Vue中各个生命周期阶段的源码探究--mounted阶段

文本将对Vue的mounted阶段进行解析。使用的测试代码和之前的一样。vue版本v2.5.17-beta.0
测试代码：

<div id="box">
	I love vue
	<ul>
		<li v-for="item in data" @click="logChange(item.NAME,item.AGE)">
			{{item.NAME}}
		li>
	ul>
div>

//js
new Vue({
	el: '#box',
	data: {
		data:[
			{NAME:'SDF',AGE:3},
			{NAME:'JIKU',AGE:6},
			{NAME:'HYF',AGE:3}
		]
	},
	methods: {
		logChange: function(name,age) {
			console.log(name,age);
		}
	}
	
});

在得到渲染函数后，就可以执行它生成虚节点。这就是在mounted阶段完成的,来看以下代码。
一、创建监听器对象

function mountComponent (
  vm,
  el,
  hydrating
) {
  vm.$el = el;
  if (!vm.$options.render) {
    vm.$options.render = createEmptyVNode;
    {
      /* istanbul ignore if */
      if ((vm.$options.template && vm.$options.template.charAt(0) !== '#') ||
        vm.$options.el || el) {
        warn(
          'You are using the runtime-only build of Vue where the template ' +
          'compiler is not available. Either pre-compile the templates into ' +
          'render functions, or use the compiler-included build.',
          vm
        );
      } else {
        warn(
          'Failed to mount component: template or render function not defined.',
          vm
        );
      }
    }
  } 
  callHook(vm, 'beforeMount');

  var updateComponent;
  /* istanbul ignore if */
  if ("development" !== 'production' && config.performance && mark) {
    updateComponent = function () {
      var name = vm._name;
      var id = vm._uid;
      var startTag = "vue-perf-start:" + id;
      var endTag = "vue-perf-end:" + id;

      mark(startTag);
      var vnode = vm._render();
      mark(endTag);
      measure(("vue " + name + " render"), startTag, endTag);

      mark(startTag);
      vm._update(vnode, hydrating);
      mark(endTag);
      measure(("vue " + name + " patch"), startTag, endTag);
    };
  } else {
    updateComponent = function () {
      vm._update(vm._render(), hydrating);
    };
  }

  // we set this to vm._watcher inside the watcher's constructor
  // since the watcher's initial patch may call $forceUpdate (e.g. inside child
  // component's mounted hook), which relies on vm._watcher being already defined
  new Watcher(vm, updateComponent, noop, {
    before: function before () {
      if (vm._isMounted) {
        callHook(vm, 'beforeUpdate');
      }
    }
  }, true /* isRenderWatcher */);
  hydrating = false;

  // manually mounted instance, call mounted on self
  // mounted is called for render-created child components in its inserted hook
  if (vm.$vnode == null) {
    vm._isMounted = true;
    callHook(vm, 'mounted');
  }
  return vm
}

这里通过创建一个监听器Watcher来执行渲染函数。
即：

new Watcher(vm, updateComponent, noop, {
    before: function before () {
      if (vm._isMounted) {
        callHook(vm, 'beforeUpdate');
      }
    }
  }, true /* isRenderWatcher */);

这个监听器函数最终会执行updateComponent回调

updateComponent = function () {
 	vm._update(vm._render(), hydrating);
};

二、执行Vue实例的_render函数，并执行渲染函数

Vue.prototype._render = function () {
    var vm = this;
    var ref = vm.$options;
    var render = ref.render;
    var _parentVnode = ref._parentVnode;

    // reset _rendered flag on slots for duplicate slot check
    {
      for (var key in vm.$slots) {
        // $flow-disable-line
        vm.$slots[key]._rendered = false;
      }
    }

    if (_parentVnode) {
      vm.$scopedSlots = _parentVnode.data.scopedSlots || emptyObject;
    }

    // set parent vnode. this allows render functions to have access
    // to the data on the placeholder node.
    vm.$vnode = _parentVnode;
    // render self
    var vnode;
    try {
      vnode = render.call(vm._renderProxy, vm.$createElement);
    } catch (e) {
      handleError(e, vm, "render");
      // return error render result,
      // or previous vnode to prevent render error causing blank component
      /* istanbul ignore else */
      {
        if (vm.$options.renderError) {
          try {
            vnode = vm.$options.renderError.call(vm._renderProxy, vm.$createElement, e);
          } catch (e) {
            handleError(e, vm, "renderError");
            vnode = vm._vnode;
          }
        } else {
          vnode = vm._vnode;
        }
      }
    }
    // return empty vnode in case the render function errored out
    if (!(vnode instanceof VNode)) {
      if ("development" !== 'production' && Array.isArray(vnode)) {
        warn(
          'Multiple root nodes returned from render function. Render function ' +
          'should return a single root node.',
          vm
        );
      }
      vnode = createEmptyVNode();
    }
    // set parent
    vnode.parent = _parentVnode;
    return vnode
  }

