Backends: SDL2/SDL3: Update monitor list when receiving a display event. (#6348)

backends/imgui_impl_sdl2.cpp

@@ -103,6 +103,7 @@
 #define SDL_HAS_USABLE_DISPLAY_BOUNDS       SDL_VERSION_ATLEAST(2,0,5)
 #define SDL_HAS_PER_MONITOR_DPI             SDL_VERSION_ATLEAST(2,0,4)
 #define SDL_HAS_VULKAN                      SDL_VERSION_ATLEAST(2,0,6)
+#define SDL_HAS_DISPLAY_EVENT               SDL_VERSION_ATLEAST(2,0,9)
 #if !SDL_HAS_VULKAN
 static const Uint32 SDL_WINDOW_VULKAN = 0x10000000;
 #endif
@@ -122,6 +123,7 @@ struct ImGui_ImplSDL2_Data
     bool            MouseCanUseGlobalState;
     bool            MouseCanReportHoveredViewport;  // This is hard to use/unreliable on SDL so we'll set ImGuiBackendFlags_HasMouseHoveredViewport dynamically based on state.
     bool            UseVulkan;
+    bool            WantUpdateMonitors;
 
     ImGui_ImplSDL2_Data()   { memset((void*)this, 0, sizeof(*this)); }
 };
@@ -364,6 +366,15 @@ bool ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event)
             io.SetKeyEventNativeData(key, event->key.keysym.sym, event->key.keysym.scancode, event->key.keysym.scancode); // To support legacy indexing (<1.87 user code). Legacy backend uses SDLK_*** as indices to IsKeyXXX() functions.
             return true;
         }
+#if SDL_HAS_DISPLAY_EVENT
+        case SDL_DISPLAYEVENT:
+        {
+            // 2.0.26 has SDL_DISPLAYEVENT_CONNECTED/SDL_DISPLAYEVENT_DISCONNECTED/SDL_DISPLAYEVENT_ORIENTATION,
+            // so change of DPI/Scaling are not reflected in this event. (SDL3 has it)
+            bd->WantUpdateMonitors = true;
+            return true;
+        }
+#endif
         case SDL_WINDOWEVENT:
         {
             // - When capturing mouse, SDL will send a bunch of conflicting LEAVE/ENTER event on every mouse move, but the final ENTER tends to be right.
@@ -436,6 +447,7 @@ static bool ImGui_ImplSDL2_Init(SDL_Window* window, SDL_Renderer* renderer, void
 #else
     bd->MouseCanReportHoveredViewport = false;
 #endif
+    bd->WantUpdateMonitors = true;
 
     io.SetClipboardTextFn = ImGui_ImplSDL2_SetClipboardText;
     io.GetClipboardTextFn = ImGui_ImplSDL2_GetClipboardText;
@@ -490,9 +502,6 @@ static bool ImGui_ImplSDL2_Init(SDL_Window* window, SDL_Renderer* renderer, void
     SDL_SetHint(SDL_HINT_MOUSE_AUTO_CAPTURE, "0");
 #endif
 
-    // Update monitors
-    ImGui_ImplSDL2_UpdateMonitors();
-
     // We need SDL_CaptureMouse(), SDL_GetGlobalMouseState() from SDL 2.0.4+ to support multiple viewports.
     // We left the call to ImGui_ImplSDL2_InitPlatformInterface() outside of #ifdef to avoid unused-function warnings.
     if ((io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable) && (io.BackendFlags & ImGuiBackendFlags_PlatformHasViewports))
@@ -692,11 +701,13 @@ static void ImGui_ImplSDL2_UpdateGamepads()
     #undef MAP_ANALOG
 }
 
-// FIXME-PLATFORM: SDL doesn't have an event to notify the application of display/monitor changes
+// FIXME: Note that doesn't update with DPI/Scaling change only as SDL2 doesn't have an event for it (SDL3 has).
 static void ImGui_ImplSDL2_UpdateMonitors()
 {
+    ImGui_ImplSDL2_Data* bd = ImGui_ImplSDL2_GetBackendData();
     ImGuiPlatformIO& platform_io = ImGui::GetPlatformIO();
     platform_io.Monitors.resize(0);
+    bd->WantUpdateMonitors = false;
     int display_count = SDL_GetNumVideoDisplays();
     for (int n = 0; n < display_count; n++)
     {
@@ -743,6 +754,10 @@ void ImGui_ImplSDL2_NewFrame()
     if (w > 0 && h > 0)
         io.DisplayFramebufferScale = ImVec2((float)display_w / w, (float)display_h / h);
 
