How to get free and total size of each StorageVolu

Background

Google (sadly) plans to ruin storage permission so that apps won't be able to access the file system using the standard File API (and file-paths). Many are against it as it changes the way apps can access the storage and in many ways it's a restricted and limited API.

As a result, we will need to use SAF (storage access framework) entirely on some future Android version (on Android Q we can, at least temporarily, use a flag to use the normal storage permission), if we wish to deal with various storage volumes and reach all files there.

So, for example, suppose you want to make a file manager and show all the storage volumes of the device, and show for each of them how many total and free bytes there are. Such a thing seems very legitimate, but as I can't find a way to do such a thing.

The problem

Starting from API 24 (here), we finally have the ability to list all of the storage volumes, as such:

    val storageManager = getSystemService(Context.STORAGE_SERVICE) as StorageManager
    val storageVolumes = storageManager.storageVolumes

Thing is, there is no function for each of the items on this list to get its size and free space.

However, somehow, Google's "Files by Google" app manages to get this information without any kind of permission being granted :

And this was tested on Galaxy Note 8 with Android 8. Not even the latest version of Android.

So this means there should be a way to get this information without any permission, even on Android 8.

What I've found

There is something similar to getting free-space, but I'm not sure if it's indeed that. It seems as such, though. Here's the code for it:

    val storageManager = getSystemService(Context.STORAGE_SERVICE) as StorageManager
    val storageVolumes = storageManager.storageVolumes
    AsyncTask.execute {
        for (storageVolume in storageVolumes) {
            val uuid: UUID = storageVolume.uuid?.let { UUID.fromString(it) } ?: StorageManager.UUID_DEFAULT
            val allocatableBytes = storageManager.getAllocatableBytes(uuid)
            Log.d("AppLog", "allocatableBytes:${android.text.format.Formatter.formatShortFileSize(this,allocatableBytes)}")
        }
    }

However, I can't find something similar for getting the total space of each of the StorageVolume instances. Assuming I'm correct on this, I've requested it here.

You can find more of what I've found in the answer I wrote to this question, but currently it's all a mix of workarounds and things that aren't workarounds but work in some cases.

The questions

Is getAllocatableBytes indeed the way to get the free space?
How can I get the free and real total space (in some cases I got lower values for some reason) of each StorageVolume, without requesting any permission, just like on Google's app?

标签： android storage-access-framework

3条回答

地球回转人心会变

2楼-- · 2020-02-09 07:45

The following uses fstatvfs(FileDescriptor) to retrieve stats without resorting to reflection or traditional file system methods.

To check the output of the program to make sure it is producing reasonable result for total, used and available space I ran the "df" command on an Android Emulator running API 29.

Output of "df" command in adb shell reporting 1K blocks:

"/data" corresponds to the "primary" UUID used when by StorageVolume#isPrimary is true.

"/storage/1D03-2E0E" corresponds to the "1D03-2E0E" UUID reported by StorageVolume#uuid.

generic_x86:/ $ df
Filesystem              1K-blocks    Used Available Use% Mounted on
/dev/root                 2203316 2140872     46060  98% /
tmpfs                     1020140     592   1019548   1% /dev
tmpfs                     1020140       0   1020140   0% /mnt
tmpfs                     1020140       0   1020140   0% /apex
/dev/block/vde1            132168   75936     53412  59% /vendor

/dev/block/vdc             793488  647652    129452  84% /data

/dev/block/loop0              232      36       192  16% /apex/com.android.apex.cts.shim@1
/data/media                793488  647652    129452  84% /storage/emulated

/mnt/media_rw/1D03-2E0E    522228      90    522138   1% /storage/1D03-2E0E

Reported by the app using fstatvfs (in 1K blocks):

For /tree/primary:/document/primary: Total=793,488 used space=647,652 available=129,452

For /tree/1D03-2E0E:/document/1D03-2E0E: Total=522,228 used space=90 available=522,138

The totals match.

fstatvfs is described here.

