hope to have a way to set scope of variable

[caikan]

Sorry, Chinease only. I'll try to translate later.

What problem does this feature solve?

有时我希望在组件某区域渲染数据，它们都来自于data上比较深层的某一个属性，当数据量比较多时，直接使用文本插值的写法会比较繁琐，比如：
(以下为了精简代码，采用了pug、es6语法，并忽略Vue组件data必须为函数的要求)
The mustache interpolations of data in some deep props make the code cumbersome:

data.js

export default {
  some: {
    deep: {
      props: {
        a: 1,
        b: 2,
      },
    },
  },
};

my-page.vue

<template lang="pug">
  div
    p {{some.deep.props.a}}
    p {{some.deep.props.b}}
</template>
<script>
  import data from './data.js'
  export default {
    data,
  }
</script>

我想到几种变通的做法：
I tried some solutions:

1. 使用计算属性：Use computed properties:

   my-page.vue

   <template lang="pug">
     div
       p {{props.a}}
       p {{props.b}}
   </template>
   <script>
     import data from './data.js'
     export default {
       data,
       computed: {
         props() {
           return this.some.deep.props;
         },
       },
     }
   </script>

   这样做相对简单，但是总要为此去定义很多计算属性似乎也有些繁琐。
   It is easy to use, but the computed properties become cumbersome.

2. 利用v-for指令来缩小作用域：Use v-for directive to define a scoped variable:

   my-page.vue

   <template lang="pug">
     div
       template(v-for="props in [some.deep.props]")
         p(ref="a") {{props.a}}
         p(ref="b") {{props.b}}
   </template>
   <script>
     import data from './data.js'
     export default {
       data,
     }
   </script>

   这是一个取巧的做法，但也有副作用：用$refs访问v-for范围内的ref引用会变成数组。
   It almost works well, but have side-effect: $refs in the v-for scope will become an array.

3. 将数据渲染区域定义为子组件，子组件接受简化后的数据：Use child component:

   data-view.vue

   <template lang="pug">
     div
       p {{props.a}}
       p {{props.b}}
   </template>
   <script>
     export default {
       props: ['props'],
     }
   </script>

   my-page.vue

   <template lang="pug">
     div
       data-view(:props="some.deep.props")
   </template>
   <script>
     import data from './data.js'
     import DataView from './data-view.vue'
     export default {
       components: {DataView},
       data,
     }
   </script>

   子组件内部的写法能简化许多，但是整体花销仍然不小。一个额外的问题是，子组件多引入了一个可能原本并不必要的包装根元素。
   Interpolations in child component become simple, but more children is also cumbersome. In addition, the child need a wrapper element as the component root, which was not needed before.

4. 利用子组件作用域插槽来封装一个通用的辅助组件：Use the scoped slot to make a reusable component:

   sub-scope.vue

   <template lang="pug">
     div
       slot(:sub="data")
   </template>
   <script>
     export default {
       props: ['data'],
     }
   </script>

   my-page.vue

   <template lang="pug">
     div
       sub-scope(:data="some.deep.props")
         template(slot-scope="scope")
           p {{scope.sub.a}}
           p {{scope.sub.b}}
       // or
       sub-scope(:data="some.deep.props")
         template(slot-scope="{sub}")
           p {{sub.a}}
           p {{sub.b}}
   </template>
   <script>
     import data from './data.js'
     import SubScope from './sub-scope.vue'
     export default {
       components: {SubScope},
       data,
     }
   </script>

   这也是取巧利用了子组件来改变访问作用域。可以复用看上去好像是一个优点，但实际使用还是比较繁琐，甚至代码看上去有些更乱了。与上一个方法同样存在的一个问题是，子组件引入了可能原本并不必要的包装根元素。
   Only the interpolations look like simpler, but other codes become more complex.

What does the proposed API look like?

我感觉利用v-for指令的办法很巧妙，就是多了一些副作用，虽然也不是非常严重的影响。
因此我希望能有一个指令，能够如同v-for一样定义一个局部作用域变量，但又不会影响$refs引用的表现。
I hope there is a directive can define scoped variables like v-for, but will not affect the performance of $refs.

div(v-scope="props of some.deep") {{props.a}}
// or
div(v-scope="(a, b) of some.deep.props") {{a}}, {{b}}

或者可以提供两种ref元素访问接口：
Or the ref API should be optimized:

• $refs仅用于访问指定引用名的第一项，类似于querySelector()。$refs should only be used for reference the first of elements with the same ref value, like querySelector().
• 而$groupedRefs(我想也许还有更好一点的名字)则用于访问包含相同引用名的全部元素数组，类似于querySelectorAll()。$groupedRefs is used for reference the array of the elements with the same ref value, like querySelectorAll().

[caikan]

其实我很希望是这样的API：

div(v-scope="some.deep.props") {{a}}, {{b}}

如果不指定要导出的变量，那么就把v-scope指定的表达式值作为优先级最高的作用域变量，相当于导出了全部属性变量。
If no exporting variable is specified, all properties should be exported.

[Solander]

You should really translate this one. I think, but I'm not sure, that this is exactly what I want in Vue as well. A way to declare a scope on a container.

