add missing file

src/helper/undo.js

@@ -0,0 +1,45 @@
+// undo functionality
+// modifed from https://github.com/memononen/stylii
+import paper from 'paper';
+
+const performSnapshot = function (dispatchPerformSnapshot) {
+    dispatchPerformSnapshot({
+        json: paper.project.exportJSON({asString: false})
+    });
+
+    // @todo enable/disable buttons
+    // updateButtonVisibility();
+};
+
+const _restore = function (entry) {
+    paper.project.clear();
+    paper.project.importJSON(entry.json);
+    paper.view.update();
+};
+
+const performUndo = function (undoState, dispatchPerformUndo) {
+    if (undoState.pointer > 0) {
+        _restore(undoState.stack[undoState.pointer - 1]);
+        dispatchPerformUndo();
+
+        // @todo enable/disable buttons
+        // updateButtonVisibility();
+    }
+};
+
+
+const performRedo = function (undoState, dispatchPerformRedo) {
+    if (undoState.pointer >= 0 && undoState.pointer < undoState.stack.length - 1) {
+        _restore(undoState.stack[undoState.pointer + 1]);
+        dispatchPerformRedo();
+        
+        // @todo enable/disable buttons
+        // updateButtonVisibility();
+    }
+};
+
+export {
+    performSnapshot,
+    performUndo,
+    performRedo
+};