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NestJS Enterprise Backend APIs · 课时

解析器与订阅

实现 GraphQL 解析器来获取和操作数据，并设置订阅以实现实时数据更新。

解析器与订阅 是 CoddyKit 上的免费 NestJS Enterprise Backend APIs 课时。 这是第 3 节课，共 3 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 NestJS Enterprise Backend APIs 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 NestJS Enterprise Backend APIs 课程共包含 3 节课。

本课时的部分内容尚未翻译，以英文显示。

Intro to GraphQL Resolvers

Welcome to the world of GraphQL resolvers in NestJS! Resolvers are the core of your GraphQL API, acting as the bridge between your GraphQL schema and your application's data sources.

  • They define how to fetch the data for a specific field in your schema.
  • Think of them as functions that execute when a corresponding field is requested in a GraphQL query or mutation.
  • Resolvers can interact with databases, REST APIs, or any other data source.

Resolver Structure in NestJS

NestJS simplifies resolver creation using decorators. You define a class as a resolver and then use specific decorators for different GraphQL operations:

  • @Resolver(): Marks a class as a GraphQL resolver.
  • @Query(): Maps a method to a GraphQL query operation (data fetching).
  • @Mutation(): Maps a method to a GraphQL mutation operation (data modification).
  • @Subscription(): Maps a method to a GraphQL subscription operation (real-time data).

Implementing a Basic Query

Let's create a simple query resolver that returns a 'Hello world!' string. This demonstrates the basic setup for a query:

import { Module, Injectable } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { GraphQLModule } from '@nestjs/graphql';
import { ApolloDriver, ApolloDriverConfig } from '@nestjs/apollo';
import { join } from 'path';
import { Query, Resolver } from '@nestjs/graphql';

@Injectable()
class AppService {
  getHello(): string {
    return 'Hello world!';
  }
}

@Resolver()
class AppResolver {
  constructor(private readonly appService: AppService) {}

  @Query(() => String)
  hello(): string {
    return this.appService.getHello();
  }
}

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: join(process.cwd(), 'src/schema.gql'),
    }),
  ],
  providers: [AppResolver, AppService],
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}
bootstrap();

Handling Query Arguments

Queries often need arguments to filter or specify data. Use the @Args() decorator to extract arguments from the GraphQL request.

Here, we'll create a query that greets a specific name.

import { Module, Injectable } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { GraphQLModule } from '@nestjs/graphql';
import { ApolloDriver, ApolloDriverConfig } from '@nestjs/apollo';
import { join } from 'path';
import { Args, Query, Resolver } from '@nestjs/graphql';

@Injectable()
class AppService {
  getGreeting(name: string): string {
    return `Hello, ${name}!`;
  }
}

@Resolver()
class AppResolver {
  constructor(private readonly appService: AppService) {}

  @Query(() => String)
  greet(@Args('name') name: string): string {
    return this.appService.getGreeting(name);
  }
}

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: join(process.cwd(), 'src/schema.gql'),
    }),
  ],
  providers: [AppResolver, AppService],
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}
bootstrap();

Introducing GraphQL Mutations

While queries fetch data, mutations are used for modifying data on the server. This includes operations like creating, updating, or deleting records.

  • Mutations are similar to queries in structure but clearly signal a side effect.
  • They typically return the modified object or a status indicating success.
  • Use the @Mutation() decorator to define them in NestJS.

Implementing a Basic Mutation

Let's create a mutation to add a simple 'Item'. We'll define an Item type and a mutation that takes a name and returns the newly created item.

import { Module, Injectable, Field, ObjectType, InputType } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { GraphQLModule } from '@nestjs/graphql';
import { ApolloDriver, ApolloDriverConfig } from '@nestjs/apollo';
import { join } from 'path';
import { Args, Mutation, Query, Resolver } from '@nestjs/graphql';

@ObjectType()
class Item {
  @Field(() => String)
  id: string;

  @Field(() => String)
  name: string;
}

@Injectable()
class ItemsService {
  private readonly items: Item[] = [];
  create(name: string): Item {
    const newItem = { id: String(this.items.length + 1), name };
    this.items.push(newItem);
    return newItem;
  }
  findAll(): Item[] { return this.items; }
}

@Resolver(() => Item)
class ItemsResolver {
  constructor(private readonly itemsService: ItemsService) {}

  @Query(() => [Item])
  items(): Item[] {
    return this.itemsService.findAll();
  }

  @Mutation(() => Item)
  createItem(@Args('name') name: string): Item {
    return this.itemsService.create(name);
  }
}

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: join(process.cwd(), 'src/schema.gql'),
    }),
  ],
  providers: [ItemsResolver, ItemsService],
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}
bootstrap();

What are GraphQL Subscriptions?

Subscriptions are a GraphQL feature that allows clients to receive real-time updates from the server. Unlike queries and mutations (which are request-response), subscriptions are long-lived connections, typically over WebSockets.

  • When a client subscribes, it opens a persistent connection.
  • The server pushes data to the client whenever a specific event occurs.
  • Perfect for chat applications, live dashboards, or notifications.

Setting up PubSub for Subscriptions

To implement subscriptions, NestJS GraphQL often uses graphql-subscriptions's PubSub (Publish-Subscribe) mechanism. The server publishes events, and subscribers listen for them.

