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.abs_diff_eq(&other.0, 0.05)
+    }
+}
+
 /// Within each tile there is a separate coordinate system. Usually this coordinate system is
 /// within [`crate::coords::EXTENT`]. Therefore, `x` and `y` must be within the bounds of
 /// [`crate::coords::EXTENT`].
@@ -146,7 +209,7 @@ impl WorldTileCoords {
     }
 
     #[tracing::instrument(skip_all)]
-    pub fn transform_for_zoom(&self, zoom: f64) -> Matrix4<f64> {
+    pub fn transform_for_zoom(&self, zoom: Zoom) -> Matrix4<f64> {
         /*
            For tile.z = zoom:
                => scale = 512
@@ -155,7 +218,7 @@ impl WorldTileCoords {
            If tile.z > zoom:
                => scale < 512
         */
-        let tile_scale = TILE_SIZE * 2.0.pow(zoom - self.z as f64);
+        let tile_scale = TILE_SIZE * Zoom::new(self.z as f64).scale_delta(&zoom);
 
         let translate = Matrix4::from_translation(Vector3::new(
             self.x as f64 * tile_scale,
@@ -309,10 +372,6 @@ pub struct WorldCoords {
     pub y: f64,
 }
 
-fn world_size_at_zoom(zoom: f64) -> f64 {
-    TILE_SIZE * 2.0.pow(zoom)
-}
-
 fn tiles_with_z(z: u8) -> f64 {
     2.0.pow(z)
 }
@@ -322,8 +381,8 @@ impl WorldCoords {
         Self { x, y }
     }
 
-    pub fn into_world_tile(self, z: u8, zoom: f64) -> WorldTileCoords {
-        let tile_scale = 2.0.pow(z as f64 - zoom) / TILE_SIZE; // TODO: Deduplicate
+    pub fn into_world_tile(self, z: u8, zoom: Zoom) -> WorldTileCoords {
+        let tile_scale = zoom.scale_to_zoom_level(z) / TILE_SIZE; // TODO: Deduplicate
         let x = self.x * tile_scale;
         let y = self.y * tile_scale;
 
@@ -371,7 +430,7 @@ pub struct ViewRegion {
 }
 
 impl ViewRegion {
-    pub fn new(view_region: Aabb2<f64>, padding: i32, zoom: f64, z: u8) -> Self {
+    pub fn new(view_region: Aabb2<f64>, padding: i32, zoom: Zoom, z: u8) -> Self {
         let min_world: WorldCoords = WorldCoords::at_ground(view_region.min.x, view_region.min.y);
         let min_world_tile: WorldTileCoords = min_world.into_world_tile(z, zoom);
         let max_world: WorldCoords = WorldCoords::at_ground(view_region.max.x, view_region.max.y);
@@ -439,6 +498,7 @@ impl fmt::Display for WorldCoords {
 #[cfg(test)]
 mod tests {
     use cgmath::{Point2, Vector4};
+
     use style_spec::source::TileAddressingScheme;
 
     use crate::coords::{Quadkey, TileCoords, ViewRegion, WorldCoords, WorldTileCoords, EXTENT};
@@ -447,7 +507,7 @@ mod tests {
     const TOP_LEFT: Vector4<f64> = Vector4::new(0.0, 0.0, 0.0, 1.0);
     const BOTTOM_RIGHT: Vector4<f64> = Vector4::new(EXTENT, EXTENT, 0.0, 1.0);
 
-    fn to_from_world(tile: (i32, i32, u8), zoom: f64) {
+    fn to_from_world(tile: (i32, i32, u8), zoom: Zoom) {
         let tile = WorldTileCoords::from(tile);
         let p1 = tile.transform_for_zoom(zoom) * TOP_LEFT;
         let p2 = tile.transform_for_zoom(zoom) * BOTTOM_RIGHT;

src/input/query_handler.rs

@@ -63,8 +63,8 @@ impl QueryHandler {
                 let view_proj = state.camera.calc_view_proj(perspective);
                 let inverted_view_proj = view_proj.invert();
 
