Android save file to directory

Saving Files

This lesson teaches you to

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Android uses a file system that’s similar to disk-based file systems on other platforms. This lesson describes how to work with the Android file system to read and write files with the File APIs.

A File object is suited to reading or writing large amounts of data in start-to-finish order without skipping around. For example, it’s good for image files or anything exchanged over a network.

This lesson shows how to perform basic file-related tasks in your app. The lesson assumes that you are familiar with the basics of the Linux file system and the standard file input/output APIs in java.io .

Choose Internal or External Storage

All Android devices have two file storage areas: «internal» and «external» storage. These names come from the early days of Android, when most devices offered built-in non-volatile memory (internal storage), plus a removable storage medium such as a micro SD card (external storage). Some devices divide the permanent storage space into «internal» and «external» partitions, so even without a removable storage medium, there are always two storage spaces and the API behavior is the same whether the external storage is removable or not. The following lists summarize the facts about each storage space.

• It’s always available.
• Files saved here are accessible by only your app by default.
• When the user uninstalls your app, the system removes all your app’s files from internal storage.

Internal storage is best when you want to be sure that neither the user nor other apps can access your files.

• It’s not always available, because the user can mount the external storage as USB storage and in some cases remove it from the device.
• It’s world-readable, so files saved here may be read outside of your control.
• When the user uninstalls your app, the system removes your app’s files from here only if you save them in the directory from getExternalFilesDir() .

External storage is the best place for files that don’t require access restrictions and for files that you want to share with other apps or allow the user to access with a computer.

Tip: Although apps are installed onto the internal storage by default, you can specify the android:installLocation attribute in your manifest so your app may be installed on external storage. Users appreciate this option when the APK size is very large and they have an external storage space that’s larger than the internal storage. For more information, see App Install Location.

Obtain Permissions for External Storage

To write to the external storage, you must request the WRITE_EXTERNAL_STORAGE permission in your manifest file:

Caution: Currently, all apps have the ability to read the external storage without a special permission. However, this will change in a future release. If your app needs to read the external storage (but not write to it), then you will need to declare the READ_EXTERNAL_STORAGE permission. To ensure that your app continues to work as expected, you should declare this permission now, before the change takes effect.

However, if your app uses the WRITE_EXTERNAL_STORAGE permission, then it implicitly has permission to read the external storage as well.

You don’t need any permissions to save files on the internal storage. Your application always has permission to read and write files in its internal storage directory.

Save a File on Internal Storage

When saving a file to internal storage, you can acquire the appropriate directory as a File by calling one of two methods:

getFilesDir() Returns a File representing an internal directory for your app. getCacheDir() Returns a File representing an internal directory for your app’s temporary cache files. Be sure to delete each file once it is no longer needed and implement a reasonable size limit for the amount of memory you use at any given time, such as 1MB. If the system begins running low on storage, it may delete your cache files without warning.

To create a new file in one of these directories, you can use the File() constructor, passing the File provided by one of the above methods that specifies your internal storage directory. For example:

Alternatively, you can call openFileOutput() to get a FileOutputStream that writes to a file in your internal directory. For example, here’s how to write some text to a file:

Or, if you need to cache some files, you should instead use createTempFile() . For example, the following method extracts the file name from a URL and creates a file with that name in your app’s internal cache directory:

Note: Your app’s internal storage directory is specified by your app’s package name in a special location of the Android file system. Technically, another app can read your internal files if you set the file mode to be readable. However, the other app would also need to know your app package name and file names. Other apps cannot browse your internal directories and do not have read or write access unless you explicitly set the files to be readable or writable. So as long as you use MODE_PRIVATE for your files on the internal storage, they are never accessible to other apps.

Save a File on External Storage

Because the external storage may be unavailable—such as when the user has mounted the storage to a PC or has removed the SD card that provides the external storage—you should always verify that the volume is available before accessing it. You can query the external storage state by calling getExternalStorageState() . If the returned state is equal to MEDIA_MOUNTED , then you can read and write your files. For example, the following methods are useful to determine the storage availability:

Although the external storage is modifiable by the user and other apps, there are two categories of files you might save here:

Public files Files that should be freely available to other apps and to the user. When the user uninstalls your app, these files should remain available to the user.