语句var ref = vm.$options; var render = ref.render;就是取出渲染函数。然后以Vue实例的一个代理对象或其本身作为上下文执行即：vnode = render.call(vm._renderProxy, vm.$createElement);，最终它会返回一个虚拟节点。
而代理实例是由initProxy函数初始化的，即vm._renderProxy = new Proxy(vm, handlers);。本例中会使用hasHandler处理函数来处理。

var getHandler = {
    get: function get (target, key) {
      if (typeof key === 'string' && !(key in target)) {
        warnNonPresent(target, key);
      }
      return target[key]
    }
  }
var hasHandler = {
    has: function has (target, key) {
      var has = key in target;
      var isAllowed = allowedGlobals(key) || (typeof key === 'string' && key.charAt(0) === '_');
      if (!has && !isAllowed) {
        warnNonPresent(target, key);
      }
      return has || !isAllowed
    }
  }

initProxy = function initProxy (vm) {
    if (hasProxy) {
      // determine which proxy handler to use
      var options = vm.$options;
      var handlers = options.render && options.render._withStripped
        ? getHandler
        : hasHandler;
      vm._renderProxy = new Proxy(vm, handlers);
    } else {
      vm._renderProxy = vm;
    }
  }

vnode = render.call(vm._renderProxy, vm.$createElement);语句会进到 下面的渲染函数中执行，这是个动态创建的一个匿名函数，其中this就是指向vm._renderProxy即代理实例或Vue实例。

(function anonymous() {
    with (this) {
        return _c('div', {
            attrs: {
                "id": "box"
            }
        }, [_v("\n\t\t\tI love vue\n\t\t\t"), _c('ul', _l((data), function(item) {
            return _c('li', {
                on: {
                    "click": function($event) {
                        logChange(item.NAME, item.AGE)
                    }
                }
            }, [_v("\n\t\t\t\t\t" + _s(item.NAME) + "--" + _s(item.AGE) + "\n\t\t\t\t")])
        }))])
    }
}
)

下面就是生成虚拟节点过程，其中虚拟节点对象结构如下：

var VNode = function VNode (
  tag,
  data,
  children,
  text,
  elm,
  context,
  componentOptions,
  asyncFactory
) {
  this.tag = tag;
  this.data = data;
  this.children = children;
  this.text = text;
  this.elm = elm;
  this.ns = undefined;
  this.context = context;
  this.fnContext = undefined;
  this.fnOptions = undefined;
  this.fnScopeId = undefined;
  this.key = data && data.key;
  this.componentOptions = componentOptions;
  this.componentInstance = undefined;
  this.parent = undefined;
  this.raw = false;
  this.isStatic = false;
  this.isRootInsert = true;
  this.isComment = false;
  this.isCloned = false;
  this.isOnce = false;
  this.asyncFactory = asyncFactory;
  this.asyncMeta = undefined;
  this.isAsyncPlaceholder = false;
}

2.1、执行渲染函数的_v函数生成关于文本节点的虚拟节点
_v工具函数其实就是createTextVNode

function createTextVNode (val) {
  return new VNode(undefined, undefined, undefined, String(val))
}

2.2、执行渲染函数的_c函数生成元素节点
该函数会执行createElement函数

function createElement (
  context,
  tag,
  data,
  children,
  normalizationType,
  alwaysNormalize
) {
  if (Array.isArray(data) || isPrimitive(data)) {
    normalizationType = children;
    children = data;
    data = undefined;
  }
  if (isTrue(alwaysNormalize)) {
    normalizationType = ALWAYS_NORMALIZE;
  }
  return _createElement(context, tag, data, children, normalizationType)
}

在这里会参数作一下处理，主要处理元素属性数据与其子节点的参数对应关系，因为data参数可能是一个数组或原始类型。比如

_c('ul', _l((data), function(item) {
    return _c('li', {
         on: {
             "click": function($event) {
                 logChange(item.NAME, item.AGE)
             }
         }
     }, [_v("\n\t\t\t\t\t" + _s(item.NAME) + "--" + _s(item.AGE) + "\n\t\t\t\t")])

