- 加载或者更新 transiton-group 组件 DOM 时，执行 render 函数，生成标签对应的节点（虚构节点 VNode）。在 render 函数中，获取 transition-group 标签内的子标签列表节点（插槽），并将 transition-group 标签上的数据顺次增加子标签节点上。同时通过和更新前的子标签节点比照，保留本次更新仍存在的子节点，以及删除的节点。最初返回 span 标签节点（或者用户定义的标签），子标签列表作为子节点增加到该标签节点中。
transition-group 动画的实现原理和 transition 统一，不同的是 transition 将标签上的数据增加到一个子标签节点上，transition-group 是将标签上的数据顺次增加到标签列表每个节点上，能够了解为 transition-group 相当于在每个子标签里面增加了 transiton 标签。
transition-group 组件选项中存在 beforeMount 周期函数，在组件加载或者更新 DOM 之前调用。在该周期函数中，从新封装了_update 函数，将在 render 执行实现返回节点时调用，首先调用__patch__，将以后仍存在的标签节点作为新节点，移除以后不存在的节点，而后在调用原始的 update 函数，将以后仍存在的标签节点作为旧节点，增加新增的节点。封装后的_update 函数执行了两次__patch__, 第一次是从 document 移除以后不存在的旧节点，第二次是增加新节点到 document 中，实现更新。
实现更新后，将会调用 transition-group 组件选项内的 updated 钩子函数，增加列表整体的过渡成果。标签节点增加到 document 中实现更新后立刻通过 transform 挪动到原先的地位，再通过动画挪动到以后地位，造成动效。
var props = extend({
tag: String,
moveClass: String
}, transitionProps);
delete props.mode;
var TransitionGroup = {
props: props,
beforeMount: function beforeMount () {
var this$1 = this;
var update = this._update;
this._update = function (vnode, hydrating) {var restoreActiveInstance = setActiveInstance(this$1);
// force removing pass
this$1.__patch__(// 移除以后不存在的旧标签
this$1._vnode,// 旧标签
this$1.kept,// 以后保留的旧标签
false, // hydrating
true // removeOnly (!important, avoids unnecessary moves)
);
this$1._vnode = this$1.kept;
restoreActiveInstance();
update.call(this$1, vnode, hydrating);// 增加以后新增的标签, 并调整节点的程序
};
},
render: function render (h) {
var tag = this.tag || this.$vnode.data.tag || 'span';
var map = Object.create(null);
var prevChildren = this.prevChildren = this.children;// 上一次组件标签内的标签节点（插槽）var rawChildren = this.$slots.default || [];// 以后组件标签内的标签节点（插槽）var children = this.children = [];
var transitionData = extractTransitionData(this);// 获取标签上的属性
for (var i = 0; i < rawChildren.length; i++) {var c = rawChildren[i];
if (c.tag) {if (c.key != null && String(c.key).indexOf('__vlist') !== 0) {children.push(c);
map[c.key] = c// 以后组件标签内的标签节点的 key 映射
;(c.data || (c.data = {})).transition = transitionData;// 将标签上的属性保留在插槽节点上
} else if (process.env.NODE_ENV !== 'production') {
var opts = c.componentOptions;
var name = opts ? (opts.Ctor.options.name || opts.tag || '') : c.tag;
warn(("<transition-group> children must be keyed: <" + name + ">"));
}
}
}
if (prevChildren) {var kept = [];
var removed = [];
for (var i$1 = 0; i$1 < prevChildren.length; i$1++) {var c$1 = prevChildren[i$1];
c$1.data.transition = transitionData;
c$1.data.pos = c$1.elm.getBoundingClientRect();
if (map[c$1.key]) {kept.push(c$1);// 本次中在上一次呈现过的插槽节点
} else {removed.push(c$1);// 本次中未呈现上一次的插槽节点
}
}
this.kept = h(tag, null, kept);// 依据上一次也呈现过的插槽节点生成组件渲染节点
this.removed = removed;
}
return h(tag, null, children)
},
updated: function updated () {// 此时地位曾经实现了更新
var children = this.prevChildren;
var moveClass = this.moveClass || ((this.name || 'v') + '-move');
if (!children.length || !this.hasMove(children[0].elm, moveClass)) {return}
// we divide the work into three loops to avoid mixing DOM reads and writes
// in each iteration - which helps prevent layout thrashing.
children.forEach(callPendingCbs);
children.forEach(recordPosition);
children.forEach(applyTranslation);// 从以后地位挪动到之前地位
// force reflow to put everything in position
// assign to this to avoid being removed in tree-shaking
// $flow-disable-line
this._reflow = document.body.offsetHeight;
children.forEach(function (c) {if (c.data.moved) {
var el = c.elm;
var s = el.style;
addTransitionClass(el, moveClass);// 增加类名开始动画
s.transform = s.WebkitTransform = s.transitionDuration = '';// 再从之前地位挪动到以后地位
el.addEventListener(transitionEndEvent, el._moveCb = function cb (e) {if (e && e.target !== el) {return}
if (!e || /transform$/.test(e.propertyName)) {el.removeEventListener(transitionEndEvent, cb);
el._moveCb = null;
removeTransitionClass(el, moveClass);// 动画实现移除类名
}
});
}
});
},
methods: {hasMove: function hasMove (el, moveClass) {
/* istanbul ignore if */
if (!hasTransition) {return false}
/* istanbul ignore if */
if (this._hasMove) {return this._hasMove}
// Detect whether an element with the move class applied has
// CSS transitions. Since the element may be inside an entering
// transition at this very moment, we make a clone of it and remove
// all other transition classes applied to ensure only the move class
// is applied.
var clone = el.cloneNode();// 通过 clone 并增加到 document 中获取 dom 款式属性
if (el._transitionClasses) {// 移除 transition 类名
el._transitionClasses.forEach(function (cls) {removeClass(clone, cls); });
}
addClass(clone, moveClass);// 增加类名
clone.style.display = 'none';
this.$el.appendChild(clone);
var info = getTransitionInfo(clone);
this.$el.removeChild(clone);
return (this._hasMove = info.hasTransform)
}
}
};