UI utilities - "User" scrollbar

A scrollbar that can be used at will rather than on windows only.

ui-utilities.cc

@@ -3,6 +3,7 @@
 #include "c-utilities.hh"
 #include "ui-utilities.hh"
 
+#define IMGUI_DEFINE_MATH_OPERATORS
 #include <imgui_internal.h>
 
 
@@ -74,6 +75,129 @@ uint32_t ImGui::ColorHSVAToU32(
 	return GetColorU32( out );
 }
 
+/*------------------------------------------------------------------------------*/
+
+// Code stolen and modified from ImGui itself
+bool ImGui::UserScrollbar(
+		const bool horizontal ,
+		const float total ,
+		const float display ,
+		float* const scrollPos ,
+		const ImVec2 topLeft ,
+		const float length ,
+		char const* const name ) noexcept
+{
+	ImGuiContext& g = *GImGui;
+	ImGuiWindow* const window = g.CurrentWindow;
+	ImDrawList* const dl = window->DrawList;
+
+	const ImGuiStyle& style = g.Style;
+	const ImGuiID id = window->GetID( name ? name :
+			( horizontal ? "#UserScrollX" : "#UserScrollY" ) );
+
+	// Render background
+	ImRect bb = horizontal
+		? ImRect( topLeft , topLeft + ImVec2( length , style.ScrollbarSize ) )
+		: ImRect( topLeft , topLeft + ImVec2( style.ScrollbarSize , length ) );
+	ImRect contentArea = window->ContentsRegionRect;
+	contentArea.Translate( window->Pos );
+	if ( !bb.Overlaps( contentArea ) ) {
+		return false;
+	}
+	dl->AddRectFilled( bb.Min , bb.Max , GetColorU32( ImGuiCol_ScrollbarBg ) );
+	bb.Expand( ImVec2(
+		-ImClamp( (float)(int)( ( bb.Max.x - bb.Min.x - 2.0f ) * 0.5f ) , 0.0f , 3.0f ) ,
+		-ImClamp( (float)(int)( ( bb.Max.y - bb.Min.y - 2.0f ) * 0.5f ) , 0.0f , 3.0f ) ) );
+
+	// V denote the main, longer axis of the scrollbar (= height for a
+	// vertical scrollbar)
+	const float scrollbarSizeV = horizontal ? bb.GetWidth( ) : bb.GetHeight( );
+	float scrollV = *scrollPos;
+
+	// Calculate the height of our grabbable box. It generally represent
+	// the amount visible (vs the total scrollable amount) But we maintain
+	// a minimum size in pixel to allow for the user to still aim inside.
+	const float maxSize = ImMax( ImMax( total , display ) , 1.0f );
+	const float grabSizePx = ImClamp( scrollbarSizeV * ( display / maxSize ) ,
+			style.GrabMinSize , scrollbarSizeV );
+	const float grabSizeNorm = grabSizePx / scrollbarSizeV;
+
+	// Handle input right away.
+	bool held = false;
+	bool hovered = false;
+	const bool previouslyHeld = ( g.ActiveId == id );
+	ButtonBehavior( bb , id , &hovered , &held );
+
+	const float scrollMax = ImMax( 1.0f , total - display );
+	float scrollRatio = ImSaturate( scrollV / scrollMax );
+	float grabNorm = scrollRatio * ( scrollbarSizeV - grabSizePx ) / scrollbarSizeV;
+	if ( held && grabSizeNorm < 1.0f ) {
+		ImGuiStorage* const storage = window->DC.StateStorage;
+		const float scrollbarPosV = horizontal ? bb.Min.x : bb.Min.y;
+		const float mousePos = horizontal ? g.IO.MousePos.x : g.IO.MousePos.y;
+		float clickDeltaToGrabCenter = storage->GetFloat( id );
+
+		// Click position in scrollbar normalized space (0.0f->1.0f)
+		const float clickedVNorm = ImSaturate(
+				( mousePos - scrollbarPosV ) / scrollbarSizeV );
+		SetHoveredID( id );
+
+		bool seekAbsolute = false;
+		if ( !previouslyHeld ) {
+			// On initial click calculate the distance between mouse
+			// and the center of the grab
+			if ( clickedVNorm >= grabNorm && clickedVNorm <= grabNorm + grabSizeNorm ) {
+				clickDeltaToGrabCenter = clickedVNorm - grabNorm - grabSizeNorm * 0.5f;
+			} else {
+				seekAbsolute = true;
+				clickDeltaToGrabCenter = 0.0f;
+			}
+		}
+
+		// Apply scroll
+		const float scrollVNorm = ImSaturate(
+			( clickedVNorm - clickDeltaToGrabCenter - grabSizeNorm * 0.5f )
+				/ ( 1.0f - grabSizeNorm ) );
+		scrollV = (float)(int)( 0.5f + scrollVNorm * scrollMax );
+
+		// Update values for rendering
+		scrollRatio = ImSaturate( scrollV / scrollMax );
+		grabNorm = scrollRatio * ( scrollbarSizeV - grabSizePx ) / scrollbarSizeV;
+
+		// Update distance to grab now that we have seeked and saturated
+		if ( seekAbsolute ) {
+			clickDeltaToGrabCenter = clickedVNorm - grabNorm - grabSizeNorm * 0.5f;
+		}
+		storage->SetFloat( id , clickDeltaToGrabCenter );
+	}
+
+	// Render
+	const ImU32 grabCol = GetColorU32(
+			held ? ImGuiCol_ScrollbarGrabActive
+			     : hovered ? ImGuiCol_ScrollbarGrabHovered
+				       : ImGuiCol_ScrollbarGrab);
+	ImRect grabRect;
+	if ( horizontal ) {
+		grabRect = ImRect(
+			ImLerp( bb.Min.x , bb.Max.x , grabNorm ) , bb.Min.y,
+			ImMin( ImLerp( bb.Min.x , bb.Max.x , grabNorm ) + grabSizePx ,
+				contentArea.Max.x ) , bb.Max.y );
+	} else {
+		grabRect = ImRect(
+			bb.Min.x, ImLerp( bb.Min.y , bb.Max.y , grabNorm ) ,
+			bb.Max.x , ImMin(
+				ImLerp( bb.Min.y , bb.Max.y , grabNorm ) + grabSizePx ,
+				contentArea.Max.y ) );
+	}
+	dl->AddRectFilled( grabRect.Min , grabRect.Max , grabCol , style.ScrollbarRounding );
+
+	if ( scrollV != *scrollPos ) {
+		*scrollPos = scrollV;
+		return true;
+	}
+	return false;
+}
+
 
