Add watcher.fail event

src/effector/event/__tests__/fail.test.js

@@ -0,0 +1,30 @@
+// @flow
+
+import {createEvent, is} from 'effector'
+import {argumentHistory} from 'effector/fixtures'
+
+test('watcher.fail is event', () => {
+  const foo = createEvent('foo')
+  const sub = foo.watch(() => {})
+  expect(is.event(sub.fail)).toBe(true)
+})
+
+it('triggers after failed .watch', () => {
+  const fn = jest.fn()
+  const foo = createEvent()
+  const sub = foo.watch(() => {
+    throw new Error('Unknown error')
+  })
+  sub.fail.watch(e => fn(e))
+
+  expect(fn).not.toBeCalled()
+  foo()
+  expect(fn).toBeCalledTimes(1)
+  expect(argumentHistory(fn)).toMatchInlineSnapshot(`
+        Array [
+          Object {
+            "error": [Error: Unknown error],
+          },
+        ]
+    `)
+})

src/effector/event/eventFabric.js

@@ -199,11 +199,15 @@ function filterEvent(
 }
 
 function watchEvent<Payload>(
-  event: Unit,
+  event: Event<Payload>,
   watcher: (payload: Payload, type: string) => any,
 ): Subscription {
-  return createLink(event, {
-    scope: {trigger: event, handler: watcher},
+  const fail = eventFabric({
+    name: 'fail ' + event.shortName,
+    parent: event.domainName,
+  })
+  const subscription = createLink(event, {
+    scope: {trigger: event, handler: watcher, fail},
     //prettier-ignore
     node: [
       noop,
@@ -212,6 +216,7 @@ function watchEvent<Payload>(
           payload,
           getDisplayName(trigger),
         ),
+        fail: (error: mixed, {fail}) => fail({error})
       }),
     ],
     meta: {
@@ -222,4 +227,7 @@ function watchEvent<Payload>(
       },
     },
   })
+  //$todo
+  subscription.fail = fail
+  return subscription
 }