Detail on what fstatvfs returns can be found here.

The following little app displays used, free and total bytes for volumes that are accessible.

MainActivity.kt

class MainActivity : AppCompatActivity() {
    private lateinit var mStorageManager: StorageManager
    private val mVolumeStats = HashMap<Uri, StructStatVfs>()
    private val mStorageVolumePathsWeHaveAccessTo = HashSet<String>()
    private lateinit var mStorageVolumes: List<StorageVolume>
    private var mHaveAccessToPrimary = false

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        mStorageManager = getSystemService(Context.STORAGE_SERVICE) as StorageManager
        mStorageVolumes = mStorageManager.storageVolumes

        requestAccessButton.setOnClickListener {
            val primaryVolume = mStorageManager.primaryStorageVolume
            val intent = primaryVolume.createOpenDocumentTreeIntent()
            startActivityForResult(intent, 1)
        }

        releaseAccessButton.setOnClickListener {
            val takeFlags =
                Intent.FLAG_GRANT_READ_URI_PERMISSION or Intent.FLAG_GRANT_WRITE_URI_PERMISSION
            val uri = buildVolumeUriFromUuid(PRIMARY_UUID)

            contentResolver.releasePersistableUriPermission(uri, takeFlags)
            val toast = Toast.makeText(
                this,
                "Primary volume permission released was released.",
                Toast.LENGTH_SHORT
            )
            toast.setGravity(Gravity.BOTTOM, 0, releaseAccessButton.height)
            toast.show()
            getVolumeStats()
            showVolumeStats()
        }
        getVolumeStats()
        showVolumeStats()

    }

    private fun getVolumeStats() {
        val persistedUriPermissions = contentResolver.persistedUriPermissions
        mStorageVolumePathsWeHaveAccessTo.clear()
        persistedUriPermissions.forEach {
            mStorageVolumePathsWeHaveAccessTo.add(it.uri.toString())
        }
        mVolumeStats.clear()
        mHaveAccessToPrimary = false
        for (storageVolume in mStorageVolumes) {
            val uuid = if (storageVolume.isPrimary) {
                // Primary storage doesn't get a UUID here.
                PRIMARY_UUID
            } else {
                storageVolume.uuid
            }

            val volumeUri = uuid?.let { buildVolumeUriFromUuid(it) }

            when {
                uuid == null ->
                    Log.d(TAG, "UUID is null for ${storageVolume.getDescription(this)}!")
                mStorageVolumePathsWeHaveAccessTo.contains(volumeUri.toString()) -> {
                    Log.d(TAG, "Have access to $uuid")
                    if (uuid == PRIMARY_UUID) {
                        mHaveAccessToPrimary = true
                    }
                    val uri = buildVolumeUriFromUuid(uuid)
                    val docTreeUri = DocumentsContract.buildDocumentUriUsingTree(
                        uri,
                        DocumentsContract.getTreeDocumentId(uri)
                    )
                    mVolumeStats[docTreeUri] = getFileStats(docTreeUri)
                }
                else -> Log.d(TAG, "Don't have access to $uuid")
            }
        }
    }

    private fun showVolumeStats() {
        val sb = StringBuilder()
        if (mVolumeStats.size == 0) {
            sb.appendln("Nothing to see here...")
        } else {
            sb.appendln("All figures are in 1K blocks.")
            sb.appendln()
        }
        mVolumeStats.forEach {
            val lastSeg = it.key.lastPathSegment
            sb.appendln("Volume: $lastSeg")
            val stats = it.value
            val blockSize = stats.f_bsize
            val totalSpace = stats.f_blocks * blockSize / 1024L
            val freeSpace = stats.f_bfree * blockSize / 1024L
            val usedSpace = totalSpace - freeSpace
            sb.appendln(" Used space: ${usedSpace.nice()}")
            sb.appendln(" Free space: ${freeSpace.nice()}")
            sb.appendln("Total space: ${totalSpace.nice()}")
            sb.appendln("----------------")
        }
        volumeStats.text = sb.toString()
        if (mHaveAccessToPrimary) {
            releaseAccessButton.visibility = View.VISIBLE
            requestAccessButton.visibility = View.GONE
        } else {
            releaseAccessButton.visibility = View.GONE
            requestAccessButton.visibility = View.VISIBLE
        }
    }

