Encapsulate undo and redo actions within history

src/history.ts

@@ -34,6 +34,29 @@ class SceneHistory {
     this.skipRecording();
   }
 
+  redoOnce(elements: ExcalidrawElement[]) {
+    const currentEntry = this.generateCurrentEntry(elements);
+    const entryToRestore = this.redoStack.pop();
+    if (entryToRestore !== undefined) {
+      this.restoreEntry(elements, entryToRestore);
+      this.stateHistory.push(currentEntry);
+    }
+  }
+
+  undoOnce(elements: ExcalidrawElement[]) {
+    const currentEntry = this.generateCurrentEntry(elements);
+    let entryToRestore = this.stateHistory.pop();
+
+    // If nothing was changed since last, take the previous one
+    if (currentEntry === entryToRestore) {
+      entryToRestore = this.stateHistory.pop();
+    }
+    if (entryToRestore !== undefined) {
+      this.restoreEntry(elements, entryToRestore);
+      this.redoStack.push(currentEntry);
+    }
+  }
+
   isRecording() {
     return this.recording;
   }

src/index.tsx

@@ -206,22 +206,10 @@ class App extends React.Component<{}, AppState> {
       const currentEntry = history.generateCurrentEntry(elements);
       if (event.shiftKey) {
         // Redo action
-        const entryToRestore = history.redoStack.pop();
+        history.redoOnce(elements);
-        if (entryToRestore !== undefined) {
-          history.restoreEntry(elements, entryToRestore);
-          history.stateHistory.push(currentEntry);
-        }
       } else {
         // undo action
-        let lastEntry = history.stateHistory.pop();
+        history.undoOnce(elements);
-        // If nothing was changed since last, take the previous one
-        if (currentEntry === lastEntry) {
-          lastEntry = history.stateHistory.pop();
-        }
-        if (lastEntry !== undefined) {
-          history.restoreEntry(elements, lastEntry);
-          history.redoStack.push(currentEntry);
-        }
       }
       this.forceUpdate();
       event.preventDefault();