这里的data参数就是一个数组。

createElement函数又会调用_createElement函数

function _createElement (
  context,
  tag,
  data,
  children,
  normalizationType
) {
  if (isDef(data) && isDef((data).__ob__)) {
    "development" !== 'production' && warn(
      "Avoid using observed data object as vnode data: " + (JSON.stringify(data)) + "\n" +
      'Always create fresh vnode data objects in each render!',
      context
    );
    return createEmptyVNode()
  }
  // object syntax in v-bind
  if (isDef(data) && isDef(data.is)) {
    tag = data.is;
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // warn against non-primitive key
  if ("development" !== 'production' &&
    isDef(data) && isDef(data.key) && !isPrimitive(data.key)
  ) {
    {
      warn(
        'Avoid using non-primitive value as key, ' +
        'use string/number value instead.',
        context
      );
    }
  }
  // support single function children as default scoped slot
  if (Array.isArray(children) &&
    typeof children[0] === 'function'
  ) {
    data = data || {};
    data.scopedSlots = { default: children[0] };
    children.length = 0;
  }
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children);
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children);
  }
  var vnode, ns;
  if (typeof tag === 'string') {
    var Ctor;
    ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag);
    if (config.isReservedTag(tag)) {
      // platform built-in elements
      vnode = new VNode(
        config.parsePlatformTagName(tag), data, children,
        undefined, undefined, context
      );
    } else if (isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {
      // component
      vnode = createComponent(Ctor, data, context, children, tag);
    } else {
      // unknown or unlisted namespaced elements
      // check at runtime because it may get assigned a namespace when its
      // parent normalizes children
      vnode = new VNode(
        tag, data, children,
        undefined, undefined, context
      );
    }
  } else {
    // direct component options / constructor
    vnode = createComponent(tag, data, context, children);
  }
  if (Array.isArray(vnode)) {
    return vnode
  } else if (isDef(vnode)) {
    if (isDef(ns)) { applyNS(vnode, ns); }
    if (isDef(data)) { registerDeepBindings(data); }
    return vnode
  } else {
    return createEmptyVNode()
  }
}

这里会根据标签名判断该元素是一个自定义组件还是普通的HTML标签，本例中，只会返回一个虚拟元素节点，本例中的不涉及组件，所以不会执行vnode = createComponent(Ctor, data, context, children, tag);语句。最后会得到一个虚拟节点树，即虚拟DOM。
3、执行path函数根据虚拟节点，生成真实DOM

function patch (oldVnode, vnode, hydrating, removeOnly) {
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) { invokeDestroyHook(oldVnode); }
      return
    }

    var isInitialPatch = false;
    var insertedVnodeQueue = [];

    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true;
      createElm(vnode, insertedVnodeQueue);
    } else {
      var isRealElement = isDef(oldVnode.nodeType);
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, removeOnly);
      } else {
        if (isRealElement) {
          // mounting to a real element
          // check if this is server-rendered content and if we can perform
          // a successful hydration.
          if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
            oldVnode.removeAttribute(SSR_ATTR);
            hydrating = true;
          }
          if (isTrue(hydrating)) {
            if (hydrate(oldVnode, vnode, insertedVnodeQueue)) {
              invokeInsertHook(vnode, insertedVnodeQueue, true);
              return oldVnode
            } else {
              warn(
                'The client-side rendered virtual DOM tree is not matching ' +
                'server-rendered content. This is likely caused by incorrect ' +
                'HTML markup, for example nesting block-level elements inside ' +
                '
, or missing . Bailing hydration and performing ' +
                'full client-side render.'
              );
            }
          }
          // either not server-rendered, or hydration failed.
          // create an empty node and replace it
          oldVnode = emptyNodeAt(oldVnode);
        }

        // replacing existing element
        var oldElm = oldVnode.elm;
        var parentElm = nodeOps.parentNode(oldElm);

        // create new node      
        createElm(
          vnode,
          insertedVnodeQueue,
          // extremely rare edge case: do not insert if old element is in a
          // leaving transition. Only happens when combining transition +
          // keep-alive + HOCs. (#4590)
          oldElm._leaveCb ? null : parentElm,
          nodeOps.nextSibling(oldElm)
        );

        // update parent placeholder node element, recursively
        if (isDef(vnode.parent)) {
          var ancestor = vnode.parent;
          var patchable = isPatchable(vnode);
          while (ancestor) {
            for (var i = 0; i < cbs.destroy.length; ++i) {
              cbs.destroy[i](ancestor);
            }
            ancestor.elm = vnode.elm;
            if (patchable) {
              for (var i$1 = 0; i$1 < cbs.create.length; ++i$1) {
                cbs.create[i$1](emptyNode, ancestor);
              }
              // #6513
              // invoke insert hooks that may have been merged by create hooks.
              // e.g. for directives that uses the "inserted" hook.
              var insert = ancestor.data.hook.insert;
              if (insert.merged) {
                // start at index 1 to avoid re-invoking component mounted hook
                for (var i$2 = 1; i$2 < insert.fns.length; i$2++) {
                  insert.fns[i$2]();
                }
              }
            } else {
              registerRef(ancestor);
            }
            ancestor = ancestor.parent;
          }
        }

        // destroy old node
        if (isDef(parentElm)) {
          removeVnodes(parentElm, [oldVnode], 0, 0);
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode);
        }
      }
    }

    invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch);
    return vnode.elm
  }

