remove most setState({}) (#959)

src/scene/createScene.ts

@@ -1,17 +0,0 @@
-import { ExcalidrawElement } from "../element/types";
-
-class SceneState {
-  constructor(private _elements: readonly ExcalidrawElement[] = []) {}
-
-  getAllElements() {
-    return this._elements;
-  }
-
-  replaceAllElements(nextElements: readonly ExcalidrawElement[]) {
-    this._elements = nextElements;
-  }
-}
-
-export const createScene = () => {
-  return new SceneState();
-};

src/scene/globalScene.ts

@@ -0,0 +1,47 @@
+import { ExcalidrawElement } from "../element/types";
+
+export interface SceneStateCallback {
+  (): void;
+}
+
+export interface SceneStateCallbackRemover {
+  (): void;
+}
+
+class SceneState {
+  private callbacks: Set<SceneStateCallback> = new Set();
+
+  constructor(private _elements: readonly ExcalidrawElement[] = []) {}
+
+  getAllElements() {
+    return this._elements;
+  }
+
+  replaceAllElements(nextElements: readonly ExcalidrawElement[]) {
+    this._elements = nextElements;
+    this.informMutation();
+  }
+
+  informMutation() {
+    for (const callback of Array.from(this.callbacks)) {
+      callback();
+    }
+  }
+
+  addCallback(cb: SceneStateCallback): SceneStateCallbackRemover {
+    if (this.callbacks.has(cb)) {
+      throw new Error();
+    }
+
+    this.callbacks.add(cb);
+
+    return () => {
+      if (!this.callbacks.has(cb)) {
+        throw new Error();
+      }
+      this.callbacks.delete(cb);
+    };
+  }
+}
+
+export const globalSceneState = new SceneState();

src/scene/index.ts

@@ -16,5 +16,5 @@ export {
   getElementContainingPosition,
   hasText,
 } from "./comparisons";
-export { createScene } from "./createScene";
 export { getZoomOrigin, getNormalizedZoom } from "./zoom";
+export { globalSceneState } from "./globalScene";