For example, photos captured by your app or other downloaded files.

Private files Files that rightfully belong to your app and should be deleted when the user uninstalls your app. Although these files are technically accessible by the user and other apps because they are on the external storage, they are files that realistically don’t provide value to the user outside your app. When the user uninstalls your app, the system deletes all files in your app’s external private directory.

For example, additional resources downloaded by your app or temporary media files.

If you want to save public files on the external storage, use the getExternalStoragePublicDirectory() method to get a File representing the appropriate directory on the external storage. The method takes an argument specifying the type of file you want to save so that they can be logically organized with other public files, such as DIRECTORY_MUSIC or DIRECTORY_PICTURES . For example:

If you want to save files that are private to your app, you can acquire the appropriate directory by calling getExternalFilesDir() and passing it a name indicating the type of directory you’d like. Each directory created this way is added to a parent directory that encapsulates all your app’s external storage files, which the system deletes when the user uninstalls your app.

For example, here’s a method you can use to create a directory for an individual photo album:

If none of the pre-defined sub-directory names suit your files, you can instead call getExternalFilesDir() and pass null . This returns the root directory for your app’s private directory on the external storage.

Remember that getExternalFilesDir() creates a directory inside a directory that is deleted when the user uninstalls your app. If the files you’re saving should remain available after the user uninstalls your app—such as when your app is a camera and the user will want to keep the photos—you should instead use getExternalStoragePublicDirectory() .

Regardless of whether you use getExternalStoragePublicDirectory() for files that are shared or getExternalFilesDir() for files that are private to your app, it’s important that you use directory names provided by API constants like DIRECTORY_PICTURES . These directory names ensure that the files are treated properly by the system. For instance, files saved in DIRECTORY_RINGTONES are categorized by the system media scanner as ringtones instead of music.

Query Free Space

If you know ahead of time how much data you’re saving, you can find out whether sufficient space is available without causing an IOException by calling getFreeSpace() or getTotalSpace() . These methods provide the current available space and the total space in the storage volume, respectively. This information is also useful to avoid filling the storage volume above a certain threshold.

However, the system does not guarantee that you can write as many bytes as are indicated by getFreeSpace() . If the number returned is a few MB more than the size of the data you want to save, or if the file system is less than 90% full, then it’s probably safe to proceed. Otherwise, you probably shouldn’t write to storage.

Note: You aren’t required to check the amount of available space before you save your file. You can instead try writing the file right away, then catch an IOException if one occurs. You may need to do this if you don’t know exactly how much space you need. For example, if you change the file’s encoding before you save it by converting a PNG image to JPEG, you won’t know the file’s size beforehand.

Delete a File

You should always delete files that you no longer need. The most straightforward way to delete a file is to have the opened file reference call delete() on itself.

If the file is saved on internal storage, you can also ask the Context to locate and delete a file by calling deleteFile() :

Note: When the user uninstalls your app, the Android system deletes the following:

• All files you saved on internal storage
• All files you saved on external storage using getExternalFilesDir() .

However, you should manually delete all cached files created with getCacheDir() on a regular basis and also regularly delete other files you no longer need.

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Saving Files to Application Folder and Gallery in Flutter

Dec 6, 2020 · 6 min read

The majority of Mobile Applications require some kind of file storage, where users can download or save files for future access. Applications like WhatsApp & Instagram save files to their own Folder in Internal Phone Storage on Android and to the Photos Library on iOS Device.

In Flutter, to get information about storage Paths you can use the path_provider package, but by default, applications do not create their own application-specific folders in Internal Storage and that is what we will learn to do in this Article. So, let’s start this by creating a New Flutter Project and do some basic Project Setup:

Project Setup:

dio (3.0.10): To download file from a URL and save it to a given path.

path_provider (1.6.10): To get Application and System Specific Storage Paths.

permission_handler (5.0.1): To check/ask for storage permissions before performing any storage task.

2.) Once these dependencies are added, we need to do some changes to the android specific folder of our Flutter Project. For this let’s navigate to the AndroidManifest.xml file:

In this file, add the following:

android: requestLegacyExternalStorage=”true” is only required if you have a targetSDKVersion of 29 or above.

