Windows, Internal: added experimental SkipRefresh mode. (#3515, #4763, #7556, #5116 , #4076, #2749, #2268)

currently: ImGui::SetNextWindowRefreshPolicy(ImGuiWindowRefreshFlags_TryToAvoidRefresh);
- This is NOT meant to replace frame-wide/app-wide idle mode.
- This is another tool: the idea that a given window could avoid refresh and reuse last frame contents.
- I think it needs to be backed by a careful and smart design overall (refresh policy, load balancing, making it easy and obvious to user).
- It's not there yet, this is currently a toy for experimenting.
My other issues with this:
- It appears to be very simple, but skipping most of Begin() logic will inevitably lead to tricky/confusing bugs. Let's see how it goes.
- I don't like very much that this opens a door to varying inconsistencies
- I don't like very much that it can lead us to situation where the lazy refresh gets disabled in bulk due to some reason (e.g. resizing a dock space) and we get sucked in the temptation to update for idle rather than update for dynamism.

imgui.cpp

@@ -6334,6 +6334,30 @@ void ImGui::UpdateWindowParentAndRootLinks(ImGuiWindow* window, ImGuiWindowFlags
     }
 }
 
+// [EXPERIMENTAL] Called by Begin(). NextWindowData is valid at this point.
+// This is designed as a toy/test-bed for
+void ImGui::UpdateWindowSkipRefresh(ImGuiWindow* window)
+{
+    ImGuiContext& g = *GImGui;
+    window->SkipRefresh = false;
+    if ((g.NextWindowData.Flags & ImGuiNextWindowDataFlags_HasRefreshPolicy) == 0)
+        return;
+    if (g.NextWindowData.RefreshFlagsVal & ImGuiWindowRefreshFlags_TryToAvoidRefresh)
+    {
+        // FIXME-IDLE: Tests for e.g. mouse clicks or keyboard while focused.
+        if (window->Appearing) // If currently appearing
+            return;
+        if (window->Hidden) // If was hidden (previous frame)
+            return;
+        if ((g.NextWindowData.RefreshFlagsVal & ImGuiWindowRefreshFlags_RefreshOnHover) && g.HoveredWindow && window->RootWindow == g.HoveredWindow->RootWindow)
+            return;
+        if ((g.NextWindowData.RefreshFlagsVal & ImGuiWindowRefreshFlags_RefreshOnFocus) && g.NavWindow && window->RootWindow == g.NavWindow->RootWindow)
+            return;
+        window->DrawList = NULL;
+        window->SkipRefresh = true;
+    }
+}
+
 // When a modal popup is open, newly created windows that want focus (i.e. are not popups and do not specify ImGuiWindowFlags_NoFocusOnAppearing)
 // should be positioned behind that modal window, unless the window was created inside the modal begin-stack.
 // In case of multiple stacked modals newly created window honors begin stack order and does not go below its own modal parent.
@@ -6532,11 +6556,14 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     if (window->Appearing)
         SetWindowConditionAllowFlags(window, ImGuiCond_Appearing, false);
 
+    // [EXPERIMENTAL] Skip Refresh mode
+    UpdateWindowSkipRefresh(window);
+
     // We intentionally set g.CurrentWindow to NULL to prevent usage until when the viewport is set, then will call SetCurrentWindow()
     g.CurrentWindow = NULL;
 
     // When reusing window again multiple times a frame, just append content (don't need to setup again)
-    if (first_begin_of_the_frame)
+    if (first_begin_of_the_frame && !window->SkipRefresh)
     {
         // Initialize
         const bool window_is_child_tooltip = (flags & ImGuiWindowFlags_ChildWindow) && (flags & ImGuiWindowFlags_Tooltip); // FIXME-WIP: Undocumented behavior of Child+Tooltip for pinned tooltip (#1345)
@@ -7027,11 +7054,16 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     }
     else
     {
+        // Skip refresh always mark active
+        if (window->SkipRefresh)
+            window->Active = true;
+
         // Append
         SetCurrentWindow(window);
         SetLastItemDataForWindow(window, window->TitleBarRect());
     }
 
+    if (!window->SkipRefresh)
         PushClipRect(window->InnerClipRect.Min, window->InnerClipRect.Max, true);
 
