ImGuiListClipper new version, detect height automatically, fix compatibility with SetScrollPosHere (#662)

imgui.h

@@ -1049,36 +1049,32 @@ struct ImColor
 };
 
 // Helper: Manually clip large list of items.
-// If you are displaying thousands of even spaced items and you have a random access to the list, you can perform clipping yourself to save on CPU.
+// If you are displaying thousands of evenly spaced items and you have a random access to the list, you can perform clipping yourself to save on CPU.
+// The clipper calculates the range of visible items and advance the cursor to compensate for the non-visible items we have skipped.
 // Usage:
-//    ImGuiListClipper clipper(count, ImGui::GetTextLineHeightWithSpacing());
-//    for (int i = clipper.DisplayStart; i < clipper.DisplayEnd; i++) // display only visible items
-//        ImGui::Text("line number %d", i);
-//    clipper.End();
-// NB: 'count' is only used to clamp the result, if you don't know your count you can use INT_MAX
+//     ImGuiListClipper clipper(1000);  // we have 1000 elements, evenly spaced.
+//     while (clipper.Step())
+//         for (int i = clipper.DisplayStart; i < clipper.DisplayEnd; i++)
+//             ImGui::Text("line number %d", i);
+// - Step 0: the clipper let you process the first element, regardless of it being visible or not, so we can measure the element height.
+// - Step 1: the clipper infer height from first element, calculate the actual range of elements to display, and position the cursor before the first element.
+// - Step 2: dummy step only required if an explicit items_height was passed to constructor or Begin() and user still call Step(). Does nothing and switch to Step 3.
+// - Step 3: the clipper validate that we have reached the expected Y position (corresponding to element DisplayEnd), advance the cursor to the end of the list and then returns 'false' to end the loop.
 struct ImGuiListClipper
 {
+    float   StartPosY;
     float   ItemsHeight;
-    int     ItemsCount, DisplayStart, DisplayEnd;
+    int     ItemsCount, StepNo, DisplayStart, DisplayEnd;
 
-    ImGuiListClipper()                         { ItemsHeight = 0.0f; ItemsCount = DisplayStart = DisplayEnd = -1; }
-    ImGuiListClipper(int count, float height)  { ItemsCount = -1; Begin(count, height); }
-    ~ImGuiListClipper()                        { IM_ASSERT(ItemsCount == -1); } // user forgot to call End()
+    // items_count:  Use -1 to ignore (you can call Begin later). Use INT_MAX if you don't know how many items you have (in which case the cursor won't be advanced in the final step).
+    // items_height: Use -1.0f to be calculated automatically on first step. Otherwise pass in the distance between your items, typically GetTextLineHeightWithSpacing() or GetItemsLineHeightWithSpacing().
+    // If you don't specify an items_height, you NEED to call Step(). If you specify items_height you may call the old Begin()/End() api directly, but prefer calling Step().
+    ImGuiListClipper(int items_count = -1, float items_height = -1.0f)  { Begin(items_count, items_height); } // NB: Begin() initialize every fields (as we allow user to call Begin/End multiple times on a same instance if they want).
+    ~ImGuiListClipper()                                                 { IM_ASSERT(ItemsCount == -1); }      // Assert if user forgot to call End() or Step() until false.
 
-    void Begin(int count, float height)        // items_height: generally pass GetTextLineHeightWithSpacing() or GetItemsLineHeightWithSpacing()
-    {
-        IM_ASSERT(ItemsCount == -1);
-        ItemsCount = count;
-        ItemsHeight = height;
-        ImGui::CalcListClipping(ItemsCount, ItemsHeight, &DisplayStart, &DisplayEnd); // calculate how many to clip/display
-        ImGui::SetCursorPosY(ImGui::GetCursorPosY() + DisplayStart * ItemsHeight);    // advance cursor
-    }
-    void End()
-    {
-        IM_ASSERT(ItemsCount >= 0);
-        ImGui::SetCursorPosY(ImGui::GetCursorPosY() + (ItemsCount - DisplayEnd) * ItemsHeight); // advance cursor
-        ItemsCount = -1;
-    }
+    IMGUI_API bool Step();                                              // Call until it returns false. The DisplayStart/DisplayEnd fields will be set and you can process/draw those items.
+    IMGUI_API void Begin(int items_count, float items_height = -1.0f);  // Automatically called by constructor if you passed 'items_count' or by Step() in Step 1.
+    IMGUI_API void End();                                               // Automatically called on the last call of Step() that returns false.
 };
 
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