Internals/experimental: BeginComboPreview(), EndComboPreview(). (#4168, #1658)

(amended)

docs/TODO.txt

@@ -184,7 +184,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
 
  - combo: use clipper: make it easier to disable clipper with a single flag.
  - combo: flag for BeginCombo to not return true when unchanged (#1182)
- - combo: a way/helper to customize the combo preview (#1658)
+ - combo: a way/helper to customize the combo preview (#1658) -> exeperimental BeginComboPreview()
  - combo/listbox: keyboard control. need InputText-like non-active focus + key handling. considering keyboard for custom listbox (pr #203)
  - listbox: multiple selection.
  - listbox: unselect option (#1208)

imgui.h

@@ -2463,6 +2463,7 @@ struct ImDrawList
     IMGUI_API void  _ResetForNewFrame();
     IMGUI_API void  _ClearFreeMemory();
     IMGUI_API void  _PopUnusedDrawCmd();
+    IMGUI_API void  _TryMergeDrawCmds();
     IMGUI_API void  _OnChangedClipRect();
     IMGUI_API void  _OnChangedTextureID();
     IMGUI_API void  _OnChangedVtxOffset();

imgui_demo.cpp

@@ -1026,8 +1026,8 @@ static void ShowDemoWindowWidgets()
         // stored in the object itself, etc.)
         const char* items[] = { "AAAA", "BBBB", "CCCC", "DDDD", "EEEE", "FFFF", "GGGG", "HHHH", "IIII", "JJJJ", "KKKK", "LLLLLLL", "MMMM", "OOOOOOO" };
         static int item_current_idx = 0; // Here we store our selection data as an index.
-        const char* combo_label = items[item_current_idx];  // Label to preview before opening the combo (technically it could be anything)
-        if (ImGui::BeginCombo("combo 1", combo_label, flags))
+        const char* combo_preview_value = items[item_current_idx];  // Pass in the preview value visible before opening the combo (it could be anything)
+        if (ImGui::BeginCombo("combo 1", combo_preview_value, flags))
         {
             for (int n = 0; n < IM_ARRAYSIZE(items); n++)
             {
@@ -1043,10 +1043,12 @@ static void ShowDemoWindowWidgets()
         }
 
         // Simplified one-liner Combo() API, using values packed in a single constant string
+        // This is a convenience for when the selection set is small and known at compile-time.
         static int item_current_2 = 0;
         ImGui::Combo("combo 2 (one-liner)", &item_current_2, "aaaa\0bbbb\0cccc\0dddd\0eeee\0\0");
 
         // Simplified one-liner Combo() using an array of const char*
+        // This is not very useful (may obsolete): prefer using BeginCombo()/EndCombo() for full control.
         static int item_current_3 = -1; // If the selection isn't within 0..count, Combo won't display a preview
         ImGui::Combo("combo 3 (array)", &item_current_3, items, IM_ARRAYSIZE(items));
 

imgui_draw.cpp

@@ -491,6 +491,18 @@ void ImDrawList::AddCallback(ImDrawCallback callback, void* callback_data)
 #define ImDrawCmd_HeaderCompare(CMD_LHS, CMD_RHS)   (memcmp(CMD_LHS, CMD_RHS, ImDrawCmd_HeaderSize))    // Compare ClipRect, TextureId, VtxOffset
 #define ImDrawCmd_HeaderCopy(CMD_DST, CMD_SRC)      (memcpy(CMD_DST, CMD_SRC, ImDrawCmd_HeaderSize))    // Copy ClipRect, TextureId, VtxOffset
 
