wip

egui/src/context.rs

@@ -19,6 +19,15 @@ use epaint::{stats::*, text::Fonts, *};
 
 // ----------------------------------------------------------------------------
 
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum PassState {
+    SinglePass,
+    SizePass,
+    LayoutPass,
+}
+
+// ----------------------------------------------------------------------------
+
 /// A wrapper around [`Arc`](std::sync::Arc)`<`[`Context`]`>`.
 /// This is how you will normally create and access a [`Context`].
 ///
@@ -102,18 +111,101 @@ impl CtxRef {
     #[must_use]
     pub fn run(
         &mut self,
-        new_input: RawInput,
-        run_ui: impl FnOnce(&CtxRef),
+        new_raw_input: RawInput,
+        run_ui: impl Fn(&CtxRef),
     ) -> (Output, Vec<ClippedShape>) {
-        let mut self_: Context = (*self.0).clone();
-        self_.begin_frame_mut(new_input);
-        *self = Self(Arc::new(self_));
+        self.memory().begin_frame(&self.input, &new_raw_input);
+
+        self.mutate(|context| {
+            if let Some(new_pixels_per_point) = context.memory.lock().new_pixels_per_point.take() {
+                context.input.pixels_per_point = new_pixels_per_point;
+            }
+            context.input.begin_frame(&new_raw_input);
+            context.update_fonts(context.input.pixels_per_point());
+        });
+
+        // Ensure we register the background area so panels and background ui can catch clicks:
+        let screen_rect = self.input.screen_rect();
+        self.memory().areas.set_state(
+            LayerId::background(),
+            containers::area::State {
+                pos: screen_rect.min,
+                size: screen_rect.size(),
+                interactable: true,
+            },
+        );
+
+        if self.memory().options.multi_pass {
+            self.frame_state.lock().begin_pass(&self.input);
+            self.mutate(|context| {
+                context.pass_state = Some(PassState::SizePass);
+            });
 
             run_ui(self);
 
+            self.drain_paint_lists();
+            self.mutate(|context| {
+                context.input.on_events(new_raw_input);
+                context.pass_state = Some(PassState::LayoutPass);
+            });
+
+            self.frame_state.lock().begin_pass(&self.input);
+            run_ui(self);
+        } else {
+            self.mutate(|context| {
+                context.input.on_events(new_raw_input);
+                context.pass_state = Some(PassState::SinglePass);
+            });
+        }
+
+        self.mutate(|context| {
+            if let Some(new_pixels_per_point) = context.memory.lock().new_pixels_per_point.take() {
+                context.input.pixels_per_point = new_pixels_per_point;
+            }
+            context.input.begin_frame(&new_raw_input);
+            context.update_fonts(context.input.pixels_per_point());
+        });
+
+        // Ensure we register the background area so panels and background ui can catch clicks:
+        let screen_rect = self.input.screen_rect();
+        self.memory().areas.set_state(
+            LayerId::background(),
+            containers::area::State {
+                pos: screen_rect.min,
+                size: screen_rect.size(),
+                interactable: true,
+            },
+        );
+
+        if self.memory().options.multi_pass {
+            self.frame_state.lock().begin_pass(&self.input);
+            run_ui(self);
+
+            self.drain_paint_lists();
+            self.mutate(|context| context.input.on_events(new_raw_input));
+
+            self.frame_state.lock().begin_pass(&self.input);
+            run_ui(self);
+        } else {
+            self.mutate(|context| context.input.on_events(new_raw_input));
+
+            self.frame_state.lock().begin_pass(&self.input);
+            run_ui(self);
+        }
+
+        self.mutate(|context| {
+            context.pass_state = None;
+        });
+
         self.end_frame()
     }
 
+    fn mutate(&mut self, mutate: impl FnOnce(&mut Context)) {
+        let mut self_: Context = (*self.0).clone();
+        mutate(&mut self_);
+        *self = Self(Arc::new(self_));
+    }
+
     // ---------------------------------------------------------------------
 
     /// If the given [`Id`] is not unique, an error will be printed at the given position.
@@ -355,6 +447,8 @@ pub struct Context {
 
