从源码看vue(v2.7.10)中的v-bind的原理

前面我们分析了v-model的原理，接下来我们看看v-bind的实现又是怎样的呢？

前置内容

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解析模板

// App.vue
var render = function render() {
  var _vm = this,
    _c = _vm._self._c
  return _c(
    "div",
    [
      _c("test", { attrs: { propTest: _vm.a } }),
      _vm._v(" "),
      _c("div", { on: { click: _vm.changeA } }, [_vm._v("点我")]),
    ],
    1
  )
}
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可以看出v-bind:propTest='a’会被解析成attrs: { propTest: _vm.a }，看过前几篇文章的都知道会触发a变量的get方法收集依赖。目前主要是看当前组件是怎么把attrs属性传递给子组件的：

function createElement$1(context, tag, data, children, normalizationType, alwaysNormalize) {
   if (isArray(data) || isPrimitive(data)) {
       normalizationType = children;
       children = data;
       data = undefined;
   }
   if (isTrue(alwaysNormalize)) {
       normalizationType = ALWAYS_NORMALIZE;
   }
   return _createElement(context, tag, data, children, normalizationType);
}
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_c(“test”, { attrs: { propTest: _vm.a } })方法主要执行createElement$1方法：

function _createElement(context, tag, data, children, normalizationType) {
 	   ...
   
       else if ((!data || !data.pre) &&
           isDef((Ctor = resolveAsset(context.$options, 'components', tag)))) {
           // component
           vnode = createComponent(Ctor, data, context, children, tag);
       }
       ...
   if (isArray(vnode)) {
       return vnode;
   }
   else if (isDef(vnode)) {
       if (isDef(ns))
           applyNS(vnode, ns);
       if (isDef(data))
           registerDeepBindings(data);
       return vnode;
   }
   else {
       return createEmptyVNode();
   }
}
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主要执行createComponent方法，传入参数如图所示：

接下来执行createComponent函数，并创建test的vm函数然后创建test的vnode：

function createComponent(Ctor, data, context, children, tag) {
    ...
    var baseCtor = context.$options._base;
    // plain options object: turn it into a constructor
    if (isObject(Ctor)) {
        Ctor = baseCtor.extend(Ctor);
    }
  ...
    
    data = data || {};
    // resolve constructor options in case global mixins are applied after
    // component constructor creation
    resolveConstructorOptions(Ctor);
    // transform component v-model data into props & events
    if (isDef(data.model)) {
        // @ts-expect-error
        transformModel(Ctor.options, data);
    }
    // extract props
    // @ts-expect-error
    var propsData = extractPropsFromVNodeData(data, Ctor, tag);
    // functional component
    // @ts-expect-error
    if (isTrue(Ctor.options.functional)) {
        return createFunctionalComponent(Ctor, propsData, data, context, children);
    }
    // extract listeners, since these needs to be treated as
    // child component listeners instead of DOM listeners
    var listeners = data.on;
    // replace with listeners with .native modifier
    // so it gets processed during parent component patch.
    data.on = data.nativeOn;
    // @ts-expect-error
    if (isTrue(Ctor.options.abstract)) {
        // abstract components do not keep anything
        // other than props & listeners & slot
        // work around flow
        var slot = data.slot;
        data = {};
        if (slot) {
            data.slot = slot;
        }
    }
    // install component management hooks onto the placeholder node
    installComponentHooks(data);
    // return a placeholder vnode
    // @ts-expect-error
    var name = getComponentName(Ctor.options) || tag;
    var vnode = new VNode(
    // @ts-expect-error
    "vue-component-".concat(Ctor.cid).concat(name ? "-".concat(name) : ''), data, undefined, undefined, undefined, context, 
    // @ts-expect-error
    { Ctor: Ctor, propsData: propsData, listeners: listeners, tag: tag, children: children }, asyncFactory);
    return vnode;
}
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创建的vnode如下图所示，其中componetnOptions是创建vm函数时的参数。这个在后面实例化test的vm函数时有用

此时所有vnode基本创建完毕。此时执行vm._update(vm._render(), hydrating)方法，该方法主要执行vm.$el = vm._patch_(prevVnode, vnode)方法，该方法执行createChildren去遍历vnode执行createElm方法：

function createElm(vnode, insertedVnodeQueue, parentElm, refElm, nested, ownerArray, index) {
   ...
    if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
        return;
    }
   ...
}
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由于第一个node是test是一个组件，所有会执行createComponent方法：

function createComponent(vnode, insertedVnodeQueue, parentElm, refElm) {
    var i = vnode.data;
    if (isDef(i)) {
        var isReactivated = isDef(vnode.componentInstance) && i.keepAlive;
        if (isDef((i = i.hook)) && isDef((i = i.init))) {
            i(vnode, false /* hydrating */);
        }
        ...
    }
}
...
init: function (vnode, hydrating) {
 ...
    else {
        var child = (vnode.componentInstance = createComponentInstanceForVnode(vnode, activeInstance));
        child.$mount(hydrating ? vnode.elm : undefined, hydrating);
    }
}
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该方法执行componentVNodeHooks的init方法的createComponentInstanceForVnode去创建test组件的vm实例：

