fix: apply ResizeObserver to camera and panels

src/Flicking.ts

@@ -218,6 +218,13 @@ class Flicking extends Component<FlickingEvents> {
    * @see Viewport
    */
   public get viewport() { return this._viewport; }
+  /**
+   * {@link AutoResizer} instance of the Flicking
+   * @ko 현재 Flicking에 활성화된 {@link AutoResizer} 인스턴스
+   * @internal
+   * @readonly
+   */
+  public get autoResizer() { return this._autoResizer; }
   // Internal States
   /**
    * Whether Flicking's {@link Flicking#init init()} is called.
@@ -906,6 +913,10 @@ class Flicking extends Component<FlickingEvents> {
   public set autoResize(val: FlickingOptions["autoResize"]) {
     this._autoResize = val;
 
+    if (!this._initialized) {
+      return;
+    }
+
     if (val) {
       this._autoResizer.enable();
     } else {
@@ -916,7 +927,7 @@ class Flicking extends Component<FlickingEvents> {
   public set useResizeObserver(val: FlickingOptions["useResizeObserver"]) {
     this._useResizeObserver = val;
 
-    if (this._autoResize) {
+    if (this._initialized && this._autoResize) {
       this._autoResizer.enable();
     }
   }

src/core/AutoResizer.ts

@@ -4,6 +4,10 @@
  */
 import Flicking from "../Flicking";
 
+/**
+ * A component that detects size change and trigger resize method when the autoResize option is used
+ * @ko autoResize 옵션을 사용할 때 크기 변화를 감지하고 Flicking의 resize를 호출하는 컴포넌트
+ */
 class AutoResizer {
   private _flicking: Flicking;
   private _enabled: boolean;
@@ -36,7 +40,11 @@ class AutoResizer {
         ? new ResizeObserver(this._skipFirstResize)
         : new ResizeObserver(this._onResize);
 
-      resizeObserver.observe(flicking.viewport.element);
+      [
+        flicking.viewport.element,
+        flicking.camera.element,
+        ...flicking.panels.map(panel => panel.element)
+      ].forEach((element) => { resizeObserver.observe(element); });
 
       this._resizeObserver = resizeObserver;
     } else {
@@ -64,6 +72,20 @@ class AutoResizer {
     return this;
   }
 
+  public observe(element: Element): this {
+    if (this._resizeObserver) {
+      this._resizeObserver.observe(element);
+    }
+    return this;
+  }
+
+  public unobserve(element: Element): this {
+    if (this._resizeObserver) {
+      this._resizeObserver.unobserve(element);
+    }
+    return this;
+  }
+
   private _onResize = () => {
     const flicking = this._flicking;
     const resizeDebounce = flicking.resizeDebounce;

src/renderer/Renderer.ts

@@ -335,6 +335,15 @@ abstract class Renderer {
     // Update camera & control
     this._updateCameraAndControl();
 
+    if (flicking.autoResize && flicking.useResizeObserver) {
+      panelsAdded.forEach((panel) => {
+        flicking.autoResizer.observe(panel.element);
+      });
+      panelsRemoved.forEach((panel) => {
+        flicking.autoResizer.unobserve(panel.element);
+      });
+    }
+
     void this.render();
 
     if (!flicking.animating) {

test/unit/Flicking.spec.ts

@@ -908,6 +908,76 @@ describe("Flicking", () => {
 
         expect(resizeSpy.calledOnce).to.be.true;
       });
+
+      ["viewport", "camera", "panel"].forEach(element => {
+        it(`should call resize when size of ${element} is changed`, async () => {
+          const flicking = await createFlicking(
+            El.viewport("1000px", "1000px").add(
+              El.camera("1000px", "1000px").add(
+                El.panel("800px", "1000px"),
+                El.panel("800px", "1000px"),
+                El.panel("800px", "1000px"),
+              )
+            ),
+            { autoResize: true, useResizeObserver: true }
+          );
+          const afterResizeSpy = sinon.spy();
+          const beforeResizeSpy = sinon.spy();
+
+          // wait for initial resize
+          await waitTime(100);
+
+          flicking.on(EVENTS.AFTER_RESIZE, afterResizeSpy);
+          flicking.on(EVENTS.BEFORE_RESIZE, beforeResizeSpy);
+
+          switch (element) {
+            case "viewport":
+              flicking.element.style.height = "3000px";
+              break;
+            case "camera":
+              flicking.camera.element.style.height = "3000px";
+              break;
+            case "panel":
+              flicking.panels[2].element.style.height = "3000px";
+              break;
+          }
+
+          await waitEvent(flicking, EVENTS.AFTER_RESIZE);
+
+          expect(afterResizeSpy.calledOnce).to.be.true;
+          expect(beforeResizeSpy.calledOnce).to.be.true;
+        });
+      });
+
+      it("should observe size of panel element if panel element is added later", async () => {
+        const flicking = await createFlicking(
+          El.viewport("1000px", "1000px").add(
+            El.camera("1000px", "1000px").add(
+              El.panel("800px", "1000px"),
+              El.panel("800px", "1000px"),
+              El.panel("800px", "1000px"),
+            )
+          ),
+          { autoResize: true, useResizeObserver: true }
+        );
+        const afterResizeSpy = sinon.spy();
+        const beforeResizeSpy = sinon.spy();
+
+        // wait for initial resize
+        await waitTime(100);
+
+        flicking.on(EVENTS.AFTER_RESIZE, afterResizeSpy);
+        flicking.on(EVENTS.BEFORE_RESIZE, beforeResizeSpy);
+
+        flicking.append(flicking.panels[0].element.outerHTML);
+
+        flicking.panels[3].element.style.height = "3000px";
+
+        await waitEvent(flicking, EVENTS.AFTER_RESIZE);
+
+        expect(afterResizeSpy.calledOnce).to.be.true;
+        expect(beforeResizeSpy.calledOnce).to.be.true;
+      });
     });
 
     describe("preventClickOnDrag", () => {

test/unit/renderer/Renderer.spec.ts

@@ -423,7 +423,7 @@ describe("Renderer", () => {
 
     it("should resize the panel with image when it's loaded", async () => {
       const flicking = await createFlicking(El.viewport("200px", "200px").add(
-        El.camera().add(
+        El.camera("100%", "100%").add(
           El.imgPanel("100%", "100%")
         )
       ), { resizeOnContentsReady: true });
@@ -438,7 +438,7 @@ describe("Renderer", () => {
 
     it("should update the camera range after the image's loaded", async () => {
       const flicking = await createFlicking(El.viewport("200px", "200px").add(
-        El.camera().add(
+        El.camera("100%", "100%").add(
           El.imgPanel("100%", "100%")
         )
       ), { resizeOnContentsReady: true });