fix: [#3128] Unused scene loader locks up engine

Closes: #3128 * Special case when no resources, consider it "loaded"
excaliburjs · Jul 20, 2024 · 4a98742 · 4a98742
1 parent ef8686e
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -70,6 +70,7 @@ are doing mtv adjustments during precollision.
 
 ### Fixed
 
+- Fixed issue where `ex.Scene.onPreLoad(loader: ex.DefaultLoader)` would lock up the engine if there was an empty loader
 - Fixed issue where `ex.Scene` scoped input events would preserve state and get stuck causing issues when switching back to the original scene.
 - Fixed issue where not all physical keys from the spec were present in `ex.Keys` including the reported `ex.Keys.Tab`
 - Fixed invalid graphics types around `ex.Graphic.tint`

diff --git a/sandbox/tests/loader-lockup/index.html b/sandbox/tests/loader-lockup/index.html
@@ -0,0 +1,12 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Loader Lockup</title>
+  </head>
+  <body>
+    <script src="../../lib/excalibur.js"></script>
+    <script src="index.js"></script>
+  </body>
+</html>
diff --git a/sandbox/tests/loader-lockup/index.ts b/sandbox/tests/loader-lockup/index.ts
@@ -0,0 +1,33 @@
+class SceneX extends ex.Scene {
+  override backgroundColor = ex.Color.DarkGray;
+  override onInitialize() {
+    this.input.pointers.on('down', () => {
+      console.warn('should change scenes');
+      this.engine.goToScene('SceneY');
+    });
+  }
+}
+
+class SceneY extends ex.Scene {
+  override backgroundColor = ex.Color.Viridian;
+  // issue still occurs if onPreLoad is not defined or
+  // if we call super.onPreLoad(loader)
+  override onPreLoad(loader: ex.DefaultLoader) {
+    // un-commenting this call fixes the issue.
+    // loader.addResource({
+    //   data: {},
+    //   isLoaded: () => true,
+    //   load: () => Promise.resolve(),
+    // });
+  }
+}
+
+var gameLoaderLockup = new ex.Engine({
+  scenes: {
+    SceneX,
+    SceneY: { scene: SceneY, loader: ex.Loader }
+  }
+});
+
+gameLoaderLockup.start();
+gameLoaderLockup.goToScene('SceneX');
diff --git a/src/engine/Director/DefaultLoader.ts b/src/engine/Director/DefaultLoader.ts
@@ -186,6 +186,10 @@ export class DefaultLoader implements Loadable<Loadable<any>[]> {
 
   private _loadingFuture = new Future<void>();
   public areResourcesLoaded() {
+    if (this._resources.length === 0) {
+      // special case no resources mean loaded;
+      return Promise.resolve();
+    }
     return this._loadingFuture.promise;
   }
 

diff --git a/src/spec/DefaultLoaderSpec.ts b/src/spec/DefaultLoaderSpec.ts
@@ -25,6 +25,12 @@ describe('A DefaultLoader', () => {
     expect(sut).toBeDefined();
   });
 
+  it('is loaded when no resources', async () => {
+    const loader = new ex.DefaultLoader();
+    expect(loader.isLoaded()).toBe(true);
+    await expectAsync(loader.areResourcesLoaded()).toBeResolved();
+  });
+
   it('can be constructed with non-defaults', () => {
     const sut = new ex.DefaultLoader({
       loadables: [new ex.ImageSource('./some/image.png')]