ImDrawList: Small refactor to create empty command when beginning the frame, allowing to simplify some functions.

+ Missing clearing two fields in ClearFreeMemory() (was hamrless)

imgui.cpp

@@ -3777,11 +3777,11 @@ void ImGui::NewFrame()
     if (g.IO.BackendFlags & ImGuiBackendFlags_RendererHasVtxOffset)
         g.DrawListSharedData.InitialFlags |= ImDrawListFlags_AllowVtxOffset;
 
-    g.BackgroundDrawList.Clear();
+    g.BackgroundDrawList.ResetForNewFrame();
     g.BackgroundDrawList.PushTextureID(g.IO.Fonts->TexID);
     g.BackgroundDrawList.PushClipRectFullScreen();
 
-    g.ForegroundDrawList.Clear();
+    g.ForegroundDrawList.ResetForNewFrame();
     g.ForegroundDrawList.PushTextureID(g.IO.Fonts->TexID);
     g.ForegroundDrawList.PushClipRectFullScreen();
 
@@ -5860,7 +5860,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         // DRAWING
 
         // Setup draw list and outer clipping rectangle
-        window->DrawList->Clear();
+        window->DrawList->ResetForNewFrame();
         window->DrawList->PushTextureID(g.Font->ContainerAtlas->TexID);
         PushClipRect(host_rect.Min, host_rect.Max, false);
 

imgui.h

@@ -1987,7 +1987,8 @@ struct ImDrawList
     ImDrawListSplitter      _Splitter;          // [Internal] for channels api (note: prefer using your own persistent instance of ImDrawListSplitter!)
 
     // If you want to create ImDrawList instances, pass them ImGui::GetDrawListSharedData() or create and use your own ImDrawListSharedData (so you can use ImDrawList without ImGui)
-    ImDrawList(const ImDrawListSharedData* shared_data) { _Data = shared_data; _OwnerName = NULL; Clear(); }
+    ImDrawList(const ImDrawListSharedData* shared_data) { _Data = shared_data; Flags = ImDrawListFlags_None; _VtxCurrentOffset = _VtxCurrentIdx = 0; _VtxWritePtr = NULL; _IdxWritePtr = NULL; _OwnerName = NULL; }
+
     ~ImDrawList() { ClearFreeMemory(); }
     IMGUI_API void  PushClipRect(ImVec2 clip_rect_min, ImVec2 clip_rect_max, bool intersect_with_current_clip_rect = false);  // Render-level scissoring. This is passed down to your render function but not used for CPU-side coarse clipping. Prefer using higher-level ImGui::PushClipRect() to affect logic (hit-testing and widget culling)
     IMGUI_API void  PushClipRectFullScreen();
@@ -2056,7 +2057,7 @@ struct ImDrawList
 
     // [Internal helpers]
     // NB: all primitives needs to be reserved via PrimReserve() beforehand!
-    IMGUI_API void  Clear();
+    IMGUI_API void  ResetForNewFrame();
     IMGUI_API void  ClearFreeMemory();
     IMGUI_API void  PrimReserve(int idx_count, int vtx_count);
     IMGUI_API void  PrimUnreserve(int idx_count, int vtx_count);

imgui_draw.cpp

@@ -376,12 +376,13 @@ void ImDrawListSharedData::SetCircleSegmentMaxError(float max_error)
     }
 }
 
-void ImDrawList::Clear()
+// Initialize before use in a new frame. We always have a command ready in the buffer.
+void ImDrawList::ResetForNewFrame()
 {
     CmdBuffer.resize(0);
     IdxBuffer.resize(0);
     VtxBuffer.resize(0);
-    Flags = _Data ? _Data->InitialFlags : ImDrawListFlags_None;
+    Flags = _Data->InitialFlags;
     _VtxCurrentOffset = 0;
     _VtxCurrentIdx = 0;
     _VtxWritePtr = NULL;
@@ -390,6 +391,7 @@ void ImDrawList::Clear()
     _TextureIdStack.resize(0);
     _Path.resize(0);
     _Splitter.Clear();
+    CmdBuffer.push_back(ImDrawCmd());
 }
 
 void ImDrawList::ClearFreeMemory()
@@ -397,6 +399,8 @@ void ImDrawList::ClearFreeMemory()
     CmdBuffer.clear();
     IdxBuffer.clear();
     VtxBuffer.clear();
+    Flags = ImDrawListFlags_None;
+    _VtxCurrentOffset = 0;
     _VtxCurrentIdx = 0;
     _VtxWritePtr = NULL;
     _IdxWritePtr = NULL;
@@ -434,8 +438,8 @@ void ImDrawList::AddDrawCmd()
 
 void ImDrawList::AddCallback(ImDrawCallback callback, void* callback_data)
 {
-    ImDrawCmd* curr_cmd = CmdBuffer.Size > 0 ? &CmdBuffer.Data[CmdBuffer.Size - 1] : NULL;
+    ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
-    if (!curr_cmd || curr_cmd->ElemCount != 0 || curr_cmd->UserCallback != NULL)
+    if (curr_cmd->ElemCount != 0 || curr_cmd->UserCallback != NULL)
     {
         AddDrawCmd();
         curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
@@ -452,8 +456,8 @@ void ImDrawList::UpdateClipRect()
 {
     // If current command is used with different settings we need to add a new command
     const ImVec4 curr_clip_rect = GetCurrentClipRect();
-    ImDrawCmd* curr_cmd = CmdBuffer.Size > 0 ? &CmdBuffer.Data[CmdBuffer.Size - 1] : NULL;
+    ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
-    if (!curr_cmd || (curr_cmd->ElemCount != 0 && memcmp(&curr_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) != 0) || curr_cmd->UserCallback != NULL)
+    if ((curr_cmd->ElemCount != 0 && memcmp(&curr_cmd->ClipRect, &curr_clip_rect, sizeof(ImVec4)) != 0) || curr_cmd->UserCallback != NULL)
     {
         AddDrawCmd();
         return;
@@ -477,7 +481,7 @@ void ImDrawList::UpdateTextureID()
 {
     // If current command is used with different settings we need to add a new command
     const ImTextureID curr_texture_id = GetCurrentTextureId();
-    ImDrawCmd* curr_cmd = CmdBuffer.Size > 0 ? &CmdBuffer.Data[CmdBuffer.Size - 1] : NULL;
+    ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
     if (!curr_cmd || (curr_cmd->ElemCount != 0 && curr_cmd->TextureId != curr_texture_id) || curr_cmd->UserCallback != NULL)
     {
         AddDrawCmd();