初始化事件系统 initEvents

源码位置：src/core/instance/events.ts

源码：

initEvents

export function initEvents(vm: Component) {
  vm._events = Object.create(null);
  vm._hasHookEvent = false;
  // init parent attached events
  const listeners = vm.$options._parentListeners;
  if (listeners) { // 若是父组件中存在
    updateComponentListeners(vm, listeners);
  }
}

updateComponentListeners

let target: any;

function add(event, fn) {
  target.$on(event, fn);
}

function remove(event, fn) {
  target.$off(event, fn);
}

function createOnceHandler(event, fn) {
  const _target = target;
  return function onceHandler() {
    const res = fn.apply(null, arguments);
    if (res !== null) {
      _target.$off(event, onceHandler);
    }
  };
}

export function updateComponentListeners(
  vm: Component,
  listeners: Object,
  oldListeners?: Object | null
) {
  target = vm;
  updateListeners(
    listeners,
    oldListeners || {},
    add,
    remove,
    createOnceHandler,
    vm
  );
  target = undefined;
}

updateListeners

export function updateListeners(
  on: Object,
  oldOn: Object,
  add: Function,
  remove: Function,
  createOnceHandler: Function,
  vm: Component
) {
  let name, cur, old, event;
  for (name in on) {
    cur = on[name];
    old = oldOn[name];
    event = normalizeEvent(name);
    if (isUndef(cur)) {
      __DEV__ &&
        warn(
          `Invalid handler for event "${event.name}": got ` + String(cur),
          vm
        );
    } else if (isUndef(old)) {
      if (isUndef(cur.fns)) {
        cur = on[name] = createFnInvoker(cur, vm);
      }
      if (isTrue(event.once)) {
        cur = on[name] = createOnceHandler(event.name, cur, event.capture);
      }
      add(event.name, cur, event.capture, event.passive, event.params);
    } else if (cur !== old) {
      old.fns = cur;
      on[name] = old;
    }
  }
  for (name in oldOn) {
    if (isUndef(on[name])) {
      event = normalizeEvent(name);
      remove(event.name, oldOn[name], event.capture);
    }
  }
}