Viewports: Lots of comments about setting up multi-viewports. (#1542)

imgui.h

@@ -32,7 +32,7 @@ Index of this file:
 // Helpers (ImGuiOnceUponAFrame, ImGuiTextFilter, ImGuiTextBuffer, ImGuiStorage, ImGuiListClipper, ImColor)
 // Draw List API (ImDrawCallback, ImDrawCmd, ImDrawIdx, ImDrawVert, ImDrawChannel, ImDrawListSplitter, ImDrawListFlags, ImDrawList, ImDrawData)
 // Font API (ImFontConfig, ImFontGlyph, ImFontGlyphRangesBuilder, ImFontAtlasFlags, ImFontAtlas, ImFont)
-// Platform interface for multi-viewport support (ImGuiPlatformMonitor, ImGuiPlatformIO, ImGuiViewport)
+// Platform interface for multi-viewport support (ImGuiPlatformIO, ImGuiPlatformMonitor, ImGuiViewportFlags, ImGuiViewport)
 
 */
 
@@ -775,11 +775,12 @@ namespace ImGui
     IMGUI_API void          MemFree(void* ptr);
 
     // (Optional) Platform/OS interface for multi-viewport support
+    // Read comments around the ImGuiPlatformIO structure for more details.
     // Note: You may use GetWindowViewport() to get the current viewport of the current window.
     IMGUI_API ImGuiPlatformIO&  GetPlatformIO();                                                // platform/renderer functions, for back-end to setup + viewports list.
     IMGUI_API ImGuiViewport*    GetMainViewport();                                              // main viewport. same as GetPlatformIO().MainViewport == GetPlatformIO().Viewports[0].
     IMGUI_API void              UpdatePlatformWindows();                                        // call in main loop. will call CreateWindow/ResizeWindow/etc. platform functions for each secondary viewport, and DestroyWindow for each inactive viewport.
-    IMGUI_API void              RenderPlatformWindowsDefault(void* platform_arg = NULL, void* renderer_arg = NULL); // call in main loop. will call RenderWindow/SwapBuffers platform functions for each secondary viewport which doesn't have the ImGuiViewportFlags_Minimized flag set. May be reimplemented by user for custom rendering needs.
+    IMGUI_API void              RenderPlatformWindowsDefault(void* platform_render_arg = NULL, void* renderer_render_arg = NULL); // call in main loop. will call RenderWindow/SwapBuffers platform functions for each secondary viewport which doesn't have the ImGuiViewportFlags_Minimized flag set. May be reimplemented by user for custom rendering needs.
     IMGUI_API void              DestroyPlatformWindows();                                       // call DestroyWindow platform functions for all viewports. call from back-end Shutdown() if you need to close platform windows before imgui shutdown. otherwise will be called by DestroyContext().
     IMGUI_API ImGuiViewport*    FindViewportByID(ImGuiID id);                                   // this is a helper for back-ends.
     IMGUI_API ImGuiViewport*    FindViewportByPlatformHandle(void* platform_handle);            // this is a helper for back-ends. the type platform_handle is decided by the back-end (e.g. HWND, MyWindow*, GLFWwindow* etc.)
@@ -2392,32 +2393,54 @@ struct ImFont
 
