add custom event emitter

packages/core/src/EventEmitter.ts

@@ -0,0 +1,42 @@
+export default class EventEmitter {
+
+  _callbacks: { [key: string]: Function[] } = {}
+
+  on(event: string, fn: Function) {
+    if (!this._callbacks[event]) {
+      this._callbacks[event] = []
+    }
+
+    this._callbacks[event].push(fn)
+    
+    return this
+  }
+
+  emit(event: string, ...args: any) {
+    const callbacks = this._callbacks[event]
+
+    if (callbacks) {
+      callbacks.forEach(callback => callback.apply(this, args))
+    }
+
+    return this
+  }
+
+  off(event: string, fn?: Function) {
+    const callbacks = this._callbacks[event]
+
+    if (callbacks) {
+      if (fn) {
+        this._callbacks[event] = callbacks.filter(callback => callback !== fn)
+      } else {
+        delete this._callbacks[event]
+      }
+    }
+
+    return this
+  }
+
+  removeAllListeners() {
+    this._callbacks = {}
+  }
+}