-                let z = state.visible_z();
-                let zoom = state.zoom;
+                let z = state.visible_z(); // FIXME: can be wrong, if tiles of different z are visible
+                let zoom = state.zoom();
 
                 if let Some(coordinates) = state
                     .camera

src/input/zoom_handler.rs

@@ -1,5 +1,6 @@
 use super::UpdateState;
 
+use crate::coords::Zoom;
 use crate::render::render_state::RenderState;
 use crate::Scheduler;
 use cgmath::num_traits::Pow;
@@ -8,20 +9,19 @@ use std::time::Duration;
 
 pub struct ZoomHandler {
     window_position: Option<Vector2<f64>>,
-    zoom_delta: f64,
+    zoom_delta: Option<Zoom>,
     sensitivity: f64,
 }
 
 impl UpdateState for ZoomHandler {
     fn update_state(&mut self, state: &mut RenderState, _scheduler: &Scheduler, _dt: Duration) {
-        if self.zoom_delta != 0.0 {
+        if let Some(zoom_delta) = self.zoom_delta {
             if let Some(window_position) = self.window_position {
-                let current_zoom = state.zoom;
-                let next_zoom = current_zoom + self.zoom_delta;
+                let current_zoom = state.zoom();
+                let next_zoom = current_zoom + zoom_delta;
 
-                state.zoom = next_zoom;
-                self.zoom_delta = 0.0;
-                println!("zoom: {}", state.zoom);
+                state.update_zoom(next_zoom);
+                self.zoom_delta = None;
 
                 let perspective = &state.perspective;
                 let view_proj = state.camera.calc_view_proj(perspective);
@@ -31,7 +31,7 @@ impl UpdateState for ZoomHandler {
                     .camera
                     .window_to_world_at_ground(&window_position, &inverted_view_proj)
                 {
-                    let scale = 2.0.pow(next_zoom - current_zoom);
+                    let scale = current_zoom.scale_delta(&next_zoom);
 
                     let delta = Vector3::new(
                         cursor_position.x * scale,
@@ -50,7 +50,7 @@ impl ZoomHandler {
     pub fn new(sensitivity: f64) -> Self {
         Self {
             window_position: None,
-            zoom_delta: 0.0,
+            zoom_delta: None,
             sensitivity,
         }
     }
@@ -64,14 +64,22 @@ impl ZoomHandler {
         true
     }
 
+    pub fn update_zoom(&mut self, delta: f64) {
+        self.zoom_delta = Some(self.zoom_delta.unwrap_or_default() + Zoom::new(delta));
+    }
+
     pub fn process_scroll(&mut self, delta: &winit::event::MouseScrollDelta) {
-        self.zoom_delta += match delta {
-            winit::event::MouseScrollDelta::LineDelta(_horizontal, vertical) => *vertical as f64,
-            winit::event::MouseScrollDelta::PixelDelta(winit::dpi::PhysicalPosition {
-                y: scroll,
-                ..
-            }) => *scroll / 100.0,
-        } * self.sensitivity;
+        self.update_zoom(
+            match delta {
+                winit::event::MouseScrollDelta::LineDelta(_horizontal, vertical) => {
+                    *vertical as f64
+                }
+                winit::event::MouseScrollDelta::PixelDelta(winit::dpi::PhysicalPosition {
+                    y: scroll,
+                    ..
+                }) => *scroll / 100.0,
+            } * self.sensitivity,
+        );
     }
 
     pub fn process_key_press(
@@ -87,11 +95,11 @@ impl ZoomHandler {
 
         match key {
             winit::event::VirtualKeyCode::Plus | winit::event::VirtualKeyCode::I => {
-                self.zoom_delta += amount;
+                self.update_zoom(amount);
                 true
             }
             winit::event::VirtualKeyCode::Minus | winit::event::VirtualKeyCode::K => {
-                self.zoom_delta -= amount;
+                self.update_zoom(-amount);
                 true
             }
             _ => false,

src/io/geometry_index.rs

@@ -9,7 +9,7 @@ use geozero::geo_types::GeoWriter;
 use geozero::{ColumnValue, FeatureProcessor, GeomProcessor, PropertyProcessor};
 use rstar::{Envelope, PointDistance, RTree, RTreeObject, AABB};
 