function createComponentInstanceForVnode(parent) {
    var options = {
        _isComponent: true,
        _parentVnode: vnode,
        parent: parent
    };
    // check inline-template render functions
    var inlineTemplate = vnode.data.inlineTemplate;
    if (isDef(inlineTemplate)) {
        options.render = inlineTemplate.render;
        options.staticRenderFns = inlineTemplate.staticRenderFns;
    }
    return new vnode.componentOptions.Ctor(options);
}
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实例化过程中使用了vnode过程中创建的vm函数，在实例化的过程中会执行initInternalComponent函数，该函数从父vnode的componentOptions中获取prop数据：

if (options && options._isComponent) {
   // optimize internal component instantiation
     // since dynamic options merging is pretty slow, and none of the
     // internal component options needs special treatment.
     initInternalComponent(vm, options);
 }
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到这里为止从App.vue中的attrs属性就已经传到test组件上了。initInternalComponent方法执行完毕继续执行initState方法：

function initState(vm) {
   var opts = vm.$options;
   if (opts.props)
       initProps$1(vm, opts.props);
   // Composition API
   initSetup(vm);
   if (opts.methods)
       initMethods(vm, opts.methods);
   if (opts.data) {
       initData(vm);
   }
   else {
       var ob = observe((vm._data = {}));
       ob && ob.vmCount++;
   }
   if (opts.computed)
       initComputed$1(vm, opts.computed);
   if (opts.watch && opts.watch !== nativeWatch) {
       initWatch(vm, opts.watch);
   }
}
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首先执行initProps$1方法：

function initProps$1(vm, propsOptions) {
   var propsData = vm.$options.propsData || {};
   var props = (vm._props = shallowReactive({}));
   // cache prop keys so that future props updates can iterate using Array
   // instead of dynamic object key enumeration.
   var keys = (vm.$options._propKeys = []);
   var isRoot = !vm.$parent;
   // root instance props should be converted
   if (!isRoot) {
       toggleObserving(false);
   }
   var _loop_1 = function (key) {
       keys.push(key);
       var value = validateProp(key, propsOptions, propsData, vm);
       /* istanbul ignore else */
       {
           var hyphenatedKey = hyphenate(key);
           if (isReservedAttribute(hyphenatedKey) ||
               config.isReservedAttr(hyphenatedKey)) {
               warn$2("\"".concat(hyphenatedKey, "\" is a reserved attribute and cannot be used as component prop."), vm);
           }
           defineReactive(props, key, value, function () {
               if (!isRoot && !isUpdatingChildComponent) {
                   warn$2("Avoid mutating a prop directly since the value will be " +
                       "overwritten whenever the parent component re-renders. " +
                       "Instead, use a data or computed property based on the prop's " +
                       "value. Prop being mutated: \"".concat(key, "\""), vm);
               }
           });
       }
       // static props are already proxied on the component's prototype
       // during Vue.extend(). We only need to proxy props defined at
       // instantiation here.
       if (!(key in vm)) {
           proxy(vm, "_props", key);
       }
   };
   for (var key in propsOptions) {
       _loop_1(key);
   }
   toggleObserving(true);
}
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获取到propsOptions并循环执行_loop_1(key)方法，该方法首先执行validateProp方法校验数据和我们在test组件中定义的props类型是否相同。然后执行defineReactive方法将该prop设置在vm._props中并设置get和set。
此时我们得出一个结论，test组件会先根据props校验propsData的类型并获取值，test组件定义的props会被设置响应式。至此App.vue中的v-on中给的值已经被传到test组件并设置了初始值。
不难看出，当App.vue中的数据发生变化时会重新执行变量中的watcher的update方法重新将值传入test组件。由于该组件已经创建了会存在prevVnode值，所以不会再次创建只会执行vm._patch_(prevVnode, vnode)去更新组件的值：

Vue.prototype._update = function (vnode, hydrating) {
    var vm = this;
    var prevEl = vm.$el;
    var prevVnode = vm._vnode;
    var restoreActiveInstance = setActiveInstance(vm);
    vm._vnode = vnode;
    // Vue.prototype.__patch__ is injected in entry points
    // based on the rendering backend used.
    if (!prevVnode) {
        // initial render
        vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */);
    }
    else {
        // updates
        vm.$el = vm.__patch__(prevVnode, vnode);
    }
    ...
};
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至此整个过程结束。

总结

1. 会将v-bind:propTest="a"解析成attrs: { propTest: _vm.a }，并在渲染组件test的时候把该值传过去。
2. 在实例化组件的时候会根据props里面创建的值和传进来的值做类型校验，然后并设置响应式同时设置初始值。
3. 父组件值变化的时候会触发该变量的set方法父组件执行updateChildren方法对比新旧子组件并更新值。