3.) Now, Let’s do the iOS specific changes. For this let’s navigate to the Info.plist file:

In this file, add the following lines at bottom:

The permission we need to add is the NSPhotoLibraryUsageDescription to describe our reason for accessing the Photos Library.

UI Code

After the basic Project Setup, let’s create a basic UI for our Application. Our Application code in main.dart will look something like this:

The code above is a basic UI Design with a FlatButton. In MyHomePageState we have 2 state variables

1. loading to indicate that the file is downloading
2. progress to act as a progress value for LinearProgressIndicator

The onPressed property of FlatButton is provided with a function of downloadFile which will call the saveVideo function (yet to implement). The way this code is written is that the downloadFile function will handle the UI changes i.e setting the loading and progress variables and getting the boolean from saveVideo function to indicate to the user if the file is downloaded successfully or not.

Now that our UI code is done, let’s take a look at the implementation of saveVideo function.

“saveVideo” Function

The code for this function looks like this:

Now, let’s take a look at this in parts and understand what everything does.

On the first line in this function, we create an instance of Directory class which comes from dart:io package. We don’t give any value to this directly because this will get its value based on what platform our app runs on. For this have to write some platform-specific code next, and this code has the potential for throwing errors, and therefore we wrap everything in a try-catch block.

To check the platform we are on, we use Platform class , again from dart:io package. Now, in both cases of if-else we need to check/request permission. Clearly this code can be written in its own function, which for this tutorial we will call _requestPermission. The code for this function is as follows:

Permission class, the instance of which we pass to the _requestPermission function comes from the permission_handler package that we earlier added to the application.

In this function, we first asynchronously check, if the permission is granted. We return true if it is already granted, or else we request the permission. If the user grants this permission we return true, or else we return false.

On Android, we request for Permission.storage and on iOS we request for Permission.photos. Once these permissions are granted we do the following:

Android

We set the value of directory to getExternalStorageDirectory() which gives us a path like this:

By default, each application has its own storage directory in “Android” folder of Device’s Internal Storage denoted by the package_name of that application.

But we do not want to save files to this directory. Instead what we want is to create our own application folder in the internal storage of our android device. One thing common in most android devices is that they have the “Android” folder in Device’s internal storage. So if we get a path leading to that “Android” directory, we can use the same path and create our application folder there. Lines 7–19 in the saveVideo function code are just for this.

On iOS Device we need to save files to Photos Library. We’ll get to this soon enough but before that, we will need to save the file in the Temporary Directory. For this, we set the directory to the result of getTemporaryDirectory().

Once the directories are set, we move out of the if-else block and we check if the directory exists? and if not, we try to create the directory. Now before moving to the next step of downloading the file, we check if the directory is successfully created or not. I am keeping this simple for this article and check the existence of the directory once again using directory.exists() which is an asynchronous function.

Now, if the directory exists we can move further and create a new File with the “filename” that we get as an argument to saveVideo function. Once this is done, we use the Dio package to download the file from a given URL and update progress using the onReceiveProgress property of dio.download function. Once the file is downloaded, on Android you can check the file in your Phone’s Internal Storage where you will find the Application Folder you specified.

Saving File to Photo Library on iOS

Once the Dio download function finishes execution, we can now save the download file to Photos Library on iOS device.

Earlier, we set the download directory in case of iOS device to Temporary Directory. This was because the package image_gallery_saver can copy the file from an existing directory to Photos Library. For this, we check if the Platform is iOS and then use saveFile function of ImageGallerySaver class and give it the path of the File we saved in Temporary Directory. One important thing to do here is to pass a “true” value to isReturnPathOfIOS parameter.

Voilà 💙

Now you can use this technique to do all sorts of custom storage behavior you want in your Flutter Apps, at least for Android.

Note: I’ve tried to keep the code for this tutorial fairly simple. For example, in the saveVideo function we just return a boolean value. In production apps, you might need to create different UI behaviour for different conditions. So take this tutorial as a source to learn the concept and implement this in your own ways.

If you like to have a visual representation for reference, check out this tutorial on my Youtube Channel and you’ll have a good idea! And while you’re there, if you find this useful, consider Subscribing and Sharing!

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