Allow simultaneous access to different draw lists

Only need to prevent accessing the same draw-list twice at one time

Closes #488

imgui-examples/examples/draw_list.rs

@@ -19,8 +19,17 @@ fn draw_text_centered(
 fn main() {
     let system = support::init(file!());
     system.main_loop(move |_, ui| {
+
+        // Get access to draw FG and BG draw lists.
+        let bg_draw_list = ui.get_background_draw_list();
+        let fg_draw_list = ui.get_foreground_draw_list();
+
+        // Note we cannot access two instances of the same draw list
+        // at once. That is to say, the following line would panic if
+        // uncommented:
+        //let bg_draw_list_2 = ui.get_background_draw_list(); // panic!
+
         {
-            let bg_draw_list = ui.get_background_draw_list();
             bg_draw_list
                 .add_circle([150.0, 150.0], 150.0, [1.0, 0.0, 0.0])
                 .thickness(4.0)
@@ -36,7 +45,6 @@ fn main() {
 
         {
             let [w, h] = ui.io().display_size;
-            let fg_draw_list = ui.get_foreground_draw_list();
             fg_draw_list
                 .add_circle([w - 150.0, h - 150.0], 150.0, [1.0, 0.0, 0.0])
                 .thickness(4.0)

imgui/src/draw_list.rs

@@ -61,27 +61,47 @@ bitflags!(
     }
 );
 
+enum DrawListType {
+    Window,
+    Background,
+    Foreground,
+}
+
 /// Object implementing the custom draw API.
 ///
 /// Called from [`Ui::get_window_draw_list`], [`Ui::get_background_draw_list`] or [`Ui::get_foreground_draw_list`].
 /// No more than one instance of this structure can live in a program at the same time.
 /// The program will panic on creating a second instance.
 pub struct DrawListMut<'ui> {
+    draw_list_type: DrawListType,
     draw_list: *mut ImDrawList,
     _phantom: PhantomData<&'ui Ui<'ui>>,
 }
 
-static DRAW_LIST_LOADED: std::sync::atomic::AtomicBool = std::sync::atomic::AtomicBool::new(false);
+// Lock for each variant of draw list. See https://github.com/imgui-rs/imgui-rs/issues/488
+static DRAW_LIST_LOADED_WINDOW: std::sync::atomic::AtomicBool = std::sync::atomic::AtomicBool::new(false);
+static DRAW_LIST_LOADED_BACKGROUND: std::sync::atomic::AtomicBool = std::sync::atomic::AtomicBool::new(false);
+static DRAW_LIST_LOADED_FOREGROUND: std::sync::atomic::AtomicBool = std::sync::atomic::AtomicBool::new(false);
 
 impl<'ui> Drop for DrawListMut<'ui> {
     fn drop(&mut self) {
-        DRAW_LIST_LOADED.store(false, std::sync::atomic::Ordering::Release);
+        match self.draw_list_type {
+            DrawListType::Window => &DRAW_LIST_LOADED_WINDOW,
+            DrawListType::Background => &DRAW_LIST_LOADED_BACKGROUND,
+            DrawListType::Foreground => &DRAW_LIST_LOADED_FOREGROUND,
+        }.store(false, std::sync::atomic::Ordering::Release);
     }
 }
 
 impl<'ui> DrawListMut<'ui> {
-    fn lock_draw_list() {
+    fn lock_draw_list(t: DrawListType) {
-        let already_loaded = DRAW_LIST_LOADED
+        let lock = match t {
+            DrawListType::Window => &DRAW_LIST_LOADED_WINDOW,
+            DrawListType::Background => &DRAW_LIST_LOADED_BACKGROUND,
+            DrawListType::Foreground => &DRAW_LIST_LOADED_FOREGROUND,
+        };
+
+        let already_loaded = lock
             .compare_exchange(
                 false,
                 true,
@@ -90,33 +110,42 @@ impl<'ui> DrawListMut<'ui> {
             )
             .is_err();
         if already_loaded {
-            panic!("DrawListMut is already loaded! You can only load one instance of it!")
+            let name = match t {
+                DrawListType::Window => "window",
+                DrawListType::Background => "background",
+                DrawListType::Foreground => "foreground",
+            };
+            panic!("The DrawListMut instance for the {} draw list is already loaded! You can only load one instance of it!", name)
         }
     }
 
     #[doc(alias = "GetWindowDrawList")]
     pub(crate) fn window(_: &Ui<'ui>) -> Self {
-        Self::lock_draw_list();
+        Self::lock_draw_list(DrawListType::Window);
+
         Self {
             draw_list: unsafe { sys::igGetWindowDrawList() },
+            draw_list_type: DrawListType::Window,
             _phantom: PhantomData,
         }
     }
 
     #[doc(alias = "GetBackgroundDrawList")]
     pub(crate) fn background(_: &Ui<'ui>) -> Self {
-        Self::lock_draw_list();
+        Self::lock_draw_list(DrawListType::Background);
         Self {
             draw_list: unsafe { sys::igGetBackgroundDrawList() },
+            draw_list_type: DrawListType::Background,
             _phantom: PhantomData,
         }
     }
 
     #[doc(alias = "GetForegroundDrawList")]
     pub(crate) fn foreground(_: &Ui<'ui>) -> Self {
-        Self::lock_draw_list();
+        Self::lock_draw_list(DrawListType::Foreground);
         Self {
             draw_list: unsafe { sys::igGetForegroundDrawList() },
+            draw_list_type: DrawListType::Foreground,
             _phantom: PhantomData,
         }
     }