+// Try to merge two last draw commands
+void ImDrawList::_TryMergeDrawCmds()
+{
+    ImDrawCmd* curr_cmd = &CmdBuffer.Data[CmdBuffer.Size - 1];
+    ImDrawCmd* prev_cmd = curr_cmd - 1;
+    if (ImDrawCmd_HeaderCompare(curr_cmd, prev_cmd) == 0 && curr_cmd->UserCallback == NULL && prev_cmd->UserCallback == NULL)
+    {
+        prev_cmd->ElemCount += curr_cmd->ElemCount;
+        CmdBuffer.pop_back();
+    }
+}
+
 // Our scheme may appears a bit unusual, basically we want the most-common calls AddLine AddRect etc. to not have to perform any check so we always have a command ready in the stack.
 // The cost of figuring out if a new command has to be added or if we can merge is paid in those Update** functions only.
 void ImDrawList::_OnChangedClipRect()

imgui_internal.h

@@ -808,6 +808,12 @@ enum ImGuiButtonFlagsPrivate_
     ImGuiButtonFlags_PressedOnDefault_      = ImGuiButtonFlags_PressedOnClickRelease
 };
 
+// Extend ImGuiComboFlags_
+enum ImGuiComboFlagsPrivate_
+{
+    ImGuiComboFlags_CustomPreview           = 1 << 20   // enable BeginComboPreview()
+};
+
 // Extend ImGuiSliderFlags_
 enum ImGuiSliderFlagsPrivate_
 {
@@ -996,6 +1002,19 @@ struct ImGuiStyleMod
     ImGuiStyleMod(ImGuiStyleVar idx, ImVec2 v)  { VarIdx = idx; BackupFloat[0] = v.x; BackupFloat[1] = v.y; }
 };
 
+// Storage data for BeginComboPreview()/EndComboPreview()
+struct IMGUI_API ImGuiComboPreviewData
+{
+    ImRect          PreviewRect;
+    ImVec2          BackupCursorPos;
+    ImVec2          BackupCursorMaxPos;
+    ImVec2          BackupCursorPosPrevLine;
+    float           BackupPrevLineTextBaseOffset;
+    ImGuiLayoutType BackupLayout;
+
+    ImGuiComboPreviewData() { memset(this, 0, sizeof(*this)); }
+};
+
 // Stacked storage data for BeginGroup()/EndGroup()
 struct IMGUI_API ImGuiGroupData
 {
@@ -1552,6 +1571,7 @@ struct ImGuiContext
     float                   ColorEditLastSat;                   // Backup of last Saturation associated to LastColor[3], so we can restore Saturation in lossy RGB<>HSV round trips
     float                   ColorEditLastColor[3];
     ImVec4                  ColorPickerRef;                     // Initial/reference color at the time of opening the color picker.
+    ImGuiComboPreviewData   ComboPreviewData;
     float                   SliderCurrentAccum;                 // Accumulated slider delta when using navigation controls.
     bool                    SliderCurrentAccumDirty;            // Has the accumulated slider delta changed since last time we tried to apply it?
     bool                    DragCurrentAccumDirty;
@@ -2416,6 +2436,8 @@ namespace ImGui
 
     // Combos
     IMGUI_API bool          BeginComboPopup(ImGuiID popup_id, const ImRect& bb, ImGuiComboFlags flags);
+    IMGUI_API bool          BeginComboPreview();
+    IMGUI_API void          EndComboPreview();
 
     // Gamepad/Keyboard Navigation
     IMGUI_API void          NavInitWindow(ImGuiWindow* window, bool force_reinit);

imgui_widgets.cpp

@@ -1541,6 +1541,8 @@ void ImGui::ShrinkWidths(ImGuiShrinkWidthItem* items, int count, float width_exc
 // - BeginCombo()
 // - BeginComboPopup() [Internal]
 // - EndCombo()
+// - BeginComboPreview() [Internal]
+// - EndComboPreview() [Internal]
 // - Combo()
 //-------------------------------------------------------------------------
 
@@ -1602,6 +1604,14 @@ bool ImGui::BeginCombo(const char* label, const char* preview_value, ImGuiComboF
     }
     RenderFrameBorder(bb.Min, bb.Max, style.FrameRounding);
 