You need to configure GraphQLModule for subscriptions and provide a PubSub instance.

import { Module, Injectable } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { GraphQLModule } from '@nestjs/graphql';
import { ApolloDriver, ApolloDriverConfig } from '@nestjs/apollo';
import { join } from 'path';
import { PubSub } from 'graphql-subscriptions';

// We need a module to provide PubSub
@Module({
  providers: [
    {
      provide: 'PUB_SUB', // Token for PubSub instance
      useValue: new PubSub(),
    },
  ],
  exports: ['PUB_SUB'], // Export to use in other modules
})
class PubSubModule {}

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: join(process.cwd(), 'src/schema.gql'),
      // Crucial for subscriptions:
      subscriptions: {
        'graphql-ws': true, // Enable WebSocket for subscriptions
        'subscriptions-transport-ws': true,
      },
    }),
    PubSubModule,
  ],
  // No resolvers needed for this example, just setup
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
  console.log('GraphQL server with subscriptions running on port 3000');
}
bootstrap();

Implementing a Subscription

Now let's build a subscription! We'll create a postAdded subscription that emits an event whenever a new post is created via a mutation.

  • Use @Subscription() to define the subscription method.
  • Inject PubSub to publish events.
  • The mutation will trigger the event.
import { Module, Injectable, Field, ObjectType, Inject } from '@nestjs/common';
import { NestFactory } from '@nestjs/core';
import { GraphQLModule } from '@nestjs/graphql';
import { ApolloDriver, ApolloDriverConfig } from '@nestjs/apollo';
import { join } from 'path';
import { Args, Mutation, Query, Resolver, Subscription } from '@nestjs/graphql';
import { PubSub } from 'graphql-subscriptions';

const POST_ADDED_EVENT = 'postAdded';

@ObjectType()
class Post {
  @Field(() => String)
  id: string;

  @Field(() => String)
  title: string;

  @Field(() => String)
  content: string;
}

@Injectable()
class PostsService {
  private readonly posts: Post[] = [];
  create(title: string, content: string): Post {
    const newPost = { id: String(this.posts.length + 1), title, content };
    this.posts.push(newPost);
    return newPost;
  }
  findAll(): Post[] { return this.posts; }
}

@Module({
  providers: [
    {
      provide: 'PUB_SUB', // Token for PubSub instance
      useValue: new PubSub(),
    },
  ],
  exports: ['PUB_SUB'], // Export to use in other modules
})
class PubSubModule {}

@Resolver(() => Post)
class PostsResolver {
  constructor(
    private readonly postsService: PostsService,
    @Inject('PUB_SUB') private pubSub: PubSub,
  ) {}

  @Query(() => [Post])
  posts(): Post[] {
    return this.postsService.findAll();
  }

  @Mutation(() => Post)
  async createPost(
    @Args('title') title: string,
    @Args('content') content: string,
  ): Promise<Post> {
    const newPost = this.postsService.create(title, content);
    await this.pubSub.publish(POST_ADDED_EVENT, { postAdded: newPost });
    return newPost;
  }

  @Subscription(() => Post, { name: POST_ADDED_EVENT })
  postAdded() {
    return this.pubSub.asyncIterator(POST_ADDED_EVENT);
  }
}

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: join(process.cwd(), 'src/schema.gql'),
      subscriptions: {
        'graphql-ws': true,
        'subscriptions-transport-ws': true,
      },
    }),
    PubSubModule,
  ],
  providers: [PostsResolver, PostsService],
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
  console.log('GraphQL server with subscriptions running on port 3000');
  console.log('Try a POST_ADDED_EVENT subscription in your client!');
}
bootstrap();

Resolver & Subscription Check

Time to test your knowledge on NestJS GraphQL resolvers and subscriptions!

Recap: Resolvers & Real-time

Congratulations! You've grasped the essentials of GraphQL resolvers and subscriptions in NestJS.

  • Resolvers are the backbone, connecting your schema to your data.
  • Queries fetch data, Mutations change data, and Subscriptions provide real-time updates.
  • We explored how to implement simple queries and mutations, and set up real-time features using PubSub for subscriptions.

Keep practicing to build dynamic and responsive GraphQL APIs!

常见问题解答

「解析器与订阅」课时是免费的吗？

是的 — 「解析器与订阅」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 NestJS Enterprise Backend APIs 课程的其余内容，请升级到 CoddyKit PRO。 NestJS Enterprise Backend APIs 课程共包含 3 节课。

「解析器与订阅」这节课中我会学到什么？

实现 GraphQL 解析器来获取和操作数据，并设置订阅以实现实时数据更新。 你通过在浏览器中直接运行的动手代码来练习 NestJS Enterprise Backend APIs，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 NestJS Enterprise Backend APIs 需要有经验吗？

无需任何先前经验。CoddyKit 上的 NestJS Enterprise Backend APIs 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 3 节课，共 3 节。

「解析器与订阅」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 NestJS Enterprise Backend APIs 课中编写并运行代码吗？

能。每节 NestJS Enterprise Backend APIs 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

  1. GraphQL 基础
  2. 设置 NestJS GraphQL
  3. 解析器与订阅
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