 //-----------------------------------------------------------------------------
 // [BETA] Platform interface for multi-viewport support
-// - completely optional, for advanced users!
-// - this is used for back-ends aiming to support the seamless creation of multiple viewport (= multiple Platform/OS windows)
-//   dear imgui manages the viewports, and the back-end create one Platform/OS windows for each secondary viewport.
-// - if you are new to dear imgui and trying to integrate it into your engine, you should probably ignore this for now.
+//-----------------------------------------------------------------------------
+// (Optional) This is completely optional, for advanced users!
+// If you are new to Dear ImGui and trying to integrate it into your engine, you can probably ignore this for now.
+//
+// This feature allows you to seamlessly drag Dear ImGui windows outside of your application viewport.
+// This is achieved by creating new Platform/OS windows on the fly, and rendering into them.
+// Dear ImGui manages the viewport structures, and the back-end create and maintain one Platform/OS window for each of those viewports.
+//
+// See Glossary https://github.com/ocornut/imgui/wiki/Glossary for details about some of the terminology.
+// See Thread https://github.com/ocornut/imgui/issues/1542 for gifs, news and questions about this evolving feature.
+//
+// About the coordinates system:
+// - When multi-viewports are enabled, all Dear ImGui coordinates become absolute coordinates (same as OS coordinates!)
+// - So e.g. ImGui::SetNextWindowPos(ImVec2(0,0)) will position a window relative to your primary monitor!
+// - If you want to position windows relative to your main application viewport, use ImGui::GetMainViewport()->Pos as a base position.
+//
+// Steps to use multi-viewports in your application, when using a default back-end from the examples/ folder:
+// - Application:  Enable feature with 'io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable'.
+// - Back-end:     The back-end initialization will setup all necessary ImGuiPlatformIO's functions and update monitors info every frame.
+// - Application:  In your main loop, call ImGui::UpdatePlatformWindows(), ImGui::RenderPlatformWindowsDefault() after EndFrame() or Render().
+// - Application:  Fix absolute coordinates used in ImGui::SetWindowPos() or ImGui::SetNextWindowPos() calls.
+//
+// Steps to use multi-viewports in your application, when using a custom back-end:
+// - Important:    THIS IS NOT EASY TO DO and comes with many subtleties not described here!
+//                 It's also an experimental feature, so some of the requirements may evolve.
+//                 Consider using default back-ends if you can. Either way, carefully follow and refer to examples/ back-ends for details.
+// - Application:  Enable feature with 'io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable'.
+// - Back-end:     Hook ImGuiPlatformIO's Platform_* and Renderer_* callbacks (see below).
+//                 Set 'io.BackendFlags |= ImGuiBackendFlags_PlatformHasViewports' and 'io.BackendFlags |= ImGuiBackendFlags_PlatformHasViewports'.
+//                 Update ImGuiPlatformIO's Monitors list every frame.
+//                 Update MousePos every frame, in absolute coordinates.
+// - Application:  In your main loop, call ImGui::UpdatePlatformWindows(), ImGui::RenderPlatformWindowsDefault() after EndFrame() or Render().
+//                 You may skip calling RenderPlatformWindowsDefault() if its API is not convenient for your needs. Read comments below.
+// - Application:  Fix absolute coordinates used in ImGui::SetWindowPos() or ImGui::SetNextWindowPos() calls.
+//
+// About ImGui::RenderPlatformWindowsDefault():
+// - This function is a mostly a _helper_ for the common-most cases, and to facilitate using default back-ends.
+// - You can check its simple source code to understand what it does.
+//   It basically iterates secondary viewports and call 4 functions that are setup in ImGuiPlatformIO, if available:
+//     Platform_RenderWindow(), Renderer_RenderWindow(), Platform_SwapBuffers(), Renderer_SwapBuffers()
+//   Those functions pointers exists only for the benefit of RenderPlatformWindowsDefault().
+// - If you have very specific rendering needs (e.g. flipping multiple swap-chain simultaneously, unusual sync/threading issues, etc.),
+//   you may be tempted to ignore RenderPlatformWindowsDefault() and write customized code to perform your renderingg.
+//   You may decide to setup the platform_io's *RenderWindow and *SwapBuffers pointers and call your functions through those pointers,
+//   or you may decide to never setup those pointers and call your code directly. They are a convenience, not an obligatory interface.
 //-----------------------------------------------------------------------------
 
-// (Optional) This is required when enabling multi-viewport. Represent the bounds of each connected monitor/display and their DPI.
-// We use this information for multiple DPI support + clamping the position of popups and tooltips so they don't straddle multiple monitors.
-struct ImGuiPlatformMonitor
-{
-    ImVec2  MainPos, MainSize;  // Coordinates of the area displayed on this monitor (Min = upper left, Max = bottom right)
-    ImVec2  WorkPos, WorkSize;  // (Optional) Coordinates without task bars / side bars / menu bars. imgui uses this to avoid positioning popups/tooltips inside this region.
-    float   DpiScale;           // 1.0f = 96 DPI
-    ImGuiPlatformMonitor() { MainPos = MainSize = WorkPos = WorkSize = ImVec2(0,0); DpiScale = 1.0f; }
-};
-
-// (Optional) Setup required only if (io.ConfigFlags & ImGuiConfigFlags_ViewportsEnable) is enabled.
-// Access via ImGui::GetPlatformIO(). This is designed so we can mix and match two imgui_impl_xxxx files,
-// one for the Platform (~window handling), one for Renderer. Custom engine back-ends will often provide
-// both Platform and Renderer interfaces and so may not need to use all functions.
-// Platform functions are typically called before their Renderer counterpart,
-// apart from Destroy which are called the other way.
-// RenderPlatformWindowsDefault() is that helper that iterate secondary viewports and call, in this order:
-//   Platform_RenderWindow(), Renderer_RenderWindow(), Platform_SwapBuffers(), Renderer_SwapBuffers()
-// You may skip using RenderPlatformWindowsDefault() and call your draw/swap functions yourself if you need
-// specific behavior for your multi-window rendering.
+// (Optional) Access via ImGui::GetPlatformIO()
 struct ImGuiPlatformIO
 {
     //------------------------------------------------------------------
@@ -2425,47 +2448,72 @@ struct ImGuiPlatformIO
     //------------------------------------------------------------------
 