     /// While positive, keep requesting repaints. Decrement at the end of each frame.
     repaint_requests: AtomicU32,
+
+    pub(crate) pass_state: Option<PassState>,
 }
 
 impl Clone for Context {
@@ -363,13 +457,14 @@ impl Clone for Context {
             fonts: self.fonts.clone(),
             memory: self.memory.clone(),
             animation_manager: self.animation_manager.clone(),
+            context_menu_system: self.context_menu_system.clone(),
             input: self.input.clone(),
             frame_state: self.frame_state.clone(),
             graphics: self.graphics.clone(),
             output: self.output.clone(),
             paint_stats: self.paint_stats.clone(),
             repaint_requests: self.repaint_requests.load(SeqCst).into(),
-            context_menu_system: self.context_menu_system.clone(),
+            pass_state: None,
         }
     }
 }
@@ -566,31 +661,6 @@ impl Context {
 
     // ---------------------------------------------------------------------
 
-    fn begin_frame_mut(&mut self, new_raw_input: RawInput) {
-        self.memory().begin_frame(&self.input, &new_raw_input);
-
-        let mut input = std::mem::take(&mut self.input);
-        if let Some(new_pixels_per_point) = self.memory().new_pixels_per_point.take() {
-            input.pixels_per_point = new_pixels_per_point;
-        }
-
-        self.input = input.begin_frame(new_raw_input);
-        self.frame_state.lock().begin_frame(&self.input);
-
-        self.update_fonts(self.input.pixels_per_point());
-
-        // Ensure we register the background area so panels and background ui can catch clicks:
-        let screen_rect = self.input.screen_rect();
-        self.memory().areas.set_state(
-            LayerId::background(),
-            containers::area::State {
-                pos: screen_rect.min,
-                size: screen_rect.size(),
-                interactable: true,
-            },
-        );
-    }
-
     /// Load fonts unless already loaded.
     fn update_fonts(&mut self, pixels_per_point: f32) {
         let new_font_definitions = self.memory().new_font_definitions.take();
@@ -627,7 +697,7 @@ impl Context {
         self.memory()
             .end_frame(&self.input, &self.frame_state().used_ids);
 
-        self.fonts().end_frame();
+        self.fonts().prune_cache();
 
         let mut output: Output = std::mem::take(&mut self.output());
         if self.repaint_requests.load(SeqCst) > 0 {

egui/src/frame_state.rs

@@ -1,10 +1,12 @@
 use crate::*;
 
-/// State that is collected during a frame and then cleared.
-/// Short-term (single frame) memory.
+// TODO: rename `FrameState` -> `PassState` ?
+/// State that is collected during a pass and then cleared.
+///
+/// One frame consists of either one or two passes.
 #[derive(Clone)]
 pub(crate) struct FrameState {
-    /// All `Id`s that were used this frame.
+    /// All `Id`s that were used this pass.
     /// Used to debug `Id` clashes of widgets.
     pub(crate) used_ids: IdMap<Rect>,
 
@@ -20,9 +22,9 @@ pub(crate) struct FrameState {
     /// How much space is used by panels.
     pub(crate) used_by_panels: Rect,
 
-    /// If a tooltip has been shown this frame, where was it?
+    /// If a tooltip has been shown this pass, where was it?
     /// This is used to prevent multiple tooltips to cover each other.
-    /// Initialized to `None` at the start of each frame.
+    /// Initialized to `None` at the start of each pass.
     pub(crate) tooltip_rect: Option<(Id, Rect, usize)>,
 
     /// Cleared by the first `ScrollArea` that makes use of it.
@@ -46,7 +48,7 @@ impl Default for FrameState {
 }
 
 impl FrameState {
-    pub(crate) fn begin_frame(&mut self, input: &InputState) {
+    pub(crate) fn begin_pass(&mut self, input: &InputState) {
         let Self {
             used_ids,
             available_rect,

egui/src/input_state.rs

@@ -93,55 +93,59 @@ impl Default for InputState {
 }
 
 impl InputState {
-    #[must_use]
-    pub fn begin_frame(mut self, new: RawInput) -> InputState {
-        let time = new
+    /// Only update the basics (time, screen_rect, dt, …)
+    pub(crate) fn begin_frame(&mut self, new: &RawInput) {
+        let old_time = self.time;
+        let new_time = new
             .time
             .unwrap_or_else(|| self.time + new.predicted_dt as f64);
-        let unstable_dt = (time - self.time) as f32;
-        let screen_rect = new.screen_rect.unwrap_or(self.screen_rect);
-        self.create_touch_states_for_new_devices(&new.events);
-        for touch_state in self.touch_states.values_mut() {
-            touch_state.begin_frame(time, &new, self.pointer.interact_pos);
-        }
-        let pointer = self.pointer.begin_frame(time, &new);
 
