expo-downloads

An Expo module for directly downloading files to the native Downloads folder on Android and iOS.

Table of Contents

• Installation
• Usage
• Demo Videos
• API Specification
  • Functions
  • Permission-related Functions
  • Exceptions
• Platform-specific Behavior

Installation

npx expo install expo-downloads

Usage

Below is a sample code which saves a text file named example.txt to the native Downloads folder. For Android versions 9 and below, the WRITE_EXTERNAL_STORAGE permission is checked.

import { saveFile, openFile, getPermissionsAsync, requestPermissionsAsync } from "expo-downloads";

const options = {
  name: "example.txt",
  type: "text/plain",
  data: "Hello, World!",
};

// For Android 9 and below, request WRITE_EXTERNAL_STORAGE permission.
const permissions = await getPermissionsAsync();
if (!permissions.granted) {
  const newPermissions = await requestPermissionsAsync();
  if (!newPermissions.granted) {
    console.error("Could not obtain permissions");
    return;
  }
}

try {
  const result = await saveFile(options);
  if (result.cancelled) {
    console.log("Download was cancelled");
  } else {
    console.log(`File saved successfully: ${result.uri}`);
    // Example of opening the file immediately after saving
    await openFile({ uri: result.uri, type: options.type });
  }
} catch (error) {
  console.error("Error saving or opening file:", error);
}

Demo Videos

Android

Text file download

android-text.mp4

Image file download

android-image.mp4

iOS

Text file download

ios-text.mp4

Image file download

ios-image.mp4

API Specification

Functions

saveFile(options: SaveFileOptions): Promise<SaveFileResponse>

• Description: Saves the Base64 encoded file data using the provided options.

• Parameters (SaveFileOptions object):

  • name: The name under which the file will be saved.
  • type: The MIME type of the file.
  • data: The file data to be saved.
  • encoding: (Optional) The encoding of the data, either "base64" or "utf8". Defaults to "utf8".

• Returns (SaveFileResponse object):

  • uri: The URI of the saved file (upon success).
  • cancelled: true if the operation was cancelled by the user (iOS only).

openFile(options: OpenFileOptions): Promise<void>

• Description: Opens the downloaded file using the native file viewer on the device.

• Parameters (OpenFileOptions object):

  • uri: The URI of the file to be opened. (Use the uri field of the SaveFileResponse object returned by saveFile.)
  • type: The MIME type of the file.

Permission-related Functions

• requestPermissionsAsync:
  Requests the WRITE_EXTERNAL_STORAGE permission for Android versions 9 and below. (This permission is not required on Android 10+ and iOS.)

• getPermissionsAsync:
  Retrieves the current status of the storage write permission.

Exceptions

Common Exceptions

• ERR_INVALID_ARGUMENT (iOS / Android)

  • Thrown if the name is empty, if the type format is invalid, or if the data is improperly formatted.

• ERR_FILE_OPEN

  • Thrown when an error occurs while attempting to open a file. This exception is raised if the specified file does not exist or if a compatible application cannot be found to open it.

iOS Specific Exceptions

• ERR_DOWNLOAD_IN_PROGRESS

  • Thrown if a download is already in progress, preventing simultaneous operations.

• ERR_MISSING_VIEW_CONTROLLER

  • Thrown if the current view controller cannot be obtained, which prevents the file-saving dialog from being displayed.

Android Specific Exceptions

• ERR_CONTENT_URI_CREATION (Android 10 and above)

  • Thrown if creating the content URI using the MediaStore API fails.

• ERR_OUTPUT_STREAM_CREATION (Android 10 and above)

  • Thrown if an OutputStream for writing to the file cannot be created.

• ERR_DIRECTORY_CREATION (Android 9 and below)

  • Thrown if the Downloads folder does not exist or if attempting to create it fails.

• ERR_OUT_OF_MEMORY

  • Thrown when the file is too large and an OutOfMemoryError occurs during saving due to insufficient memory.

Platform-specific Behavior

iOS

• Implementation:
  Uses UIDocumentPickerViewController to allow the user to select a save location.

Android

Android 10 and above (Q+)

• Implementation:
  Uses the MediaStore API to save files to the system-managed Downloads folder.
• Permissions:
  No special storage permissions are required on API level 29 and above.

Android 9 and below

• Implementation:
  Writes directly to the device's Downloads folder using legacy methods.
• Permissions:
  The WRITE_EXTERNAL_STORAGE permission is required. Use requestPermissionsAsync and getPermissionsAsync to handle permissions.

Web

• Implementation:
  Utilizes HTML's Blob and <a> tag to download the file.