-use crate::coords::{InnerCoords, Quadkey, WorldCoords, WorldTileCoords, EXTENT, TILE_SIZE};
+use crate::coords::{InnerCoords, Quadkey, WorldCoords, WorldTileCoords, Zoom, EXTENT, TILE_SIZE};
 use crate::util::math::bounds_from_points;
 
 pub struct GeometryIndex {
@@ -33,7 +33,7 @@ impl GeometryIndex {
         &self,
         world_coords: &WorldCoords,
         z: u8,
-        zoom: f64,
+        zoom: Zoom,
     ) -> Option<Vec<&IndexedGeometry<f64>>> {
         let world_tile_coords = world_coords.into_world_tile(z, zoom);
 
@@ -41,7 +41,7 @@ impl GeometryIndex {
             .build_quad_key()
             .and_then(|key| self.index.get(&key))
         {
-            let scale = 2.0f64.pow(z as f64 - zoom); // TODO deduplicate
+            let scale = zoom.scale_delta(&Zoom::new(z as f64)); // FIXME: can be wrong, if tiles of different z are visible
 
             let delta_x = world_coords.x / TILE_SIZE * scale - world_tile_coords.x as f64;
             let delta_y = world_coords.y / TILE_SIZE * scale - world_tile_coords.y as f64;

src/io/scheduler.rs

@@ -9,7 +9,7 @@ use std::sync::{Arc, Mutex};
 use vector_tile::parse_tile_bytes;
 
 /// Describes through which channels work-requests travel. It describes the flow of work.
-use crate::coords::{WorldCoords, WorldTileCoords};
+use crate::coords::{WorldCoords, WorldTileCoords, Zoom};
 use crate::io::tile_cache::TileCache;
 use crate::io::{
     LayerTessellateMessage, TessellateMessage, TileFetchResult, TileRequest, TileRequestID,
@@ -155,7 +155,7 @@ impl ThreadLocalState {
         &self,
         world_coords: &WorldCoords,
         z: u8,
-        zoom: f64,
+        zoom: Zoom,
     ) -> Option<Vec<IndexedGeometry<f64>>> {
         if let Ok(mut geometry_index) = self.geometry_index.lock() {
             geometry_index
@@ -256,7 +256,7 @@ impl ThreadLocalState {
             }
         }
 
-        tracing::info!("tile at {} finished", &coords);
+        tracing::info!("tile at {} finished", &tile_request.coords);
 
         self.tessellate_result_sender
             .send(TessellateMessage::Tile(TileTessellateMessage {
@@ -330,9 +330,9 @@ impl Scheduler {
         &mut self,
         coords: &WorldTileCoords,
         layers: &HashSet<String>,
-    ) -> Result<(), SendError<TileRequest>> {
+    ) -> Result<bool, SendError<TileRequest>> {
         if !self.tile_cache.is_layers_missing(coords, layers) {
-            return Ok(());
+            return Ok(false);
         }
 
         if let Ok(mut tile_request_state) = self.tile_request_state.try_lock() {
@@ -372,9 +372,11 @@ impl Scheduler {
                     self.schedule_method.schedule(self, future_fn).unwrap();
                 }
             }
-        }
 
-        Ok(())
+            Ok(false)
+        } else {
+            Ok(true)
+        }
     }
 
     pub fn get_tile_cache(&self) -> &TileCache {

src/render/camera.rs

@@ -1,5 +1,5 @@
 use cgmath::prelude::*;
-use cgmath::{Matrix4, Point2, Point3, Vector2, Vector3, Vector4};
+use cgmath::{AbsDiffEq, Matrix4, Point2, Point3, Vector2, Vector3, Vector4};
 
 use crate::render::shaders::ShaderCamera;
 use crate::util::math::{bounds_from_points, Aabb2, Aabb3, Plane};
@@ -73,6 +73,15 @@ pub struct Camera {
     pub height: f64,
 }
 