-        let mut keys_down = self.keys_down;
-        let mut scroll_delta = Vec2::ZERO;
-        let mut zoom_factor_delta = 1.0;
+        self.pixels_per_point = new.pixels_per_point.unwrap_or(self.pixels_per_point);
+        self.screen_rect = new.screen_rect.unwrap_or(self.screen_rect);
+        self.time = new_time;
+        self.pointer.time = new_time;
+        self.pointer.delta = Vec2::ZERO;
+        self.pointer.pointer_events.clear();
+        self.unstable_dt = (new_time - old_time) as f32;
+        self.predicted_dt = new.predicted_dt;
+        self.modifiers = new.modifiers;
+
+        self.scroll_delta = Vec2::ZERO;
+        self.zoom_factor_delta = 1.0;
+        self.events.clear();
+    }
+
+    /// Take all events
+    pub(crate) fn on_events(&mut self, new: RawInput) {
+        self.create_touch_states_for_new_devices(&new.events);
+        self.pointer.on_events(&new.events);
+        for touch_state in self.touch_states.values_mut() {
+            touch_state.on_events(self.time, &new.events, self.pointer.interact_pos);
+        }
+
+        self.scroll_delta = Vec2::ZERO;
+        self.zoom_factor_delta = 1.0;
         for event in &new.events {
             match event {
                 Event::Key { key, pressed, .. } => {
                     if *pressed {
-                        keys_down.insert(*key);
+                        self.keys_down.insert(*key);
                     } else {
-                        keys_down.remove(key);
+                        self.keys_down.remove(key);
                     }
                 }
                 Event::Scroll(delta) => {
-                    scroll_delta += *delta;
+                    self.scroll_delta += *delta;
                 }
                 Event::Zoom(factor) => {
-                    zoom_factor_delta *= *factor;
+                    self.zoom_factor_delta *= *factor;
                 }
                 _ => {}
             }
         }
-        InputState {
-            pointer,
-            touch_states: self.touch_states,
-            scroll_delta,
-            zoom_factor_delta,
-            screen_rect,
-            pixels_per_point: new.pixels_per_point.unwrap_or(self.pixels_per_point),
-            time,
-            unstable_dt,
-            predicted_dt: new.predicted_dt,
-            modifiers: new.modifiers,
-            keys_down,
-            events: new.events.clone(), // TODO: remove clone() and use raw.events
-            raw: new,
-        }
+
+        self.events = new.events.clone(); // TODO: remove clone() and use raw.events, or remove [`Self::raw].
+        self.raw = new;
     }
 
     #[inline(always)]
@@ -417,16 +421,13 @@ impl Default for PointerState {
 }
 
 impl PointerState {
-    #[must_use]
-    pub(crate) fn begin_frame(mut self, time: f64, new: &RawInput) -> PointerState {
-        self.time = time;
-
+    pub(crate) fn on_events(&mut self, events: &[Event]) {
         self.pointer_events.clear();
 
         let old_pos = self.latest_pos;
         self.interact_pos = self.latest_pos;
 
-        for event in &new.events {
+        for event in events {
             match event {
                 Event::PointerMoved(pos) => {
                     let pos = *pos;
@@ -463,7 +464,7 @@ impl PointerState {
 
                     if pressed {
                         self.press_origin = Some(pos);
-                        self.press_start_time = Some(time);
+                        self.press_start_time = Some(self.time);
                         self.has_moved_too_much_for_a_click = false;
                         self.pointer_events.push(PointerEvent::Pressed(pos));
                     } else {
@@ -471,10 +472,10 @@ impl PointerState {
 
                         let click = if clicked {
                             let double_click =
-                                (time - self.last_click_time) < MAX_DOUBLE_CLICK_DELAY;
+                                (self.time - self.last_click_time) < MAX_DOUBLE_CLICK_DELAY;
                             let count = if double_click { 2 } else { 1 };
 
-                            self.last_click_time = time;
+                            self.last_click_time = self.time;
 
                             Some(Click {
                                 pos,
@@ -509,22 +510,20 @@ impl PointerState {
         };
 
         if let Some(pos) = self.latest_pos {
-            self.pos_history.add(time, pos);
+            self.pos_history.add(self.time, pos);
         } else {
             // we do not clear the `pos_history` here, because it is exactly when a finger has
             // released from the touch screen that we may want to assign a velocity to whatever
             // the user tried to throw.
         }
 