+    // Update monitors
+    if (bd->WantUpdateMonitors)
+        ImGui_ImplSDL2_UpdateMonitors();
+
     // Setup time step (we don't use SDL_GetTicks() because it is using millisecond resolution)
     // (Accept SDL_GetPerformanceCounter() not returning a monotonically increasing value. Happens in VMs and Emscripten, see #6189, #6114, #3644)
     static Uint64 frequency = SDL_GetPerformanceFrequency();

backends/imgui_impl_sdl3.cpp

@@ -64,6 +64,7 @@ struct ImGui_ImplSDL3_Data
     bool            MouseCanUseGlobalState;
     bool            MouseCanReportHoveredViewport;  // This is hard to use/unreliable on SDL so we'll set ImGuiBackendFlags_HasMouseHoveredViewport dynamically based on state.
     bool            UseVulkan;
+    bool            WantUpdateMonitors;
 
     ImGui_ImplSDL3_Data()   { memset((void*)this, 0, sizeof(*this)); }
 };
@@ -300,6 +301,15 @@ bool ImGui_ImplSDL3_ProcessEvent(const SDL_Event* event)
             io.SetKeyEventNativeData(key, event->key.keysym.sym, event->key.keysym.scancode, event->key.keysym.scancode); // To support legacy indexing (<1.87 user code). Legacy backend uses SDLK_*** as indices to IsKeyXXX() functions.
             return true;
         }
+        case SDL_EVENT_DISPLAY_ORIENTATION:
+        case SDL_EVENT_DISPLAY_CONNECTED:
+        case SDL_EVENT_DISPLAY_DISCONNECTED:
+        case SDL_EVENT_DISPLAY_MOVED:
+        case SDL_EVENT_DISPLAY_SCALE_CHANGED:
+        {
+            bd->WantUpdateMonitors = true;
+            return true;
+        }
         case SDL_EVENT_WINDOW_MOUSE_ENTER:
         {
             bd->MouseWindowID = event->window.windowID;
@@ -375,6 +385,7 @@ static bool ImGui_ImplSDL3_Init(SDL_Window* window, SDL_Renderer* renderer, void
 #else
     bd->MouseCanReportHoveredViewport = false;
 #endif
+    bd->WantUpdateMonitors = true;
 
     io.SetClipboardTextFn = ImGui_ImplSDL3_SetClipboardText;
     io.GetClipboardTextFn = ImGui_ImplSDL3_GetClipboardText;
@@ -420,9 +431,6 @@ static bool ImGui_ImplSDL3_Init(SDL_Window* window, SDL_Renderer* renderer, void
     // SDL 3.x : see https://github.com/libsdl-org/SDL/issues/6659
     SDL_SetHint("SDL_BORDERLESS_WINDOWED_STYLE", "0");
 
-    // Update monitors
-    ImGui_ImplSDL3_UpdateMonitors();
-
     // We need SDL_CaptureMouse(), SDL_GetGlobalMouseState() from SDL 2.0.4+ to support multiple viewports.
     // We left the call to ImGui_ImplSDL3_InitPlatformInterface() outside of #ifdef to avoid unused-function warnings.
     if ((io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable) && (io.BackendFlags & ImGuiBackendFlags_PlatformHasViewports))
@@ -619,11 +627,12 @@ static void ImGui_ImplSDL3_UpdateGamepads()
     #undef MAP_ANALOG
 }
 
-// FIXME-PLATFORM: SDL doesn't have an event to notify the application of display/monitor changes
 static void ImGui_ImplSDL3_UpdateMonitors()
 {
+    ImGui_ImplSDL3_Data* bd = ImGui_ImplSDL3_GetBackendData();
     ImGuiPlatformIO& platform_io = ImGui::GetPlatformIO();
     platform_io.Monitors.resize(0);
+    bd->WantUpdateMonitors = false;
 
     int display_count;
     SDL_DisplayID* displays = SDL_GetDisplays(&display_count);
@@ -665,6 +674,10 @@ void ImGui_ImplSDL3_NewFrame()
     if (w > 0 && h > 0)
         io.DisplayFramebufferScale = ImVec2((float)display_w / w, (float)display_h / h);
 
+    // Update monitors
+    if (bd->WantUpdateMonitors)
+        ImGui_ImplSDL3_UpdateMonitors();
+
     // Setup time step (we don't use SDL_GetTicks() because it is using millisecond resolution)
     // (Accept SDL_GetPerformanceCounter() not returning a monotonically increasing value. Happens in VMs and Emscripten, see #6189, #6114, #3644)
     static Uint64 frequency = SDL_GetPerformanceFrequency();

docs/CHANGELOG.txt

@@ -126,6 +126,10 @@ Docking+Viewports Branch:
 
 - Viewports: added void* ImGuiPlatformMonitor::PlatformHandle field (backend-dependant),
   for usage by user code.
+- Backends: SDL2: Update monitor list when receiving a display event. (#6348)
+  Note however that SDL2 currently doesn't have an event for a DPI/Scaling change,
+  so monitor data won't be updated in this situation.
+- Backends: SDL3: Update monitor list when receiving a display event. (#6348)
 
 
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