This post discusses how to download files to your device from your API in an Ionic 3 application. By using the File and File Transfer plugins that are available for Ionic you can accomplish this task.First, you install the necessary plugins — File and File Transfer. You can refer to Ionic’s documentation here to install them.

• https://ionicframework.com/docs/v3/native/file/
• https://ionicframework.com/docs/v3/native/file-transfer/

Make certain that you add the plugins to your providers in the main app module. Next, you create a service to handle the download. Using a service takes advantage of Angular’s modularity and reusability. Remember to add the following service to your providers in the main app module.

1import { Injectable } from "@angular/core";
2import { Observable } from "rxjs/Observable";
3import { fromPromise } from "rxjs/observable/fromPromise";
4import { FileTransfer, FileTransferObject } from '@ionic-native/file-transfer';
5import { Platform, AlertController } from "ionic-angular";
6import { File } from "@ionic-native/file";
7import { AndroidPermissions } from '@ionic-native/android-permissions';
8 
9@Injectable()
10export class DownloadService {
11    constructor(
12        private transfer: FileTransfer,
13        private file: File,
14        private platform: Platform,
15        private androidPermissions: AndroidPermissions,
16        private alertCtrl: AlertController
17    ) {
18    }
19 
20    public downloadFile(fileId: number, fileName: string): Observable<any> {
21        return fromPromise(this.performDownload(fileId, fileName, fileExtension));
22    }
23 
24    protected async performDownload(fileId: number, fileName: string){
25				// We added this check since this is only intended to work on devices and emulators 
26        if (!this.platform.is('cordova')) {
27            console.warn('Cannot download in local environment!');
28            return;
29        }
30 
31        const fileTransfer: FileTransferObject = this.transfer.create();
32 
33        let uri = encodeURI(`www.yourapi.com/file/download?id=fileId`);
34 
35        let path = await this.getDownloadPath();
36 
37        let fullFileName = fileName + '.' + fileExtension;
38 
39	// Depending on your needs, you might want to use some form of authentication for your API endpoints
40	// In this case, we are using bearer tokens
41	let bearerToken = 'yourToken';
42 
43        return fileTransfer.download(
44            uri,
45            path + fileName,
46            false,
47            {
48                headers: {
49                    "Authorization": `Bearer ${bearerToken}`
50                }
51            }
52        ).then(
53            result => {
54                this.showAlert(true, fileName);
55            },
56            error => {
57                this.showAlert(false, fileName);
58            }
59        )
60    }
61 
62    public async getDownloadPath() {
63        if (this.platform.is('ios')) {
64            return this.file.documentsDirectory;
65        }
66	
67			// To be able to save files on Android, we first need to ask the user for permission. 
68			// We do not let the download proceed until they grant access
69        await this.androidPermissions.checkPermission(this.androidPermissions.PERMISSION.WRITE_EXTERNAL_STORAGE).then(
70            result => {
71                if (!result.hasPermission) {
72                    return this.androidPermissions.requestPermission(this.androidPermissions.PERMISSION.WRITE_EXTERNAL_STORAGE);
73                }
74            }
75        );
76 
77        return this.file.externalRootDirectory + "/Download/";
78    }
79 
80    // Here we are using simple alerts to show the user if the download was successful or not
81    public showAlert(hasPassed: boolean, fileName: string) {
82        let title = hasPassed ? "Download complete!" : "Download failed!";
83 
84        let subTitle = hasPassed ? `Successfully downloaded ${fileName}.` : `There was a problem while downloading ${fileName}`;
85 
86        const alert = this.alertCtrl.create({
87            title: title,
88            subTitle: subTitle,
89            buttons: ['OK']
90        });
91        alert.present();
92    }
93}
94

Next, you inject the download service into the page and add a function to be called from your template. That code looks as follows:

1import { Component} from '@angular/core';
2import { IonicPage, NavController } from 'ionic-angular';
3import { DownloadService } from '../../providers/download/download-service';
4 
5@IonicPage()
6@Component({
7  selector: 'page-downloads',
8  templateUrl: 'downloads.html',
9})
10export class DownloadsPage {
11 
12  // Here we are injecting our download service into our page
13  constructor(private downloadService: DownloadService{ }
14 
15  // This is the function that will trigger the download
16  public downloadFile(fileId: number, fileName: string) {
17    this.downloadService.downloadFile(fileId, fileName).subscribe();
18  }
19 
20}

Finally, add the necessary code to call the function from page template. The assumption is that you have a list of items to be downloaded.

1<ion-list>
2  <ng-container *ngFor="let item of downloadableItems">
3    <button (click)="downloadFile(item.id, item.name)">
4      <div>
5        <h2>{{ content.name }}</h2>
6      </div>
7    </button>
8  </ng-container>
9</ion-list>

In summary, you create a service to handle the download. Then, you inject the service into the page and add a function to be called from the template. Finally, you call the function. Now, you can successfully download files from an API to your device.