     // (Optional) Platform functions (e.g. Win32, GLFW, SDL2)
-    // Most of them are called by ImGui::UpdatePlatformWindows() and ImGui::RenderPlatformWindowsDefault().
-    void    (*Platform_CreateWindow)(ImGuiViewport* vp);                    // Create a new platform window for the given viewport
-    void    (*Platform_DestroyWindow)(ImGuiViewport* vp);
-    void    (*Platform_ShowWindow)(ImGuiViewport* vp);                      // Newly created windows are initially hidden so SetWindowPos/Size/Title can be called on them first
-    void    (*Platform_SetWindowPos)(ImGuiViewport* vp, ImVec2 pos);
-    ImVec2  (*Platform_GetWindowPos)(ImGuiViewport* vp);
-    void    (*Platform_SetWindowSize)(ImGuiViewport* vp, ImVec2 size);
-    ImVec2  (*Platform_GetWindowSize)(ImGuiViewport* vp);
-    void    (*Platform_SetWindowFocus)(ImGuiViewport* vp);                  // Move window to front and set input focus
-    bool    (*Platform_GetWindowFocus)(ImGuiViewport* vp);
-    bool    (*Platform_GetWindowMinimized)(ImGuiViewport* vp);
-    void    (*Platform_SetWindowTitle)(ImGuiViewport* vp, const char* title);
-    void    (*Platform_SetWindowAlpha)(ImGuiViewport* vp, float alpha);     // (Optional) Setup window transparency
-    void    (*Platform_UpdateWindow)(ImGuiViewport* vp);                    // (Optional) Called in UpdatePlatforms(). Optional hook to allow the platform back-end from doing general book-keeping every frame.
-    void    (*Platform_RenderWindow)(ImGuiViewport* vp, void* render_arg);  // (Optional) Setup for render
-    void    (*Platform_SwapBuffers)(ImGuiViewport* vp, void* render_arg);   // (Optional) Call Present/SwapBuffers (platform side)
-    float   (*Platform_GetWindowDpiScale)(ImGuiViewport* vp);               // (Optional) [BETA] (FIXME-DPI) DPI handling: Return DPI scale for this viewport. 1.0f = 96 DPI.
-    void    (*Platform_OnChangedViewport)(ImGuiViewport* vp);               // (Optional) [BETA] (FIXME-DPI) DPI handling: Called during Begin() every time the viewport we are outputting into changes, so back-end has a chance to swap fonts to adjust style.
-    void    (*Platform_SetImeInputPos)(ImGuiViewport* vp, ImVec2 pos);      // (Optional) Set IME (Input Method Editor, e.g. for Asian languages) input position, so text preview appears over the imgui input box.
-    int     (*Platform_CreateVkSurface)(ImGuiViewport* vp, ImU64 vk_inst, const void* vk_allocators, ImU64* out_vk_surface); // (Optional) For Renderer to call into Platform code
-
-    // (Optional) Renderer functions (e.g. DirectX, OpenGL3, Vulkan)
-    void    (*Renderer_CreateWindow)(ImGuiViewport* vp);                    // Create swap chains, frame buffers etc.
-    void    (*Renderer_DestroyWindow)(ImGuiViewport* vp);
-    void    (*Renderer_SetWindowSize)(ImGuiViewport* vp, ImVec2 size);      // Resize swap chain, frame buffers etc.
-    void    (*Renderer_RenderWindow)(ImGuiViewport* vp, void* render_arg);  // (Optional) Clear targets, Render viewport->DrawData
-    void    (*Renderer_SwapBuffers)(ImGuiViewport* vp, void* render_arg);   // (Optional) Call Present/SwapBuffers (renderer side)
-
-    // (Optional) List of monitors (updated by: app/back-end, used by: imgui to clamp popups/tooltips within same monitor and not have them straddle monitors)
+    // For reference, the second column shows which function are generally calling the Platform Functions:
+    //   N = ImGui::NewFrame()                        ~ beginning of the dear imgui frame: read info from platform/OS windows (latest size/position)
+    //   F = ImGui::Begin(), ImGui::EndFrame()        ~ during the dear imgui frame
+    //   U = ImGui::UpdatePlatformWindows()           ~ after the dear imgui frame: create and update all platform/OS windows
+    //   R = ImGui::RenderPlatformWindowsDefault()    ~ render
+    //   D = ImGui::DestroyPlatformWindows()          ~ shutdown
+    // The general idea is that NewFrame() we will read the current Platform/OS state, and UpdatePlatformWindows() will write to it.
+    //
+    // The functions are designed so we can mix and match 2 imgui_impl_xxxx files, one for the Platform (~window/input handling), one for Renderer.
+    // Custom engine back-ends will often provide both Platform and Renderer interfaces and so may not need to use all functions.
+    // Platform functions are typically called before their Renderer counterpart, apart from Destroy which are called the other way.
+
+    // Platform function --------------------------------------------------- Called by -----
+    void    (*Platform_CreateWindow)(ImGuiViewport* vp);                    // . . U . .  // Create a new platform window for the given viewport