     // Clear 'accessed' flag last thing (After PushClipRect which will set the flag. We want the flag to stay false when the default "Debug" window is unused)
@@ -7040,7 +7072,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     g.NextWindowData.ClearFlags();
 
     // Update visibility
-    if (first_begin_of_the_frame)
+    if (first_begin_of_the_frame && !window->SkipRefresh)
     {
         if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_ChildMenu))
         {
@@ -7086,6 +7118,11 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
                 skip_items = true;
         window->SkipItems = skip_items;
     }
+    else if (first_begin_of_the_frame)
+    {
+        // Skip refresh mode
+        window->SkipItems = true;
+    }
 
     // [DEBUG] io.ConfigDebugBeginReturnValue override return value to test Begin/End and BeginChild/EndChild behaviors.
     // (The implicit fallback window is NOT automatically ended allowing it to always be able to receive commands without crashing)
@@ -7128,9 +7165,16 @@ void ImGui::End()
     // Close anything that is open
     if (window->DC.CurrentColumns)
         EndColumns();
+    if (!window->SkipRefresh)
         PopClipRect();   // Inner window clip rectangle
     PopFocusScope();
 
+    if (window->SkipRefresh)
+    {
+        IM_ASSERT(window->DrawList == NULL);
+        window->DrawList = &window->DrawListInst;
+    }
+
     // Stop logging
     if (!(window->Flags & ImGuiWindowFlags_ChildWindow))    // FIXME: add more options for scope of logging
         LogFinish();
@@ -7815,6 +7859,14 @@ void ImGui::SetNextWindowBgAlpha(float alpha)
     g.NextWindowData.BgAlphaVal = alpha;
 }
 
+// This is experimental and meant to be a toy for exploring a future/wider range of features.
+void ImGui::SetNextWindowRefreshPolicy(ImGuiWindowRefreshFlags flags)
+{
+    ImGuiContext& g = *GImGui;
+    g.NextWindowData.Flags |= ImGuiNextWindowDataFlags_HasRefreshPolicy;
+    g.NextWindowData.RefreshFlagsVal = flags;
+}
+
 ImDrawList* ImGui::GetWindowDrawList()
 {
     ImGuiWindow* window = GetCurrentWindow();

imgui_demo.cpp

@@ -10,6 +10,11 @@
 // Read top of imgui.cpp and imgui.h for many details, documentation, comments, links.
 // Get the latest version at https://github.com/ocornut/imgui
 
+// How to easily locate code?
+// - Use the Item Picker to debug break in code by clicking any widgets: https://github.com/ocornut/imgui/wiki/Debug-Tools
+// - Browse an online version the demo with code linked to hovered widgets: https://pthom.github.io/imgui_manual_online/manual/imgui_manual.html
+// - Find a visible string and search for it in the code!
+
 //---------------------------------------------------
 // PLEASE DO NOT REMOVE THIS FILE FROM YOUR PROJECT!
 //---------------------------------------------------

imgui_draw.cpp

@@ -385,6 +385,7 @@ void ImDrawListSharedData::SetCircleTessellationMaxError(float max_error)
 }
 
 // Initialize before use in a new frame. We always have a command ready in the buffer.
+// In the majority of cases, you would want to call PushClipRect() and PushTextureID() after this.
 void ImDrawList::_ResetForNewFrame()
 {
     // Verify that the ImDrawCmd fields we want to memcmp() are contiguous in memory.

imgui_internal.h

@@ -182,6 +182,7 @@ typedef int ImGuiSeparatorFlags;        // -> enum ImGuiSeparatorFlags_     // F
 typedef int ImGuiTextFlags;             // -> enum ImGuiTextFlags_          // Flags: for TextEx()
 typedef int ImGuiTooltipFlags;          // -> enum ImGuiTooltipFlags_       // Flags: for BeginTooltipEx()
 typedef int ImGuiTypingSelectFlags;     // -> enum ImGuiTypingSelectFlags_  // Flags: for GetTypingSelectRequest()
+typedef int ImGuiWindowRefreshFlags;    // -> enum ImGuiWindowRefreshFlags_ // Flags: for SetNextWindowRefreshPolicy()
 
 typedef void (*ImGuiErrorLogCallback)(void* user_data, const char* fmt, ...);
 
@@ -1117,6 +1118,15 @@ struct IMGUI_API ImGuiInputTextState
 
 };
 