该path函数会执行createElm函数，生成真实DOM，
createElm函数首先会生成父级DOM节点，然后再生成子级DOM节点，再把生成的子级DOM节点，插到它的直接父级DOM节点中，这里是通过调用createChildren函数生成子级DOM节点。

function createElm (
    vnode,
    insertedVnodeQueue,
    parentElm,
    refElm,
    nested,
    ownerArray,
    index
  ) {
    if (isDef(vnode.elm) && isDef(ownerArray)) {
      // This vnode was used in a previous render!
      // now it's used as a new node, overwriting its elm would cause
      // potential patch errors down the road when it's used as an insertion
      // reference node. Instead, we clone the node on-demand before creating
      // associated DOM element for it.
      vnode = ownerArray[index] = cloneVNode(vnode);
    }

    vnode.isRootInsert = !nested; // for transition enter check
    if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
      return
    }

    var data = vnode.data;
    var children = vnode.children;
    var tag = vnode.tag;
    if (isDef(tag)) {
      {
        if (data && data.pre) {
          creatingElmInVPre++;
        }
        if (isUnknownElement$$1(vnode, creatingElmInVPre)) {
          warn(
            'Unknown custom element: <' + tag + '> - did you ' +
            'register the component correctly? For recursive components, ' +
            'make sure to provide the "name" option.',
            vnode.context
          );
        }
      }

      vnode.elm = vnode.ns
        ? nodeOps.createElementNS(vnode.ns, tag)
        : nodeOps.createElement(tag, vnode);
      setScope(vnode);

      /* istanbul ignore if */
      {
        createChildren(vnode, children, insertedVnodeQueue);
        if (isDef(data)) {
          invokeCreateHooks(vnode, insertedVnodeQueue);
        }
        insert(parentElm, vnode.elm, refElm);
      }

      if ("development" !== 'production' && data && data.pre) {
        creatingElmInVPre--;
      }
    } else if (isTrue(vnode.isComment)) {
      vnode.elm = nodeOps.createComment(vnode.text);
      insert(parentElm, vnode.elm, refElm);
    } else {
      vnode.elm = nodeOps.createTextNode(vnode.text);
      insert(parentElm, vnode.elm, refElm);
    }
  }

 function createChildren (vnode, children, insertedVnodeQueue) {
    if (Array.isArray(children)) {
      {
        checkDuplicateKeys(children);
      }
      for (var i = 0; i < children.length; ++i) {
        createElm(children[i], insertedVnodeQueue, vnode.elm, null, true, children, i);
      }
    } else if (isPrimitive(vnode.text)) {
      nodeOps.appendChild(vnode.elm, nodeOps.createTextNode(String(vnode.text)));
    }
  }

createChildren函数中遍历子虚拟节点，生成子级DOM节点。

其中最终所用到生成DOM节点函数如下：

function createElement$1 (tagName, vnode) {
  var elm = document.createElement(tagName);
  if (tagName !== 'select') {
    return elm
  }
  // false or null will remove the attribute but undefined will not
  if (vnode.data && vnode.data.attrs && vnode.data.attrs.multiple !== undefined) {
    elm.setAttribute('multiple', 'multiple');
  }
  return elm
}

往父级DOM插入子元素的工具函数如下：

 function insert (parent, elm, ref$$1) {
    if (isDef(parent)) {
      if (isDef(ref$$1)) {
        if (ref$$1.parentNode === parent) {
          nodeOps.insertBefore(parent, elm, ref$$1);
        }
      } else {
        nodeOps.appendChild(parent, elm);
      }
    }
  }

生成所有DOM节点后，就会把它挂载到页面上了。最后就是清除页面上原有的HTML模板，调用removeAndInvokeRemoveHook函数处理。

function removeAndInvokeRemoveHook (vnode, rm) {
    if (isDef(rm) || isDef(vnode.data)) {
      var i;
      var listeners = cbs.remove.length + 1;
      if (isDef(rm)) {
        // we have a recursively passed down rm callback
        // increase the listeners count
        rm.listeners += listeners;
      } else {
        // directly removing
        rm = createRmCb(vnode.elm, listeners);
      }
      // recursively invoke hooks on child component root node
      if (isDef(i = vnode.componentInstance) && isDef(i = i._vnode) && isDef(i.data)) {
        removeAndInvokeRemoveHook(i, rm);
      }
      for (i = 0; i < cbs.remove.length; ++i) {
        cbs.remove[i](vnode, rm);
      }
      if (isDef(i = vnode.data.hook) && isDef(i = i.remove)) {
        i(vnode, rm);
      } else {
        rm();
      }
    } else {
      removeNode(vnode.elm);
    }
  }

总之 ，这一阶段大致流程就是执行渲染函数得到虚拟DOM，然后根据虚拟DOM生成真实DOM，最后将其挂载到页面中。