refactor: compose protocol-neutral event dispatch

src/events.test.ts

@@ -0,0 +1,82 @@
+import { describe, expect, it, spyOn } from "bun:test";
+
+import { EventDispatcher } from "./events";
+
+type TestEventMap = {
+  change: { value: number };
+};
+
+describe("EventDispatcher", () => {
+  it("replaces the singular handler", () => {
+    const dispatcher = new EventDispatcher<TestEventMap>();
+    const calls: string[] = [];
+    const first = () => calls.push("first");
+    const second = () => calls.push("second");
+    const warn = spyOn(console, "warn").mockImplementation(() => {});
+
+    try {
+      dispatcher.setHandler("change", first);
+      dispatcher.setHandler("change", second);
+
+      expect(dispatcher.getHandler("change")).toBe(second);
+      dispatcher.dispatch("change", { value: 1 });
+      expect(calls).toEqual(["second"]);
+    } finally {
+      warn.mockRestore();
+    }
+  });
+
+  it("dispatches the singular handler before listeners in insertion order", () => {
+    const dispatcher = new EventDispatcher<TestEventMap>();
+    const calls: string[] = [];
+
+    dispatcher.setHandler("change", () => calls.push("handler"));
+    dispatcher.addEventListener("change", () => calls.push("listener-1"));
+    dispatcher.addEventListener("change", () => calls.push("listener-2"));
+
+    dispatcher.dispatch("change", { value: 1 });
+
+    expect(calls).toEqual(["handler", "listener-1", "listener-2"]);
+  });
+
+  it("uses a listener snapshot when a listener is removed during dispatch", () => {
+    const dispatcher = new EventDispatcher<TestEventMap>();
+    const calls: string[] = [];
+    const second = () => calls.push("second");
+    const first = () => {
+      calls.push("first");
+      dispatcher.removeEventListener("change", second);
+    };
+
+    dispatcher.addEventListener("change", first);
+    dispatcher.addEventListener("change", second);
+
+    dispatcher.dispatch("change", { value: 1 });
+    dispatcher.dispatch("change", { value: 2 });
+
+    expect(calls).toEqual(["first", "second", "first"]);
+  });
+
+  it("warns only when one singular handler replaces another", () => {
+    const dispatcher = new EventDispatcher<TestEventMap>();
+    const warn = spyOn(console, "warn").mockImplementation(() => {});
+    const first = () => {};
+    const second = () => {};
+
+    try {
+      dispatcher.setHandler("change", first);
+      dispatcher.setHandler("change", undefined);
+      dispatcher.setHandler("change", first);
+      expect(warn).not.toHaveBeenCalled();
+
+      dispatcher.setHandler("change", second);
+
+      expect(warn).toHaveBeenCalledTimes(1);
+      expect(String(warn.mock.calls[0]?.[0])).toContain(
+        "onchange handler replaced",
+      );
+    } finally {
+      warn.mockRestore();
+    }
+  });
+});