+impl Eq for Camera {}
+impl PartialEq for Camera {
+    fn eq(&self, other: &Self) -> bool {
+        self.position.abs_diff_eq(&other.position, 0.05)
+            && self.yaw.abs_diff_eq(&other.yaw, 0.05)
+            && self.pitch.abs_diff_eq(&other.pitch, 0.05)
+    }
+}
+
 impl Camera {
     pub fn new<
         V: Into<cgmath::Point3<f64>>,

src/render/render_state.rs

@@ -1,6 +1,7 @@
 use std::collections::HashSet;
 use std::default::Default;
 
+use cgmath::AbsDiffEq;
 use std::{cmp, iter};
 
 use tracing;
@@ -10,7 +11,7 @@ use winit::window::Window;
 
 use style_spec::Style;
 
-use crate::coords::{ViewRegion, TILE_SIZE};
+use crate::coords::{ViewRegion, Zoom, TILE_SIZE};
 use crate::io::scheduler::Scheduler;
 use crate::io::LayerTessellateMessage;
 use crate::platform::{COLOR_TEXTURE_FORMAT, MIN_BUFFER_SIZE};
@@ -23,7 +24,7 @@ use crate::render::options::{
 };
 use crate::render::tile_view_pattern::{TileInView, TileViewPattern};
 use crate::tessellation::IndexDataType;
-use crate::util::FPSMeter;
+use crate::util::{ChangeObserver, FPSMeter};
 
 use super::piplines::*;
 use super::shaders;
@@ -66,14 +67,24 @@ pub struct RenderState {
 
     tile_view_pattern: TileViewPattern<Queue, Buffer>,
 
-    pub camera: camera::Camera,
+    pub camera: ChangeObserver<camera::Camera>,
     pub perspective: camera::Perspective,
-    pub zoom: f64,
+    zoom: ChangeObserver<Zoom>,
+    try_failed: bool,
 
     style: Box<Style>,
 }
 
 impl RenderState {
+    pub fn zoom(&self) -> Zoom {
+        *self.zoom
+    }
+
+    pub fn update_zoom(&mut self, new_zoom: Zoom) {
+        *self.zoom = new_zoom;
+        log::info!("zoom: {}", new_zoom);
+    }
+
     pub async fn new(window: &Window, style: Box<Style>) -> Self {
         let sample_count = 4;
 
@@ -292,7 +303,7 @@ impl RenderState {
             sample_count,
             globals_uniform_buffer,
             fps_meter: FPSMeter::new(),
-            camera,
+            camera: ChangeObserver::new(camera),
             perspective: projection,
             suspended: false, // Initially the app is not suspended
             buffer_pool: BufferPool::new(
@@ -305,7 +316,8 @@ impl RenderState {
                 tile_view_buffer,
                 TILE_VIEW_BUFFER_SIZE,
             )),
-            zoom: 0.0,
+            zoom: ChangeObserver::default(),
+            try_failed: false,
             style,
         }
     }
@@ -356,12 +368,13 @@ impl RenderState {
     }
 
     pub fn visible_z(&self) -> u8 {
-        self.zoom.floor() as u8
+        self.zoom.level()
     }
 
     /// Request tiles which are currently in view
     #[tracing::instrument(skip_all)]
-    fn request_tiles_in_view(&self, view_region: &ViewRegion, scheduler: &mut Scheduler) {
+    fn request_tiles_in_view(&self, view_region: &ViewRegion, scheduler: &mut Scheduler) -> bool {
+        let mut try_failed = false;
         let source_layers: HashSet<String> = self
             .style
             .layers
@@ -372,9 +385,10 @@ impl RenderState {
         for coords in view_region.iter() {
             if coords.build_quad_key().is_some() {
                 // TODO: Make tesselation depend on style?
-                scheduler.try_request_tile(&coords, &source_layers).unwrap();
+                try_failed = scheduler.try_request_tile(&coords, &source_layers).unwrap();
             }
         }
+        try_failed
     }
 
