GraphQL简介

GraphQL是API的查询语言,是一组服务器端运行时(以各种后端语言实现),用于执行查询

什么是GraphQL

GraphQL是API的新领域(应用程序编程接口)设计,以及我们如何构建和使用它们。

它是一种查询语言,还有一组用于执行查询的服务器端运行时(以各种后端语言实现)。它与特定技术无关,但是您可以使用任何语言来实现它。

这是一种方法与REST直接竞争代表性国家转移)API,就像REST一样肥皂首先。

正如我们将看到的,它与REST有很大的不同。它为API设计创建了一个全新的维度。

GraphQL原为在Facebook开发,就像最近震动JavaScript世界的许多技术一样,反应和React Native,这是公开的launched in 2015-尽管Facebook几年前已在内部使用它。

许多大公司正在Facebook旁边采用GraphQL,包括GitHub,Pinterest,Twitter,Sky,《纽约时报》,Shopify,Yelp等数千种。

当GitHub决定使用该技术实现其API的v4时,我第一次与GraphQL联系,然后加入了他们的beta程序。从那时起,我发现它在许多方面都改变了游戏规则。

怎么运行的

GraphQL公开单个端点从您的服务器。

向该端点发送查询通过使用特殊的查询语言语法。该查询是只是一个字符串

服务器通过提供JSON对象来响应查询。

让我们看看这种查询的第一个例子。该查询获取具有以下名称的人的姓名:id=1

GET /graphql?query={ person(id: "1") { name } }

or:

{
  person(id: "1") {
    name
  }
}

We’ll get this JSON response back:

{
  "name": "Tony"
}

Let’s add a bit more complexity: we get the name of the person, and the city where the person lives, by extracting it from the address object. We don’t care about other details of the address, and the server does not return them back to us because we didn’t ask for them:

GET /graphql?query={ person(id: "1") { name, address { city } } }

or

{
  person(id: "1") {
    name
    address {
      city
    }
  }
}

This is what we get back:

{
  "name": "Tony",
  "address": {
    "city": "York"
  }
}

As you can see the data we get is basically the same structure of the request we sent, filled with values that were fetched.

GraphQL Queries

In this section you’ll learn how is a GraphQL query composed.

The concepts I’ll introduce are

  • fields and arguments
  • aliases
  • fragments

Fields and arguments

Take this simple GraphQL query:

{
  person(id: "1") {
    name
  }
}

In this query you see 2 fields, person and name, and 1 argument.

The field person returns an Object which has another field in it, a String.

The argument allows us to specify which person we want to reference. We pass an id, but we could as well pass a name argument, if the API we talk to has the option to find a person by name.

Arguments are not limited to any particular field. We could have a friends field in person that lists the friends of that person, and it could have a limit argument, to specify how many we want the API to return:

{
  person(id: "1") {
    name
    friends(limit: 100)
  }
}

Aliases

You can ask the API to return a field with a different name. For example here you request the name field, but you want it returned as fullname:

{
  owner: person(id: "1") {
    fullname: name
  }
}

will return

{
  "data": {
    "owner": {
      "fullname": "Tony"
    }
  }
}

This feature, beside creating more ad-hoc naming for your client code, in case you need, is the only thing that can make the query work if you need to reference the same endpoint 2 times in the same query:

{
  owner: person(id: "1") {
    fullname: name
  }
  first_employee: person(id: "2") {
    fullname: name
  }
}

Fragments

In the above query we replicated the person structure. Fragments allow us to specify the structure just once (a very useful thing when you have many similar fields):

{
  owner: person(id: "1") {
    ...personFields
  }
  first_employee: person(id: "2") {
    ...personFields
  }
}

fragment personFields on person { fullname: name }

GraphQL Variables

More complex GraphQL queries need to use variables, a way to dynamically specify a value that is used inside a query.

In this case we added the person id as a string inside the query:

{
  owner: person(id: "1") {
    fullname: name
  }
}

The id will most probably change dynamically in our program, so we need a way to pass it, and not with string interpolation.

With variables, the same query can be written as this:

query GetOwner($id: String) {
  owner: person(id: $id) {
    fullname: name
  }
}

{ “id”: “1” }

In this snippet we have assigned the GetOwner name to our query. Think of it as named functions, while previously you had an anonymous function. Named queries are useful when you have lots of queries in your application.

The query definition with the variables looks like a function definition, and it works in an equivalent way.

Making variables required

Appending a ! to the type:

query GetOwner($id: String!)

instead of $id: String will make the $id variable required.

Specifying a default value for a variable

You can specify a default value using this syntax:

query GetOwner($id: String = "1")

GraphQL Directives

Directives let you include or exclude a field if a variable is true or false.

query GetPerson($id: String) {
  person(id: $id) {
    fullname: name,
    address: @include(if: $getAddress) {
      city
      street
      country
    }
  }
}

{ “id”: “1”, “getAddress”: false }

In this case if getAddress variable we pass is true, we also get the address field, otherwise not.

We have 2 directives available: include, which we have just seen (includes if true), and skip, which is the opposite (skips if true)

@include(if: Boolean)

query GetPerson($id: String) {
  person(id: $id) {
    fullname: name,
    address: @include(if: $getAddress) {
      city
      street
      country
    }
  }
}

{ “id”: “1”, “getAddress”: false }

@skip(if: Boolean)

query GetPerson($id: String) {
  person(id: $id) {
    fullname: name,
    address: @skip(if: $excludeAddress) {
      city
      street
      country
    }
  }
}

{ “id”: “1”, “excludeAddress”: false }


More graphql tutorials: