xref: /docs/en/application-dev/file-management/file-access-across-devices.md

# Accessing Files Across Devices

The distributed file system provides applications the capability for accessing files across devices. If the same application has been installed on two devices, you can use the [ohos.file.fs APIs](app-file-access.md) to read and write the application files in the [distributed file directory](app-sandbox-directory.md#mapping-between-application-sandbox-paths-and-physical-paths) (**/data/storage/el2/distributedfiles/**) on the other device. For example, an application is installed on both device A and device B. After device A and device B are connected to form a Super Device, the application on device A can access the files in the distributed directory of the same application on device B.

## How to Develop

1. Connect the devices to form a Super Device.<br>
   Perform unified account authentication for the devices. The devices are not necessarily in the same LAN.

2. Implement cross-device access to the files of your application.<br>
   Place the files in the **distributedfiles/** directory of the application sandbox directory to implement access from difference devices.

   For example, create a file in the **distributedfiles/** directory on device A and write data to the file. For details about how to obtain the application context, see [Obtaining the Context of UIAbility](../application-models/uiability-usage.md#obtaining-the-context-of-uiability).

   ```ts
   import { fileIo as fs } from '@kit.CoreFileKit';
   import { common } from '@kit.AbilityKit';
   import { BusinessError } from '@kit.BasicServicesKit';
 
   let context = getContext(this) as common.UIAbilityContext; // Obtain the UIAbilityContext of device A.
   let pathDir: string = context.distributedFilesDir;
   // Obtain the file path of the distributed directory.
   let filePath: string = pathDir + '/test.txt';
   
   try {
     // Create a file in the distributed directory.
     let file = fs.openSync(filePath, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE);
     console.info('Succeeded in createing.');
     // Write data to the file.
     fs.writeSync(file.fd, 'content');
     // Close the file.
     fs.closeSync(file.fd);
   } catch (error: BusinessError) {
     let err: BusinessError = error as BusinessError;
     console.error(`Failed to openSync / writeSync / closeSync. Code: ${err.code}, message: ${err.message}`);
   } 
   ```

   Device B initiates a link setup request to device A. After the link is set up, device B can read the test file in the distributed file directory.

   ```ts
   import { fileIo as fs } from '@kit.CoreFileKit';
   import { common } from '@kit.AbilityKit';
   import { BusinessError } from '@kit.BasicServicesKit';
   import { buffer } from '@kit.ArkTS';
   import { distributedDeviceManager } from '@kit.DistributedServiceKit'
 
   // Obtain the network ID of device A.
   let dmInstance = distributedDeviceManager.createDeviceManager("com.example.hap");
   let deviceInfoList: Array<distributedDeviceManager.DeviceBasicInfo> = dmInstance.getAvailableDeviceListSync();
   let networkId = deviceInfoList[0].networkId;
 
   // Define the callback for accessing the user directory.
   let listeners : fs.DfsListeners = {
     onStatus: (networkId: string, status: number): void => {
       console.info('Failed to access public directory');
     }
   }
 
   // Access and mount the user directory.
   fs.connectDfs(networkId, listeners).then(() => {
     console.info("Success to connectDfs");
     let context = getContext(this) as common.UIAbilityContext; // Obtain the UIAbilityContext of device B.
     let pathDir: string = context.distributedFilesDir;
     // Obtain the file path of the distributed directory.
     let filePath: string = pathDir + '/test.txt';
   
     try {
       // Open the file in the distributed directory.
       let file = fs.openSync(filePath, fs.OpenMode.READ_WRITE);
       // Set the buffer for receiving the read data.
       let arrayBuffer = new ArrayBuffer(4096);
       // Read the file. The return value is the number of read bytes.
       class Option {
           public offset: number = 0;
           public length: number = 0;
       }
       let option = new Option();
       option.length = arrayBuffer.byteLength;
       let num = fs.readSync(file.fd, arrayBuffer, option);
       // Print the read data.
       let buf = buffer.from(arrayBuffer, 0, num);
       console.info('read result: ' + buf.toString());
     } catch (error: BusinessError) {
       let err: BusinessError = error as BusinessError;
       console.error(`Failed to openSync / readSync. Code: ${err.code}, message: ${err.message}`);
     }
   }).catch((error: BusinessError) => {
     let err: BusinessError = error as BusinessError;
     console.error(`Failed to connectDfs Code: ${err.code}, message: ${err.message}`);
   });
   ```

3. Disconnect the link for device B.

   ```ts
   import { BusinessError } from '@kit.BasicServicesKit';
   import { distributedDeviceManager } from '@kit.DistributedServiceKit'
   
   // Obtain the network ID of device A.
   let dmInstance = distributedDeviceManager.createDeviceManager("com.example.hap");
   let deviceInfoList: Array<distributedDeviceManager.DeviceBasicInfo> = dmInstance.getAvailableDeviceListSync();
   let networkId = deviceInfoList[0].networkId;
 
   // Disconnect from the user directory.
   fs.disconnectDfs(networkId).then(() => {
     console.info("Success to disconnectDfs");
   }).catch((error: BusinessError) => {
     let err: BusinessError = error as BusinessError;
     console.error(`Failed to disconnectDfs Code: ${err.code}, message: ${err.message}`)
   })
   ```