+    void    (*Platform_DestroyWindow)(ImGuiViewport* vp);                   // N . U . D  //
+    void    (*Platform_ShowWindow)(ImGuiViewport* vp);                      // . . U . .  // Newly created windows are initially hidden so SetWindowPos/Size/Title can be called on them before showing the window
+    void    (*Platform_SetWindowPos)(ImGuiViewport* vp, ImVec2 pos);        // . . U . .  // Set platform window position (given the upper-left corner of client area)
+    ImVec2  (*Platform_GetWindowPos)(ImGuiViewport* vp);                    // N . . . .  //
+    void    (*Platform_SetWindowSize)(ImGuiViewport* vp, ImVec2 size);      // . . U . .  // Set platform window client area size (ignoring OS decorations such as OS title bar etc.)
+    ImVec2  (*Platform_GetWindowSize)(ImGuiViewport* vp);                   // N . . . .  // Get platform window client area size
+    void    (*Platform_SetWindowFocus)(ImGuiViewport* vp);                  // N . . . .  // Move window to front and set input focus
+    bool    (*Platform_GetWindowFocus)(ImGuiViewport* vp);                  // . . U . .  //
+    bool    (*Platform_GetWindowMinimized)(ImGuiViewport* vp);              // N . . . .  // Get platform window minimized state. When minimized, we generally won't attempt to get/set size and contents will be culled more easily
+    void    (*Platform_SetWindowTitle)(ImGuiViewport* vp, const char* str); // . . U . .  // Set platform window title (given an UTF-8 string)
+    void    (*Platform_SetWindowAlpha)(ImGuiViewport* vp, float alpha);     // . . U . .  // (Optional) Setup window transparency
+    void    (*Platform_UpdateWindow)(ImGuiViewport* vp);                    // . . U . .  // (Optional) Called by UpdatePlatformWindows(). Optional hook to allow the platform back-end from doing general book-keeping every frame.
+    void    (*Platform_RenderWindow)(ImGuiViewport* vp, void* render_arg);  // . . . R .  // (Optional) Main rendering (platform side! This is often unused, or just setting a "current" context for OpenGL bindings). 'render_arg' is the value passed to RenderPlatformWindowsDefault().
+    void    (*Platform_SwapBuffers)(ImGuiViewport* vp, void* render_arg);   // . . . R .  // (Optional) Call Present/SwapBuffers (platform side! This is often unused!). 'render_arg' is the value passed to RenderPlatformWindowsDefault().
+    float   (*Platform_GetWindowDpiScale)(ImGuiViewport* vp);               // N . . . .  // (Optional) [BETA] FIXME-DPI: DPI handling: Return DPI scale for this viewport. 1.0f = 96 DPI.
+    void    (*Platform_OnChangedViewport)(ImGuiViewport* vp);               // . F . . .  // (Optional) [BETA] FIXME-DPI: DPI handling: Called during Begin() every time the viewport we are outputting into changes, so back-end has a chance to swap fonts to adjust style.
+    void    (*Platform_SetImeInputPos)(ImGuiViewport* vp, ImVec2 pos);      // . F . . .  // (Optional) Set IME (Input Method Editor, e.g. for Asian languages) input position, so text preview appears over the imgui input box. FIXME: The call timing of this is inconsistent because we want to support without multi-viewports.
+    int     (*Platform_CreateVkSurface)(ImGuiViewport* vp, ImU64 vk_inst, const void* vk_allocators, ImU64* out_vk_surface); // (Optional) For a Vulkan Renderer to call into Platform code (since the surface creation needs to tie them both).
+
+    // (Optional) Renderer functions (e.g. DirectX, OpenGL, Vulkan)
+    void    (*Renderer_CreateWindow)(ImGuiViewport* vp);                    // . . U . .  // Create swap chain, frame buffers etc. (called after Platform_CreateWindow)
+    void    (*Renderer_DestroyWindow)(ImGuiViewport* vp);                   // N . U . D  // Destroy swap chain, frame buffers etc. (called before Platform_DestroyWindow)
+    void    (*Renderer_SetWindowSize)(ImGuiViewport* vp, ImVec2 size);      // . . U . .  // Resize swap chain, frame buffers etc. (called after Platform_SetWindowSize)
+    void    (*Renderer_RenderWindow)(ImGuiViewport* vp, void* render_arg);  // . . . R .  // (Optional) Clear framebuffer, setup render target, then render the viewport->DrawData. 'render_arg' is the value passed to RenderPlatformWindowsDefault().
+    void    (*Renderer_SwapBuffers)(ImGuiViewport* vp, void* render_arg);   // . . . R .  // (Optional) Call Present/SwapBuffers. 'render_arg' is the value passed to RenderPlatformWindowsDefault().
+
+    // (Optional) Monitor list
+    // - Updated by: app/back-end. Update every frame to dynamically support changing monitor or DPI configuration.
+    // - Used by: dear imgui to query DPI info, clamp popups/tooltips within same monitor and not have them straddle monitors.
     ImVector<ImGuiPlatformMonitor>  Monitors;
 
