为什么data属性是一个函数而不是一个对象？

const app = new Vue({
    el:"#app",
    // 对象格式
    data:{
        foo:"foo"
    },
    // 函数格式
    data(){
        return {
             foo:"foo"
        }
    }
})

Vue.component('component1',{
    template:`<div>组件</div>`,
    data:{
        foo:"foo"
    }
})

function Component(){
}
Component.prototype.data = {
  count : 0
}

const componentA = new Component()
const componentB = new Component()

console.log(componentB.data.count)  // 0
componentA.data.count = 1
console.log(componentB.data.count)  // 1

function Component(){
  this.data = this.data()
}
Component.prototype.data = function (){
    return {
      count : 0
    }
}

console.log(componentB.data.count)  // 0
componentA.data.count = 1
console.log(componentB.data.count)  // 0

function initData (vm: Component) {
  let data = vm.$options.data
  data = vm._data = typeof data === 'function'
    ? getData(data, vm)
    : data || {}
    ...
}

Vue.prototype._init = function (options?: Object) {
    ...
    // merge options
    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)
    } else {
      vm.$options = mergeOptions(
        resolveConstructorOptions(vm.constructor),
        options || {},
        vm
      )
    }
    ...
  }

strats.data = function (
  parentVal: any,
  childVal: any,
  vm?: Component
): ?Function {
  if (!vm) {
    if (childVal && typeof childVal !== "function") {
      process.env.NODE_ENV !== "production" &&
        warn(
          'The "data" option should be a function ' +
            "that returns a per-instance value in component " +
            "definitions.",
          vm
        );
      return parentVal;
    }
    return mergeDataOrFn(parentVal, childVal);
  }
  return mergeDataOrFn(parentVal, childVal, vm);
};