Finish event.

packages/xo-collection/README.md

@@ -163,6 +163,17 @@ col.on('remove', (removed) => {
 })
 ```
 
+**End of update**
+
+> Emitted when all the update process is finished and all the update
+> events has been emitted.
+
+```javascript
+col.on('finish', () => {
+  console.log('the collection has been updated')
+})
+```
+
 ### Iteration
 
 ```javascript

packages/xo-collection/src/collection.js

@@ -248,6 +248,13 @@ export default class Collection extends EventEmitter {
         return
       }
 
+      const {_buffer: buffer} = this
+
+      // Due to deduplication there could be nothing in the buffer.
+      if (isEmpty(buffer)) {
+        return
+      }
+
       const data = {
         add: Object.create(null),
         remove: Object.create(null),
@@ -255,7 +262,7 @@ export default class Collection extends EventEmitter {
       }
 
       for (let key in this._buffer) {
-        data[this._buffer[key]][key] = this._items[key]
+        data[buffer[key]][key] = this._items[key]
       }
 
       forEach(data, (items, action) => {
@@ -264,6 +271,12 @@ export default class Collection extends EventEmitter {
         }
       })
 
+      // Indicates the end of the update.
+      //
+      // This name has been chosen because it is used in Node writable
+      // streams when the data has been successfully committed.
+      this.emit('finish')
+
       this._buffer = Object.create(null)
     }
   }