 /*= T_CameraMouseControl =======================================================*/
 

ui-utilities.hh

@@ -38,6 +38,35 @@ namespace ImGui {
 			float value ,
 			float alpha ) noexcept;
 
+	/*--------------------------------------------------------------------*/
+
+	/* "User" scrollbar
+	 *
+	 * Code ripped from ImGui and modified so it can be used in other
+	 * circumstances.
+	 *
+	 * Parameters:
+	 *	horizontal	Horizontal bar if true, vertical bar if false
+	 *	total		Total size of the scrolled area
+	 *	display		Size of the displayed area
+	 *	pos		Pointer to the position, will be updated when
+	 *			scrolled
+	 *	topLeft		Top-left corner of the scroll bar
+	 *	length		Width of the scrollbar (if horizontal)
+	 *			Height of the scrollbar (if vertical)
+	 *	name		Name of the widget; if NULL, #UserScrollX will
+	 *			be used by default for horizontal scrollbars,
+	 *			or #UserScrollY for vertical scrollbars
+	 */
+	bool UserScrollbar(
+			bool horizontal ,
+			float total ,
+			float display ,
+			float* pos ,
+			ImVec2 topLeft ,
+			float length ,
+			char const* name = nullptr ) noexcept;
+
 } // namespace ImGui
 
 /*----------------------------------------------------------------------------*/