vue2 源码片段

 patch的基础函数：

function createRmCb (childElm, listeners) {
      function remove () {
        if (--remove.listeners === 0) {
          removeNode(childElm);
        }
      }
      remove.listeners = listeners;
      return remove
 
return function patch (oldVnode, vnode, hydrating, removeOnly) {
  //oldVode一般是原生dom, vnode 是虚拟dom
  // 把oldVnode转换成虚拟dom.
   oldVnode = emptyNodeAt(oldVnode)
   // 获取虚拟dom对原生dom的引用
   var oldElm = oldVnode.elm;
}
}

once函数：

// 对通过once包装的fn,仅仅调用一次
once (fn) {
    var called = false;
    return function () {
      if (!called) {
        called = true;
        fn.apply(this, arguments);
      }
    }
  } 

cbs.remove函数

remove: function remove (vnode, rm) {
      /* istanbul ignore else */
      if (vnode.data.show !== true) {
        leave(vnode, rm);
      } else {
        rm(); // createRmCb中的回调中的remove。

} }

leave函数：

  function leave (vnode, rm) {
    var el = vnode.elm;

    // call enter callback now
    if (isDef(el._enterCb)) {
      el._enterCb.cancelled = true;
      el._enterCb();
    }

    var data = resolveTransition(vnode.data.transition);
    if (isUndef(data) || el.nodeType !== 1) {
      return rm()
    }

    /* istanbul ignore if */
    if (isDef(el._leaveCb)) {
      return
    }

    var css = data.css;
    var type = data.type;
    var leaveClass = data.leaveClass;
    var leaveToClass = data.leaveToClass;
    var leaveActiveClass = data.leaveActiveClass;
    var beforeLeave = data.beforeLeave;
    var leave = data.leave;
    var afterLeave = data.afterLeave;
    var leaveCancelled = data.leaveCancelled;
    var delayLeave = data.delayLeave;
    var duration = data.duration;

    var expectsCSS = css !== false && !isIE9;
    var userWantsControl = getHookArgumentsLength(leave);

    var explicitLeaveDuration = toNumber(
      isObject(duration)
        ? duration.leave
        : duration
    );

    if ( isDef(explicitLeaveDuration)) {
      checkDuration(explicitLeaveDuration, 'leave', vnode);
    }

    var cb = el._leaveCb = once(function () {
      if (el.parentNode && el.parentNode._pending) {
        el.parentNode._pending[vnode.key] = null;
      }
      if (expectsCSS) {
        removeTransitionClass(el, leaveToClass);
        removeTransitionClass(el, leaveActiveClass);
      }
      if (cb.cancelled) {
        if (expectsCSS) {
          removeTransitionClass(el, leaveClass);
        }
        leaveCancelled && leaveCancelled(el);
      } else {
        rm();
        afterLeave && afterLeave(el);
      }
      el._leaveCb = null;
    });

    if (delayLeave) {
      delayLeave(performLeave);
    } else {
      performLeave();
    }