+enum ImGuiWindowRefreshFlags_
+{
+    ImGuiWindowRefreshFlags_None                = 0,
+    ImGuiWindowRefreshFlags_TryToAvoidRefresh   = 1 << 0,   // [EXPERIMENTAL] Try to keep existing contents, USER MUST NOT HONOR BEGIN() RETURNING FALSE AND NOT APPEND.
+    ImGuiWindowRefreshFlags_RefreshOnHover      = 1 << 1,   // [EXPERIMENTAL] Always refresh on hover
+    ImGuiWindowRefreshFlags_RefreshOnFocus      = 1 << 2,   // [EXPERIMENTAL] Always refresh on focus
+    // Refresh policy/frequency, Load Balancing etc.
+};
+
 enum ImGuiNextWindowDataFlags_
 {
     ImGuiNextWindowDataFlags_None               = 0,
@@ -1129,6 +1139,7 @@ enum ImGuiNextWindowDataFlags_
     ImGuiNextWindowDataFlags_HasBgAlpha         = 1 << 6,
     ImGuiNextWindowDataFlags_HasScroll          = 1 << 7,
     ImGuiNextWindowDataFlags_HasChildFlags      = 1 << 8,
+    ImGuiNextWindowDataFlags_HasRefreshPolicy   = 1 << 9,
 };
 
 // Storage for SetNexWindow** functions
@@ -1150,6 +1161,7 @@ struct ImGuiNextWindowData
     void*                       SizeCallbackUserData;
     float                       BgAlphaVal;             // Override background alpha
     ImVec2                      MenuBarOffsetMinVal;    // (Always on) This is not exposed publicly, so we don't clear it and it doesn't have a corresponding flag (could we? for consistency?)
+    ImGuiWindowRefreshFlags     RefreshFlagsVal;
 
     ImGuiNextWindowData()       { memset(this, 0, sizeof(*this)); }
     inline void ClearFlags()    { Flags = ImGuiNextWindowDataFlags_None; }
@@ -2534,6 +2546,7 @@ struct IMGUI_API ImGuiWindow
     bool                    Collapsed;                          // Set when collapsing window to become only title-bar
     bool                    WantCollapseToggle;
     bool                    SkipItems;                          // Set when items can safely be all clipped (e.g. window not visible or collapsed)
+    bool                    SkipRefresh;                        // [EXPERIMENTAL] Reuse previous frame drawn contents, Begin() returns false.
     bool                    Appearing;                          // Set during the frame where the window is appearing (or re-appearing)
     bool                    Hidden;                             // Do not display (== HiddenFrames*** > 0)
     bool                    IsFallbackWindow;                   // Set on the "Debug##Default" window.
@@ -3007,6 +3020,7 @@ namespace ImGui
     IMGUI_API ImGuiWindow*  FindWindowByID(ImGuiID id);
     IMGUI_API ImGuiWindow*  FindWindowByName(const char* name);
     IMGUI_API void          UpdateWindowParentAndRootLinks(ImGuiWindow* window, ImGuiWindowFlags flags, ImGuiWindow* parent_window);
+    IMGUI_API void          UpdateWindowSkipRefresh(ImGuiWindow* window);
     IMGUI_API ImVec2        CalcWindowNextAutoFitSize(ImGuiWindow* window);
     IMGUI_API bool          IsWindowChildOf(ImGuiWindow* window, ImGuiWindow* potential_parent, bool popup_hierarchy);
     IMGUI_API bool          IsWindowWithinBeginStackOf(ImGuiWindow* window, ImGuiWindow* potential_parent);
@@ -3032,6 +3046,9 @@ namespace ImGui
     IMGUI_API int           FindWindowDisplayIndex(ImGuiWindow* window);
     IMGUI_API ImGuiWindow*  FindBottomMostVisibleWindowWithinBeginStack(ImGuiWindow* window);
 
+    // Windows: Idle, Refresh Policies [EXPERIMENTAL]
+    IMGUI_API void          SetNextWindowRefreshPolicy(ImGuiWindowRefreshFlags flags);
+
     // Fonts, drawing
     IMGUI_API void          SetCurrentFont(ImFont* font);
     inline ImFont*          GetDefaultFont() { ImGuiContext& g = *GImGui; return g.IO.FontDefault ? g.IO.FontDefault : g.IO.Fonts->Fonts[0]; }