首先会通过module.hot.accept监听文件变化，并传入该文件的渲染函数：
module.hot.accept(/*! ./App.vue?vue&type=template&id=472cff63&scoped=true& */ "./App.vue?vue&type=template&id=472cff63&scoped=true&", __WEBPACK_OUTDATED_DEPENDENCIES__ => { /* harmony import */ _App_vue_vue_type_template_id_472cff63_scoped_true___WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__(/*! ./App.vue?vue&type=template&id=472cff63&scoped=true& */ "./App.vue?vue&type=template&id=472cff63&scoped=true&");
(function () {
api.rerender('472cff63', {
render: _App_vue_vue_type_template_id_472cff63_scoped_true___WEBPACK_IMPORTED_MODULE_0__.render,
staticRenderFns: _App_vue_vue_type_template_id_472cff63_scoped_true___WEBPACK_IMPORTED_MODULE_0__.staticRenderFns
})
})(__WEBPACK_OUTDATED_DEPENDENCIES__); })
随后执行rerender方法，只贴重点部分代码：
record.Ctor.options.render = options.render
record.Ctor.options.staticRenderFns = options.staticRenderFns
record.instances.slice().forEach(function (instance) {
instance.$options.render = options.render
instance.$options.staticRenderFns = options.staticRenderFns
// reset static trees
// pre 2.5, all static trees are cached together on the instance
if (instance._staticTrees) {
instance._staticTrees = []
}
// 2.5.0
if (Array.isArray(record.Ctor.options.cached)) {
record.Ctor.options.cached = []
}
// 2.5.3
if (Array.isArray(instance.$options.cached)) {
instance.$options.cached = []
}
// post 2.5.4: v-once trees are cached on instance._staticTrees.
// Pure static trees are cached on the staticRenderFns array
// (both already reset above)
// 2.6: temporarily mark rendered scoped slots as unstable so that
// child components can be forced to update
var restore = patchScopedSlots(instance)
instance.$forceUpdate()
instance.$nextTick(restore)
})
首先会给当前的VueComponent的Options和实例里面的渲染函数替换为最新，然后执行实例上的forceUpdate方法：
Vue.prototype.$forceUpdate = function () {
var vm = this;
if (vm._watcher) {
vm._watcher.update();
}
};
该方法是执行watcher实例的update方法：
Watcher.prototype.update = function () {
/* istanbul ignore else */
if (this.lazy) {
this.dirty = true;
}
else if (this.sync) {
this.run();
}
else {
queueWatcher(this);
}
};
function queueWatcher(watcher) {
var id = watcher.id;
if (has[id] != null) {
return;
}
if (watcher === Dep.target && watcher.noRecurse) {
return;
}
has[id] = true;
if (!flushing) {
queue.push(watcher);
}
else {
// if already flushing, splice the watcher based on its id
// if already past its id, it will be run next immediately.
var i = queue.length - 1;
while (i > index$1 && queue[i].id > watcher.id) {
i--;
}
queue.splice(i + 1, 0, watcher);
}
// queue the flush
if (!waiting) {
waiting = true;
if (!config.async) {
flushSchedulerQueue();
return;
}
nextTick(flushSchedulerQueue);
}
}
重点看 queueWatcher(this)，将最新的watcher放入quene队列并且将flushSchedulerQueue函数传给nextTick。
function nextTick(cb, ctx) {
var _resolve;
callbacks.push(function () {
if (cb) {
try {
cb.call(ctx);
}
catch (e) {
handleError(e, ctx, 'nextTick');
}
}
else if (_resolve) {
_resolve(ctx);
}
});
if (!pending) {
pending = true;
timerFunc();
}
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(function (resolve) {
_resolve = resolve;
});
}
}
在callbacks里面放入一个执行回调的函数。并执行timerFunc()：
timerFunc = function () {
p_1.then(flushCallbacks);
// In problematic UIWebViews, Promise.then doesn't completely break, but
// it can get stuck in a weird state where callbacks are pushed into the
// microtask queue but the queue isn't being flushed, until the browser
// needs to do some other work, e.g. handle a timer. Therefore we can
// "force" the microtask queue to be flushed by adding an empty timer.
if (isIOS)
setTimeout(noop);
};
该方法异步执行flushCallbacks：
function flushCallbacks() {
pending = false;
var copies = callbacks.slice(0);
callbacks.length = 0;
for (var i = 0; i < copies.length; i++) {
copies[i]();
}
}
开始执行回调函数我们第一次放进去的函数flushSchedulerQueue：
function flushSchedulerQueue() {
currentFlushTimestamp = getNow();
flushing = true;
var watcher, id;
// Sort queue before flush.
// This ensures that:
// 1. Components are updated from parent to child. (because parent is always
// created before the child)
// 2. A component's user watchers are run before its render watcher (because
// user watchers are created before the render watcher)
// 3. If a component is destroyed during a parent component's watcher run,
// its watchers can be skipped.
queue.sort(sortCompareFn);
// do not cache length because more watchers might be pushed
// as we run existing watchers
for (index$1 = 0; index$1 < queue.length; index$1++) {
watcher = queue[index$1];
if (watcher.before) {
watcher.before();
}
id = watcher.id;
has[id] = null;
watcher.run();
// in dev build, check and stop circular updates.
if (has[id] != null) {
circular[id] = (circular[id] || 0) + 1;
if (circular[id] > MAX_UPDATE_COUNT) {
warn$2('You may have an infinite update loop ' +
(watcher.user
? "in watcher with expression \"".concat(watcher.expression, "\"")
: "in a component render function."), watcher.vm);
break;
}
}
}
// keep copies of post queues before resetting state
var activatedQueue = activatedChildren.slice();
var updatedQueue = queue.slice();
resetSchedulerState();
// call component updated and activated hooks
callActivatedHooks(activatedQueue);
callUpdatedHooks(updatedQueue);
// devtool hook
/* istanbul ignore if */
if (devtools && config.devtools) {
devtools.emit('flush');
}
}
首先触发watcher.before()，该方法是beforeUpdate的hook。然后执行watcher.run()方法：
Watcher.prototype.run = function () {
if (this.active) {
var value = this.get();
if (value !== this.value ||
// Deep watchers and watchers on Object/Arrays should fire even
// when the value is the same, because the value may
// have mutated.
isObject(value) ||
this.deep) {
// set new value
var oldValue = this.value;
this.value = value;
if (this.user) {
var info = "callback for watcher \"".concat(this.expression, "\"");
invokeWithErrorHandling(this.cb, this.vm, [value, oldValue], this.vm, info);
}
else {
this.cb.call(this.vm, value, oldValue);
}
}
}
};
该方法watcher.get方法，该方法会重新执行render方法生成vnode，然后调用update方法更新节点：
updateComponent = function () {
vm._update(vm._render(), hydrating);
};
总结：
1.获取监听文件最新的render和staticRenderFns并赋值给当前的VueComponent和当前vm实例。
2.使用$forceUpdate添加当前的vm的watcher并在queue中，最后异步执行flushSchedulerQueue，该函数遍历quene执行watcher的run方法，该方法会执行vm实例的_update方法完成更新。