LoopBack 4 Todo Application Tutorial - Integrate with a geo-coding service
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Services

To call other APIs and web services from LoopBack applications, we recommend to use Service Proxies as a design pattern for encapsulating low-level implementation details of communication with 3rd-party services and providing JavaScript/TypeScript API that’s easy to consume e.g. from Controllers. See Calling other APIs and web services for more details.

In LoopBack, each service proxy is backed by a DataSource, this datasource leverages one of the service connectors to make outgoing requests and parse responses returned by the service.

In our tutorial, we will leverage US Census Geocoder API to convert textual US addresses into GPS coordinates, thus enabling client applications of our Todo API to display location-based reminders,

Configure the backing datasource

Run lb4 datasource to define a new datasource connecting to Geocoder REST service. When prompted for a connector to use, select “REST services”.

$ lb4 datasource
? Datasource name: geocoder
? Select the connector for geocoder: REST services (supported by StrongLoop)
? Base URL for the REST service:
? Default options for the request:
? An array of operation templates:
? Use default CRUD mapping: No
   create src/datasources/geocoder.datasource.config.json
   create src/datasources/geocoder.datasource.ts
 # npm will install dependencies now
    update src/datasources/index.ts

Datasource Geocoder was created in src/datasources/

Edit the newly created datasource configuration to configure Geocoder API endpoints. Configuration options provided by REST Connector are described in our docs here: REST connector.

/src/datasources/geocoder.datasource.config.json

{
  "name": "geocoder",
  "connector": "rest",
  "options": {
    "headers": {
      "accept": "application/json",
      "content-type": "application/json"
    }
  },
  "operations": [
    {
      "template": {
        "method": "GET",
        "url": "https://geocoding.geo.census.gov/geocoder/locations/onelineaddress",
        "query": {
          "format": "{format=json}",
          "benchmark": "Public_AR_Current",
          "address": "{address}"
        },
        "responsePath": "$.result.addressMatches[*].coordinates"
      },
      "functions": {
        "geocode": ["address"]
      }
    }
  ]
}

Implement a service provider

Use the lb4 service command and the following inputs to create a geocoder service:

lb4 service
? Service type: Remote service proxy backed by a data source
? Please select the datasource GeocoderDatasource
? Service name: geocoder
   create src/services/geocoder.service.ts
   update src/services/index.ts

Service Geocoder was created in src/services/

In the src/services/geocoder.service.ts, we’ll add a GeoPoint interface and a geocode function to the Geocoder interface as follows:

src/services/geocoder.service.ts

import {inject, Provider} from '@loopback/core';
import {getService} from '@loopback/service-proxy';
import {GeocoderDataSource} from '../datasources';

// Add the following interface
export interface GeoPoint {
  /**
   * latitude
   */
  y: number;

  /**
   * longitude
   */
  x: number;
}

export interface Geocoder {
  // Add the following property
  geocode(address: string): Promise<GeoPoint[]>;
}

export class GeocoderProvider implements Provider<Geocoder> {
  constructor(
    // geocoder must match the name property in the datasource json file
    @inject('datasources.geocoder')
    protected dataSource: GeocoderDataSource = new GeocoderDataSource(),
  ) {}

  value(): Promise<Geocoder> {
    return getService(this.dataSource);
  }
}

Enhance Todo model with location data

Add two new properties to our Todo model: remindAtAddress and remindAtGeo.

src/models/todo.model.ts

@model()
export class Todo extends Entity {
  // original code remains unchanged, add the following two properties:

  @property({
    type: 'string',
  })
  remindAtAddress?: string; // address,city,zipcode

  @property({
    type: 'string',
  })
  remindAtGeo?: string; // latitude,longitude
}

Look up address location in the controller

Finally, modify TodoController to look up the address and convert it to GPS coordinates when a new Todo item is created.

Import Geocoder interface into the TodoController and then modify the Controller constructor to receive Geocoder as a new dependency.

src/controllers/todo.controller.ts

import {inject} from '@loopback/core';
import {Geocoder} from '../services';

export class TodoController {
  constructor(
    @repository(TodoRepository)
    public todoRepository: TodoRepository,
    @inject('services.Geocoder') protected geoService: Geocoder,
  ) {}

  // etc.
}

Modify the create method to look up the address provided in remindAtAddress property and convert it to GPS coordinates stored in remindAtGeo.

src/controllers/todo.controller.ts

export class TodoController {
  // constructor, etc.

  @post('/todos', {
    responses: {
      '200': {
        description: 'Todo model instance',
        content: {'application/json': {schema: getModelSchemaRef(Todo)}},
      },
    },
  })
  async create(
    @requestBody({
      content: {
        'application/json': {
          schema: getModelSchemaRef(Todo, {title: 'NewTodo', exclude: ['id']}),
        },
      },
    })
    todo: Omit<Todo, 'id'>,
  ): Promise<Todo> {
    if (todo.remindAtAddress) {
      const geo = await this.geoService.geocode(todo.remindAtAddress);

      if (!geo[0]) {
        // address not found
        throw new HttpErrors.BadRequest(
          `Address not found: ${todo.remindAtAddress}`,
        );
      }

      // Encode the coordinates as "lat,lng" (Google Maps API format). See also
      // https://stackoverflow.com/q/7309121/69868
      // https://gis.stackexchange.com/q/7379
      todo.remindAtGeo = `${geo[0].y},${geo[0].x}`;
    }
    return this.todoRepository.create(todo);
  }

  // other endpoints remain unchanged
}

Congratulations! Now your Todo API makes it easy to enter an address for a reminder and have the client application show the reminder when the device reaches close proximity of that address based on GPS location.

Previous step: Putting it all together