     /// Update tile metadata for all required tiles on the GPU according to current zoom, camera and perspective
@@ -383,16 +397,12 @@ impl RenderState {
     #[tracing::instrument(skip_all)]
     fn update_metadata(
         &mut self,
-        scheduler: &mut Scheduler,
+        _scheduler: &mut Scheduler,
         view_region: &ViewRegion,
         view_proj: &ViewProjection,
     ) {
-        self.tile_view_pattern.update_pattern(
-            view_region,
-            scheduler.get_tile_cache(),
-            &self.buffer_pool,
-            self.zoom,
-        );
+        self.tile_view_pattern
+            .update_pattern(view_region, &self.buffer_pool, *self.zoom);
         self.tile_view_pattern
             .upload_pattern(&self.queue, view_proj);
 
@@ -511,6 +521,12 @@ impl RenderState {
                                 buffer,
                                 ..
                             } => {
+                                let allocate_feature_metadata = tracing::span!(
+                                    tracing::Level::TRACE,
+                                    "allocate_feature_metadata"
+                                );
+
+                                let guard = allocate_feature_metadata.enter();
                                 let feature_metadata = layer_data
                                     .features()
                                     .iter()
@@ -522,6 +538,7 @@ impl RenderState {
                                         .take(feature_indices[i] as usize)
                                     })
                                     .collect::<Vec<_>>();
+                                drop(guard);
 
                                 tracing::trace!("Allocating geometry at {}", &coords);
                                 self.buffer_pool.allocate_layer_geometry(
@@ -552,27 +569,36 @@ impl RenderState {
         let view_region = self
             .camera
             .view_region_bounding_box(&view_proj.invert())
-            .map(|bounding_box| ViewRegion::new(bounding_box, 1, self.zoom, visible_z));
+            .map(|bounding_box| ViewRegion::new(bounding_box, 0, *self.zoom, visible_z));
 
         drop(_guard);
 
         if let Some(view_region) = &view_region {
             self.upload_tile_geometry(&view_proj, view_region, scheduler);
             self.update_metadata(scheduler, view_region, &view_proj);
-            self.request_tiles_in_view(view_region, scheduler);
         }
 
         // TODO: Could we draw inspiration from StagingBelt (https://docs.rs/wgpu/latest/wgpu/util/struct.StagingBelt.html)?
         // TODO: What is StagingBelt for?
 
-        // Update globals
-        self.queue.write_buffer(
-            &self.globals_uniform_buffer,
-            0,
-            bytemuck::cast_slice(&[ShaderGlobals::new(
-                self.camera.create_camera_uniform(&self.perspective),
-            )]),
-        );
+        if self.camera.did_change() || self.zoom.did_change() || self.try_failed {
+            if let Some(view_region) = &view_region {
+                // FIXME: We also need to request tiles from layers above if we are over the maximum zoom level
+                self.try_failed = self.request_tiles_in_view(view_region, scheduler);
+            }
+
+            // Update globals
+            self.queue.write_buffer(
+                &self.globals_uniform_buffer,
+                0,
+                bytemuck::cast_slice(&[ShaderGlobals::new(
+                    self.camera.create_camera_uniform(&self.perspective),
+                )]),
+            );
+        }
+
+        self.camera.finished_observing();
+        self.zoom.finished_observing();
     }
 
     #[tracing::instrument(skip_all)]

src/render/tile_view_pattern.rs

@@ -1,4 +1,4 @@
-use crate::coords::{ViewRegion, WorldTileCoords};
+use crate::coords::{ViewRegion, WorldTileCoords, Zoom};
 use crate::io::tile_cache::TileCache;
 use crate::render::buffer_pool::{BackingBufferDescriptor, BufferPool, Queue};
 use crate::render::camera::ViewProjection;
@@ -28,6 +28,18 @@ pub struct TileShape {
     pub buffer_range: Range<wgpu::BufferAddress>,
 }
 
