Implemented IME preview text for CJK characters in SDL2 backend

backends/imgui_impl_sdl2.cpp

@@ -354,6 +354,7 @@ bool ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event)
         case SDL_TEXTINPUT:
         {
             io.AddInputCharactersUTF8(event->text.text);
+            io.ClearPreEditText();
             return true;
         }
         case SDL_KEYDOWN:
@@ -379,13 +380,37 @@ bool ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event)
                 bd->MouseLastLeaveFrame = 0;
             }
             if (window_event == SDL_WINDOWEVENT_LEAVE)
+			{
                 bd->MouseLastLeaveFrame = ImGui::GetFrameCount() + 1;
+                ImGui::GetIO().ClearPreEditText();
+			}
             if (window_event == SDL_WINDOWEVENT_FOCUS_GAINED)
                 io.AddFocusEvent(true);
             else if (event->window.event == SDL_WINDOWEVENT_FOCUS_LOST)
+			{
                 io.AddFocusEvent(false);
+                ImGui::GetIO().ClearPreEditText();
+			}
             return true;
         }
+        case SDL_TEXTEDITING: 
+		{
+            if(strlen(event->edit.text) > 0)
+                ImGui::GetIO().SetPreEditText(event->edit.text);
+            else
+                io.ClearPreEditText();
+            break;
+        }
+#if defined(SDL_HINT_IME_SUPPORT_EXTENDED_TEXT)
+        case SDL_TEXTEDITING_EXT: 
+		{
+            if( event->editExt.text != nullptr && strlen(event->editExt.text) > 0)
+                ImGui::GetIO().SetPreEditText( event->editExt.text );
+            else
+                ImGui::GetIO().ClearPreEditText();
+            break;
+        }
+#endif
         case SDL_CONTROLLERDEVICEADDED:
         case SDL_CONTROLLERDEVICEREMOVED:
         {

imgui.cpp

@@ -1337,6 +1337,17 @@ ImGuiIO::ImGuiIO()
     AppAcceptingEvents = true;
     BackendUsingLegacyKeyArrays = (ImS8)-1;
     BackendUsingLegacyNavInputArray = true; // assume using legacy array until proven wrong
+    PreEditText[0] = '\0';
+}
+
+void ImGuiIO::SetPreEditText(const char *str)
+{
+    strcpy(PreEditText, str);
+}
+
+void ImGuiIO::ClearPreEditText()
+{
+    PreEditText[0] = '\0';
 }
 
 // Pass in translated ASCII characters for text input.

imgui.h

@@ -2168,6 +2168,8 @@ struct ImGuiIO
     IMGUI_API void  AddInputCharacter(unsigned int c);                      // Queue a new character input
     IMGUI_API void  AddInputCharacterUTF16(ImWchar16 c);                    // Queue a new character input from a UTF-16 character, it can be a surrogate
     IMGUI_API void  AddInputCharactersUTF8(const char* str);                // Queue a new characters input from a UTF-8 string
+    IMGUI_API void  SetPreEditText(const char *str);
+    IMGUI_API void  ClearPreEditText();
 
     IMGUI_API void  SetKeyEventNativeData(ImGuiKey key, int native_keycode, int native_scancode, int native_legacy_index = -1); // [Optional] Specify index for legacy <1.87 IsKeyXXX() functions with native indices + specify native keycode, scancode.
     IMGUI_API void  SetAppAcceptingEvents(bool accepting_events);           // Set master flag for accepting key/mouse/text events (default to true). Useful if you have native dialog boxes that are interrupting your application loop/refresh, and you want to disable events being queued while your app is frozen.
@@ -2251,6 +2253,7 @@ struct ImGuiIO
     bool        BackendUsingLegacyNavInputArray;    // 0: using AddKeyAnalogEvent(), 1: writing to legacy io.NavInputs[] directly
     ImWchar16   InputQueueSurrogate;                // For AddInputCharacterUTF16()
     ImVector<ImWchar> InputQueueCharacters;         // Queue of _characters_ input (obtained by platform backend). Fill using AddInputCharacter() helper.
+    char PreEditText[255];
 
     IMGUI_API   ImGuiIO();
 };

imgui_widgets.cpp

@@ -4973,7 +4973,30 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
         if (is_multiline || (buf_display_end - buf_display) < buf_display_max_length)
         {
             ImU32 col = GetColorU32(is_displaying_hint ? ImGuiCol_TextDisabled : ImGuiCol_Text);
-            draw_window->DrawList->AddText(g.Font, g.FontSize, draw_pos - draw_scroll, col, buf_display, buf_display_end, 0.0f, is_multiline ? NULL : &clip_rect);
+			if(GImGui->IO.PreEditText && !is_multiline)
+            {
+                const char *preEditTextEnd = GImGui->IO.PreEditText + strlen(GImGui->IO.PreEditText);
+                ImWchar wcharBuf[255] = { 0 };
+                int wlen = ImTextStrFromUtf8(wcharBuf, 255, GImGui->IO.PreEditText, preEditTextEnd);
+                char tmpBuf[255] = "\0";
+
+                ImVec2 drawPosStart = draw_pos - draw_scroll;
+                size_t tmpBufLen = ImTextStrToUtf8( tmpBuf, 255, text_begin, text_begin + state->Stb.cursor );
+                draw_window->DrawList->AddText(g.Font, g.FontSize, drawPosStart, col, tmpBuf, tmpBuf + tmpBufLen, 0.0f, &clip_rect);
+                drawPosStart.x += InputTextCalcTextSizeW(GImGui, text_begin, text_begin + state->Stb.cursor).x;
+                ImU32 bg_color = GetColorU32( ImGuiCol_TextSelectedBg, 1.f );
+                ImVec2 highlightRectMax = drawPosStart;
+                highlightRectMax += InputTextCalcTextSizeW(GImGui, wcharBuf, wcharBuf + wlen);
+                draw_window->DrawList->AddRectFilled( drawPosStart, highlightRectMax, bg_color );
+                draw_window->DrawList->AddText(g.Font, g.FontSize, drawPosStart, col, GImGui->IO.PreEditText, preEditTextEnd, 0.0f, &clip_rect);
+                if( buf_display_end != NULL && buf_display + tmpBufLen < buf_display_end )
+                {
+                    drawPosStart.x = highlightRectMax.x;
+                    draw_window->DrawList->AddText(g.Font, g.FontSize, drawPosStart, col, buf_display + tmpBufLen, buf_display_end, 0.0f, &clip_rect);
+                }
+            }
+            else
+                draw_window->DrawList->AddText(g.Font, g.FontSize, draw_pos - draw_scroll, col, buf_display, buf_display_end, 0.0f, is_multiline ? NULL : &clip_rect);
         }
 
         // Draw blinking cursor