     //------------------------------------------------------------------
     // Output - List of viewports to render into platform windows
     //------------------------------------------------------------------
 
-    // List of viewports (the list is updated by calling ImGui::EndFrame or ImGui::Render)
+    // Viewports list (the list is updated by calling ImGui::EndFrame or ImGui::Render)
+    // (in the future we will attempt to organize this feature to remove the need for a "main viewport")
     ImGuiViewport*                  MainViewport;                           // Guaranteed to be == Viewports[0]
     ImVector<ImGuiViewport*>        Viewports;                              // Main viewports, followed by all secondary viewports.
     ImGuiPlatformIO()               { memset(this, 0, sizeof(*this)); }     // Zero clear
 };
 
+// (Optional) This is required when enabling multi-viewport. Represent the bounds of each connected monitor/display and their DPI.
+// We use this information for multiple DPI support + clamping the position of popups and tooltips so they don't straddle multiple monitors.
+struct ImGuiPlatformMonitor
+{
+    ImVec2  MainPos, MainSize;      // Coordinates of the area displayed on this monitor (Min = upper left, Max = bottom right)
+    ImVec2  WorkPos, WorkSize;      // (Optional) Coordinates without task bars / side bars / menu bars. imgui uses this to avoid positioning popups/tooltips inside this region.
+    float   DpiScale;               // 1.0f = 96 DPI
+    ImGuiPlatformMonitor()          { MainPos = MainSize = WorkPos = WorkSize = ImVec2(0, 0); DpiScale = 1.0f; }
+};
+
 // Flags stored in ImGuiViewport::Flags, giving indications to the platform back-ends.
 enum ImGuiViewportFlags_
 {
@@ -2498,9 +2546,11 @@ struct ImGuiViewport
     ImGuiID             ParentViewportId;       // (Advanced) 0: no parent. Instruct the platform back-end to setup a parent/child relationship between platform windows.
 
     // Our design separate the Renderer and Platform back-ends to facilitate combining default back-ends with each others.
-    // When our create your own back-end for a custom engine, it is possible that both Renderer and Platform will be handled by the same system and you may not need to use all the UserData/Handle fields.
-    void*               RendererUserData;       // void* to hold custom data structure for the renderer (e.g. swap chain, framebuffers etc.).
-    void*               PlatformUserData;       // void* to hold custom data structure for the OS / platform (e.g. windowing info, render context).
+    // When our create your own back-end for a custom engine, it is possible that both Renderer and Platform will be handled
+    // by the same system and you may not need to use all the UserData/Handle fields.
+    // The library never uses those fields, they are merely storage to facilitate back-end implementation.
+    void*               RendererUserData;       // void* to hold custom data structure for the renderer (e.g. swap chain, framebuffers etc.). generally set by your Renderer_CreateWindow function.
+    void*               PlatformUserData;       // void* to hold custom data structure for the OS / platform (e.g. windowing info, render context). generally set by your Platform_CreateWindow function.
     void*               PlatformHandle;         // void* for FindViewportByPlatformHandle(). (e.g. suggested to use natural platform handle such as HWND, GLFWWindow*, SDL_Window*)
     void*               PlatformHandleRaw;      // void* to hold lower-level, platform-native window handle (e.g. the HWND) when using an abstraction layer like GLFW or SDL (where PlatformHandle would be a SDL_Window*)
     bool                PlatformRequestMove;    // Platform window requested move (e.g. window was moved by the OS / host window manager, authoritative position will be OS window position)