refactor: standardize event argument

README.md

@@ -220,6 +220,8 @@ navigation.emit({
 });
 ```
 
+The `data` is available under the `data` property in the `event` object, i.e. `event.data`.
+
 The `target` property determines the screen that will receive the event. If the `target` property is omitted, the event is dispatched to all screens in the navigator.
 
 Screens cannot emit events as there is no `emit` method on a screen's `navigation` prop.

packages/core/src/__tests__/useEventEmitter.test.tsx

@@ -247,20 +247,23 @@ it('fires custom events', () => {
   expect(secondCallback).toBeCalledTimes(0);
   expect(thirdCallback).toBeCalledTimes(0);
 
-  act(() =>
+  act(() => {
     ref.current.navigation.emit({
       type: eventName,
       target: ref.current.state.routes[ref.current.state.routes.length - 1].key,
       data: 42,
-    })
-  );
+    });
+  });
 
   expect(firstCallback).toBeCalledTimes(0);
   expect(secondCallback).toBeCalledTimes(0);
   expect(thirdCallback).toBeCalledTimes(1);
-  expect(thirdCallback.mock.calls[0][0]).toBe(42);
+  expect(thirdCallback.mock.calls[0][0].type).toBe('someSuperCoolEvent');
+  expect(thirdCallback.mock.calls[0][0].data).toBe(42);
 
-  act(() => ref.current.navigation.emit({ type: eventName }));
+  act(() => {
+    ref.current.navigation.emit({ type: eventName });
+  });
 
   expect(firstCallback).toBeCalledTimes(1);
   expect(secondCallback).toBeCalledTimes(1);

packages/core/src/types.tsx

@@ -160,18 +160,24 @@ export type EventMapBase = {
   blur: undefined;
 };
 
-export type EventListenerCallback<Data> = Data extends undefined
-  ? () => void
-  : (data: Data) => void;
+export type EventArg<EventName extends string, Data> = {
+  readonly type: EventName;
+  readonly defaultPrevented: boolean;
+  preventDefault(): void;
+} & (Data extends undefined ? {} : { readonly data: Data });
+
+export type EventListenerCallback<EventName extends string, Data> = (
+  e: EventArg<EventName, Data>
+) => void;
 
 export type EventConsumer<EventMap extends { [key: string]: any }> = {
   addListener<EventName extends Extract<keyof EventMap, string>>(
     type: EventName,
-    callback: EventListenerCallback<EventMap[EventName]>
+    callback: EventListenerCallback<EventName, EventMap[EventName]>
   ): () => void;
   removeListener<EventName extends Extract<keyof EventMap, string>>(
     type: EventName,
-    callback: EventListenerCallback<EventMap[EventName]>
+    callback: EventListenerCallback<EventName, EventMap[EventName]>
   ): void;
 };
 
@@ -180,7 +186,7 @@ export type EventEmitter<EventMap extends { [key: string]: any }> = {
     type: EventName;
     data?: EventMap[EventName];
     target?: string;
-  }): void;
+  }): EventArg<EventName, EventMap[EventName]>;
 };
 
 class PrivateValueStore<A, B, C> {

packages/core/src/useEventEmitter.tsx

@@ -1,5 +1,5 @@
 import * as React from 'react';
-import { EventEmitter, EventConsumer } from './types';
+import { EventEmitter, EventConsumer, EventArg } from './types';
 
 export type NavigationEventEmitter = EventEmitter<{ [key: string]: any }> & {
   create: (target: string) => EventConsumer<{ [key: string]: any }>;
@@ -51,7 +51,26 @@ export default function useEventEmitter(): NavigationEventEmitter {
           ? items[target] && items[target].slice()
           : ([] as Listeners).concat(...Object.keys(items).map(t => items[t]));
 
-      callbacks && callbacks.forEach(cb => cb(data));
+      let defaultPrevented = false;
+
+      const event: EventArg<any, any> = {
+        get type() {
+          return type;
+        },
+        get data() {
+          return data;
+        },
+        get defaultPrevented() {
+          return defaultPrevented;
+        },
+        preventDefault() {
+          defaultPrevented = true;
+        },
+      };
+
+      callbacks && callbacks.forEach(cb => cb(event));
+
+      return event;
     },
     []
   );