+impl TileShape {
+    fn new(coords: WorldTileCoords, zoom: Zoom, index: u64) -> Self {
+        const STRIDE: u64 = size_of::<ShaderTileMetadata>() as u64;
+        Self {
+            coords,
+            zoom_factor: zoom.scale_to_tile(&coords),
+            transform: coords.transform_for_zoom(zoom),
+            buffer_range: index as u64 * STRIDE..(index as u64 + 1) * STRIDE,
+        }
+    }
+}
+
 pub struct TileInView {
     pub shape: TileShape,
 
@@ -61,7 +73,6 @@ impl<Q: Queue<B>, B> TileViewPattern<Q, B> {
     pub fn update_pattern(
         &mut self,
         view_region: &ViewRegion,
-        tile_cache: &TileCache,
         buffer_pool: &BufferPool<
             wgpu::Queue,
             Buffer,
@@ -70,43 +81,31 @@ impl<Q: Queue<B>, B> TileViewPattern<Q, B> {
             ShaderLayerMetadata,
             ShaderFeatureStyle,
         >,
-        zoom: f64,
+        zoom: Zoom,
     ) {
         self.in_view.clear();
 
-        let stride = size_of::<ShaderTileMetadata>() as u64;
-
         let mut index = 0;
 
+        let pool_index = buffer_pool.index();
+
         for coords in view_region.iter() {
             if coords.build_quad_key().is_none() {
                 continue;
             }
 
-            let shape = TileShape {
-                coords,
-                zoom_factor: 2.0_f64.powf(coords.z as f64 - zoom),
-                transform: coords.transform_for_zoom(zoom),
-                buffer_range: index as u64 * stride..(index as u64 + 1) * stride,
-            };
+            let shape = TileShape::new(coords, zoom, index);
 
             index += 1;
 
             let fallback = {
-                if !buffer_pool.index().has_tile(&coords) {
-                    if let Some(fallback_coords) =
-                        buffer_pool.index().get_tile_coords_fallback(&coords)
-                    {
+                if !pool_index.has_tile(&coords) {
+                    if let Some(fallback_coords) = pool_index.get_tile_coords_fallback(&coords) {
                         tracing::trace!(
                             "Could not find data at {coords}. Falling back to {fallback_coords}"
                         );
 
-                        let shape = TileShape {
-                            coords: fallback_coords,
-                            zoom_factor: 2.0_f64.powf(fallback_coords.z as f64 - zoom),
-                            transform: fallback_coords.transform_for_zoom(zoom),
-                            buffer_range: index as u64 * stride..(index as u64 + 1) * stride,
-                        };
+                        let shape = TileShape::new(fallback_coords, zoom, index);
 
                         index += 1;
                         Some(shape)

src/util/mod.rs

@@ -5,6 +5,7 @@ pub mod math;
 
 use crate::coords::WorldTileCoords;
 pub use fps_meter::FPSMeter;
+use std::ops::{Deref, DerefMut};
 
 struct MinMaxBoundingBox {
     min_x: i32,
@@ -49,3 +50,61 @@ impl MinMaxBoundingBox {
         }
     }
 }
+
+pub struct ChangeObserver<T> {
+    inner: T,
+    last_value: Option<T>,
+}
+
+impl<T> ChangeObserver<T> {
+    pub fn new(value: T) -> Self {
+        Self {
+            inner: value,
+            last_value: None,
+        }
+    }
+}
+
+impl<T> ChangeObserver<T>
+where
+    T: Clone + Eq,
+{
+    pub fn finished_observing(&mut self) {
+        self.last_value = Some(self.inner.clone());
+    }
+
+    pub fn did_change(&self) -> bool {
+        if let Some(last_value) = &self.last_value {
+            if !last_value.eq(&self.inner) {
+                true
+            } else {
+                false
+            }
+        } else {
+            true
+        }
+    }
+}
+
+impl<T> Default for ChangeObserver<T>
+where
+    T: Default,
+{
+    fn default() -> Self {
+        ChangeObserver::new(T::default())
+    }
+}
+
+impl<T> Deref for ChangeObserver<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl<T> DerefMut for ChangeObserver<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.inner
+    }
+}