+    // Custom preview
+    if (flags & ImGuiComboFlags_CustomPreview)
+    {
+        g.ComboPreviewData.PreviewRect = ImRect(bb.Min.x, bb.Min.y, value_x2, bb.Max.y);
+        IM_ASSERT(preview_value == NULL || preview_value[0] == 0);
+        preview_value = NULL;
+    }
+
     // Render preview and label
     if (preview_value != NULL && !(flags & ImGuiComboFlags_NoPreview))
     {
@@ -1683,6 +1693,57 @@ void ImGui::EndCombo()
     EndPopup();
 }
 
+// Call directly after the BeginCombo/EndCombo block. The preview is designed to only host non-interactive elements
+// (Experimental, see GitHub issues: #1658, #4168)
+bool ImGui::BeginComboPreview()
+{
+    ImGuiContext& g = *GImGui;
+    ImGuiWindow* window = g.CurrentWindow;
+    ImGuiComboPreviewData* preview_data = &g.ComboPreviewData;
+
+    if (window->SkipItems || !window->ClipRect.Overlaps(window->DC.LastItemRect)) // FIXME: Because we don't have a ImGuiItemStatusFlags_Visible flag to test last ItemAdd() result
+        return false;
+    IM_ASSERT(window->DC.LastItemRect.Min.x == preview_data->PreviewRect.Min.x && window->DC.LastItemRect.Min.y == preview_data->PreviewRect.Min.y); // Didn't call after BeginCombo/EndCombo block or forgot to pass ImGuiComboFlags_CustomPreview flag?
+    if (!window->ClipRect.Contains(preview_data->PreviewRect)) // Narrower test (optional)
+        return false;
+
+    // FIXME: This could be contained in a PushWorkRect() api
+    preview_data->BackupCursorPos = window->DC.CursorPos;
+    preview_data->BackupCursorMaxPos = window->DC.CursorMaxPos;
+    preview_data->BackupCursorPosPrevLine = window->DC.CursorPosPrevLine;
+    preview_data->BackupPrevLineTextBaseOffset = window->DC.PrevLineTextBaseOffset;
+    preview_data->BackupLayout = window->DC.LayoutType;
+    window->DC.CursorPos = preview_data->PreviewRect.Min + g.Style.FramePadding;
+    window->DC.CursorMaxPos = window->DC.CursorPos;
+    window->DC.LayoutType = ImGuiLayoutType_Horizontal;
+    PushClipRect(preview_data->PreviewRect.Min, preview_data->PreviewRect.Max, true);
+
+    return true;
+}
+
+void ImGui::EndComboPreview()
+{
+    ImGuiContext& g = *GImGui;
+    ImGuiWindow* window = g.CurrentWindow;
+    ImGuiComboPreviewData* preview_data = &g.ComboPreviewData;
+
+    // FIXME: Using CursorMaxPos approximation instead of correct AABB which we will store in ImDrawCmd in the future
+    ImDrawList* draw_list = window->DrawList;
+    if (window->DC.CursorMaxPos.x < preview_data->PreviewRect.Max.x && window->DC.CursorMaxPos.y < preview_data->PreviewRect.Max.y)
+        if (draw_list->CmdBuffer.Size > 1) // Unlikely case that the PushClipRect() didn't create a command
+        {
+            draw_list->_CmdHeader.ClipRect = draw_list->CmdBuffer[draw_list->CmdBuffer.Size - 1].ClipRect = draw_list->CmdBuffer[draw_list->CmdBuffer.Size - 2].ClipRect;
+            draw_list->_TryMergeDrawCmds();
+        }
+    PopClipRect();
+    window->DC.CursorPos = preview_data->BackupCursorPos;
+    window->DC.CursorMaxPos = ImMax(window->DC.CursorMaxPos, preview_data->BackupCursorMaxPos);
+    window->DC.CursorPosPrevLine = preview_data->BackupCursorPosPrevLine;
+    window->DC.PrevLineTextBaseOffset = preview_data->BackupPrevLineTextBaseOffset;
+    window->DC.LayoutType = preview_data->BackupLayout;
+    preview_data->PreviewRect = ImRect();
+}
+
 // Getter for the old Combo() API: const char*[]
 static bool Items_ArrayGetter(void* data, int idx, const char** out_text)
 {