feat(@tinacms/react-core): useCMSEvent makes subscribing to events ea…

…sier

packages/@tinacms/react-core/src/index.ts

@@ -21,4 +21,5 @@ export * from './use-form'
 export * from './use-plugin'
 export * from './use-subscribable'
 export * from './use-watch-form-values'
+export * from './use-cms-event'
 export * from './with-plugin'

packages/@tinacms/react-core/src/use-cms-event.ts

@@ -0,0 +1,33 @@
+/**
+
+Copyright 2019 Forestry.io Inc
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+*/
+
+import * as React from 'react'
+import { Callback } from '@tinacms/core'
+import { useCMS } from './use-cms'
+
+export function useCMSEvent(
+  event: string | string[],
+  callback: Callback,
+  deps: React.DependencyList
+) {
+  const cms = useCMS()
+
+  React.useEffect(function() {
+    return cms.events.subscribe(event, callback)
+  }, deps)
+}