-        self.pos_history.flush(time);
+        self.pos_history.flush(self.time);
 
         self.velocity = if self.pos_history.len() >= 3 && self.pos_history.duration() > 0.01 {
             self.pos_history.velocity().unwrap_or_default()
         } else {
             Vec2::default()
         };
-
-        self
     }
 
     fn wants_repaint(&self) -> bool {

egui/src/input_state/touch_state.rs

@@ -3,7 +3,7 @@ use std::{collections::BTreeMap, fmt::Debug};
 use crate::{
     data::input::TouchDeviceId,
     emath::{normalized_angle, Pos2, Vec2},
-    Event, RawInput, TouchId, TouchPhase,
+    Event, TouchId, TouchPhase,
 };
 
 /// All you probably need to know about a multi-touch gesture.
@@ -122,9 +122,9 @@ impl TouchState {
         }
     }
 
-    pub fn begin_frame(&mut self, time: f64, new: &RawInput, pointer_pos: Option<Pos2>) {
+    pub(crate) fn on_events(&mut self, time: f64, events: &[Event], pointer_pos: Option<Pos2>) {
         let mut added_or_removed_touches = false;
-        for event in &new.events {
+        for event in events {
             match *event {
                 Event::Touch {
                     device_id,

egui/src/layout.rs

@@ -230,6 +230,18 @@ impl Layout {
         }
     }
 
+    /// change layout to one suitable for size pass
+    pub(crate) fn for_size_pass(self) -> Self {
+        Self {
+            main_dir: self.main_dir,
+            main_wrap: self.main_wrap,
+            main_align: Align::Min,
+            main_justify: false,
+            cross_align: Align::Min,
+            cross_justify: false,
+        }
+    }
+
     #[inline(always)]
     pub fn with_main_wrap(self, main_wrap: bool) -> Self {
         Self { main_wrap, ..self }

egui/src/lib.rs

@@ -396,6 +396,8 @@ pub mod text {
     };
 }
 
+pub(crate) use context::PassState;
+
 pub use {
     containers::*,
     context::{Context, CtxRef},

egui/src/memory.rs

@@ -100,6 +100,11 @@ pub struct Options {
     /// but is a signal to any backend that we want the [`crate::Output::events`] read out loud.
     /// Screen readers is an experimental feature of egui, and not supported on all platforms.
     pub screen_reader: bool,
+
+    /// Enable experimental two-pass layout.
+    ///
+    /// See https://github.com/emilk/egui/issues/843
+    pub multi_pass: bool,
 }
 
 // ----------------------------------------------------------------------------

egui/src/ui.rs

@@ -92,10 +92,15 @@ impl Ui {
     pub fn child_ui_with_id_source(
         &mut self,
         max_rect: Rect,
-        layout: Layout,
+        mut layout: Layout,
         id_source: impl Hash,
     ) -> Self {
         crate::egui_assert!(!max_rect.any_nan());
+
+        if self.ctx().pass_state == Some(PassState::SizePass) {
+            layout = layout.for_size_pass();
+        }
+
         let next_auto_id_source = Id::new(self.next_auto_id_source).with("child").value();
         self.next_auto_id_source = self.next_auto_id_source.wrapping_add(1);
         let menu_state = self.get_menu_state();

egui_demo_lib/src/backend_panel.rs

@@ -132,6 +132,13 @@ impl BackendPanel {
             ui.ctx().memory().options.screen_reader = screen_reader;
         }
 
+        {
+            let mut multi_pass = ui.ctx().memory().options.multi_pass;
+            ui.checkbox(&mut multi_pass, "☯ Multipass (experimental)")
+                .on_hover_text("Experimental two-pass layout");
+            ui.ctx().memory().options.multi_pass = multi_pass;
+        }
+
         if !frame.is_web() {
             ui.separator();
             if ui.button("Quit").clicked() {

epaint/src/text/fonts.rs

@@ -398,8 +398,8 @@ impl Fonts {
     }
 
     /// Must be called once per frame to clear the [`Galley`] cache.
-    pub fn end_frame(&self) {
-        self.galley_cache.lock().end_frame();
+    pub fn prune_cache(&self) {
+        self.galley_cache.lock().prune();
     }
 }
 
@@ -453,8 +453,8 @@ impl GalleyCache {
         self.cache.len()
     }
 
-    /// Must be called once per frame to clear the [`Galley`] cache.
-    pub fn end_frame(&mut self) {
+    /// Must be called once per frame to prune the [`Galley`] cache.
+    pub fn prune(&mut self) {
         let current_generation = self.generation;
         self.cache.retain(|_key, cached| {
             cached.last_used == current_generation // only keep those that were used this frame