[caikan]

Translation updated. @Solander

[flip111]

What about changing

<script>
  import data from './data.js'
  export default {
    data,
  }
</script>

into

<script>
  import data from './data.js'
  export default {
    data.some.deep.props,
  }
</script>

[Kingwl]

@atinux Thanks for your comments

1. I'm also cautious about this feature, I did not provide a shortcut ( such as $v="" ) and export all props in the implementation

2. This feature is sometimes useful:

<div v-for="item in items" v-local:result="doSomeComplicated(item)">
    <span>{{result.a}}</span>
    <span>{{result.b}}</span>
    <input :value="item.some">
</div>

3. this is seems not a heavy or complicated implementation

hope to get more feedback about this feature :XD

[jkzing]

@atinux Thanks for your comments

@Austio, just help to @ the right person. 😁

[ortega-io]

I also would like to be able to use the mentioned syntax:

<div v-for="item in items" v-local:result="doSomeComplicated(item)">
    <span>{{result.a}}</span>
    <span>{{result.b}}</span>
    <input :value="item.some">
</div>

To be able to access deep properties without having to use long names like:

object.child.property

[sirlancelot]

How about some sort of v-let style notation to be more like JavaScript? My thoughts:

<template>
    <div>
        <!-- Object-style syntax, similar to `v-bind` -->
        <template v-let="{ props: some.deep.props }">
            <span>{{ props.a }}</span>
            <span>{{ props.a }}</span>
        </template>
    </div>
</template>

<template>
    <div>
        <!-- Single-value syntax, similar to `v-bind:arg` -->
        <template v-let:props="some.deep.props">
            <span>{{ props.a }}</span>
            <span>{{ props.a }}</span>
        </template>
    </div>
</template>

JavaScript's let keyword is block-scoped, v-let should be consistent with that, so the leted values should only be accessible in the child nodes.

[AlexandreBonneau]

If you declare a v-let, then shouldn't there be a v-const?

[sirlancelot]

@AlexandreBonneau No, that doesn't make sense.

[Austio]

My thoughts are that you would still need a computed property or method in most cases for this where the data is coming from some api b/c there will be possibility of null/undefined. Keeping a separation between the template and presenting the data will be marginally more verbose but the gains are clarity and also reduction in size of vue core.

[OEvgeny]

Userland solution which works, but I highly recommend not to use it "as is".

(May be useful when you can move some logic into separate component which will make template more cleaner)

[AlexandreBonneau]

@OEvgeny This is a nice workaround, thanks.

However, a simpler v-let directive could prevent cluttering the template with those additional tags.
I think this feature request is still valid.

[FranckFreiburger]

note that xsl has the concept of const variable.
Splitting templates or using computed or method is not always possible without breaking logical blocks or reducing readability.

[daqin0525]

个人习惯和风格可能都不太一样，清晰明朗，简洁易用，就是好框架，感觉用法还是不错的，很符合现在的代码编程习惯

[vpiskunov]

Any updates on this? Just ran into this as well...

[zippaaa]

I was looking for a solution too... While I found only this:

<template>
<div v-for="item in items" v-myvar="myvar = doSomeComplicated(item)">
    <span>{{myvar.a}}</span>
    <span>{{myvar.b}}</span>
    <input :value="item.some">
</div>
</template>

<script>
export default {
	//...
	//fake
	directives: {
		myvar: {}
	}
};
</script>

[AlexandreBonneau]

Using the directives object is counterintuitive ; if you would want to declare those local variables, then it ought to be better to use:

export default {
	//...
	//fake
	localVariables: {
		myvar: {}
	}
};

However, having to name very local var in the component seems a bit too verbose.
If you have multiple <v-for> in your component, you'll have to name each temporary variable differently, while perhaps they would manipulate the same data.

v-let is still my preferred way to go, and would be scoped in the template to the very html element it is declared on.

[zippaaa]

if you would want to declare those local variables, then it ought to be better to use:
export default {
localVariables: {
myvar: {}
}
};

Is it works now? Can you show a complete example with local variable in template?

[AlexandreBonneau]

No, it does not work now, as we are discussing options to implement (or not) local variables!

The point of my previous comment was to explain how I think having to declare the local variables in the <script> part is a bad idea, and how v-let could solve this elegantly and concisely.

[zippaaa]

I agree with you.
I wrote an example of how I temporarily solved the problem.

[Vovencia]

I was looking for a solution too... While I found only this:

<template>
<div v-for="item in items" v-myvar="myvar = doSomeComplicated(item)">
    <span>{{myvar.a}}</span>
    <span>{{myvar.b}}</span>
    <input :value="item.some">
</div>
</template>

<script>
export default {
	//...
	//fake
	directives: {
		myvar: {}
	}
};
</script>

Another way:
be careful, values are assigned to the instance
(this.bar = my.very.deeply.nested.bar), (this.secondBar = my.very.deeply.nested.secondBar), (this.foo = my.deeply.nested.foo)

<template>
  <div
        :bar="((bar = my.very.deeply.nested.bar), (secondBar = my.very.deeply.nested.secondBar), undefined)"
        v-bind="((foo = my.deeply.nested.foo), {})"
  >
    I am static! <span v-text="foo + ' '"></span> <span v-text="bar"></span>{{ secondBar }}
  </div>
</template>

<script>
import * as Vue from "vue";

export default {
  data: () => ({
    my: {
      deeply: { nested: { foo: "hello" } },
      very: { deeply: { nested: { bar: "world", secondBar: "!!!" } } }
    }
  })
};
</script>

https://codesandbox.io/s/l55551j0xq

[doncatnip]

A v-let or v-local or something similar would be pretty awesome. Some constellations force me to write a myriad of components with only a very small template and maybe one computed getter, introducing lots of boilerplate that could be avoided with a v-let.

[HerringtonDarkholme]

Our new RFC repo https://github.com/vuejs/rfcs is dedicated to such feature request.