Vue数据变化后页面更新详细介绍

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。

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