From 455295011d1d04d2c9a87f22af4b8b7f99315c04 Mon Sep 17 00:00:00 2001
From: "Li, Zhen" <zhenli.craig@gatech.edu>
Date: Tue, 20 Feb 2024 21:48:23 -0500
Subject: [PATCH] Partial fix issue-42: allow drag on canvas

---
 Sources/Grape/Views/ForceDirectedGraph+Gesture.swift | 6 ++++--
 Sources/Grape/Views/ForceDirectedGraphModel.swift    | 5 +++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/Sources/Grape/Views/ForceDirectedGraph+Gesture.swift b/Sources/Grape/Views/ForceDirectedGraph+Gesture.swift
index 781d8cc..ee42d7b 100644
--- a/Sources/Grape/Views/ForceDirectedGraph+Gesture.swift
+++ b/Sources/Grape/Views/ForceDirectedGraph+Gesture.swift
@@ -9,13 +9,15 @@ extension ForceDirectedGraph {
     internal func onDragChange(
         _ value: SwiftUI.DragGesture.Value
     ) {
-        if model.draggingNodeID == nil {
+        if !model.isDragStartStateRecorded {
             if let nodeID = model.findNode(at: value.startLocation) {
                 model.draggingNodeID = nodeID
             } else {
                 model.backgroundDragStart = value.location.simd
             }
+            assert(model.isDragStartStateRecorded == true)
         }
+        
         guard let nodeID = model.draggingNodeID else {
             if let dragStart = model.backgroundDragStart {
                 let delta = value.location.simd - dragStart
@@ -51,7 +53,7 @@ extension ForceDirectedGraph {
             if let dragStart = model.backgroundDragStart {
                 let delta = value.location.simd - dragStart
                 model.modelTransform.translate += delta
-                model.backgroundDragStart = value.location.simd
+                model.backgroundDragStart = nil
             }
             return
         }
diff --git a/Sources/Grape/Views/ForceDirectedGraphModel.swift b/Sources/Grape/Views/ForceDirectedGraphModel.swift
index 11fb74b..4d55bb8 100644
--- a/Sources/Grape/Views/ForceDirectedGraphModel.swift
+++ b/Sources/Grape/Views/ForceDirectedGraphModel.swift
@@ -50,6 +50,11 @@ public final class ForceDirectedGraphModel<Content: GraphContent> {
     @usableFromInline
     var backgroundDragStart: SIMD2<Double>? = nil
 
+    @inlinable
+    var isDragStartStateRecorded: Bool {
+        return draggingNodeID != nil || backgroundDragStart != nil 
+    }
+
     @usableFromInline
     let velocityDecay: Double