    private fun buildVolumeUriFromUuid(uuid: String): Uri {
        return DocumentsContract.buildTreeDocumentUri(
            EXTERNAL_STORAGE_AUTHORITY,
            "$uuid:"
        )
    }

    private fun getFileStats(docTreeUri: Uri): StructStatVfs {
        val pfd = contentResolver.openFileDescriptor(docTreeUri, "r")!!
        return fstatvfs(pfd.fileDescriptor)
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        Log.d(TAG, "resultCode:$resultCode")
        val uri = data?.data ?: return
        val takeFlags =
            Intent.FLAG_GRANT_READ_URI_PERMISSION or Intent.FLAG_GRANT_WRITE_URI_PERMISSION
        contentResolver.takePersistableUriPermission(uri, takeFlags)
        Log.d(TAG, "granted uri: ${uri.path}")
        getVolumeStats()
        showVolumeStats()
    }

    companion object {
        fun Long.nice(fieldLength: Int = 12): String = String.format(Locale.US, "%,${fieldLength}d", this)

        const val EXTERNAL_STORAGE_AUTHORITY = "com.android.externalstorage.documents"
        const val PRIMARY_UUID = "primary"
        const val TAG = "AppLog"
    }
}

activity_main.xml

<LinearLayout 
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        tools:context=".MainActivity">

    <TextView
            android:id="@+id/volumeStats"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_marginBottom="16dp"
            android:layout_weight="1"
            android:fontFamily="monospace"
            android:padding="16dp" />

    <Button
            android:id="@+id/requestAccessButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:layout_marginBottom="16dp"
            android:visibility="gone"
            android:text="Request Access to Primary" />

    <Button
            android:id="@+id/releaseAccessButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:layout_marginBottom="16dp"
            android:text="Release Access to Primary" />
</LinearLayout>

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女痞

3楼-- · 2020-02-09 07:45

Found a workaround, by using what I wrote here , and mapping each StorageVolume with a real file as I wrote here. Sadly this might not work in the future, as it uses a lot of "tricks" :

        for (storageVolume in storageVolumes) {
            val volumePath = FileUtilEx.getVolumePath(storageVolume)
            if (volumePath == null) {
                Log.d("AppLog", "storageVolume \"${storageVolume.getDescription(this)}\" - failed to get volumePath")
            } else {
                val statFs = StatFs(volumePath)
                val availableSizeInBytes = statFs.availableBytes
                val totalBytes = statFs.totalBytes
                val formattedResult = "availableSizeInBytes:${android.text.format.Formatter.formatShortFileSize(this, availableSizeInBytes)} totalBytes:${android.text.format.Formatter.formatShortFileSize(this, totalBytes)}"
                Log.d("AppLog", "storageVolume \"${storageVolume.getDescription(this)}\" - volumePath:$volumePath - $formattedResult")
            }
        }

Seems to work on both emulator (that has primary storage and SD-card) and real device (Pixel 2), both on Android Q beta 4.

A bit better solution which wouldn't use reflection, could be to put a unique file in each of the paths we get on ContextCompat.getExternalCacheDirs, and then try to find them via each of the StorageVolume instances. It is tricky though because you don't know when to start the search, so you will need to check various paths till you reach the destination. Not only that, but as I wrote here, I don't think there is an official way to get the Uri or DocumentFile or File or file-path of each StorageVolume.

Anyway, weird thing is that the total space is lower than the real one. Probably as it's a partition of what's the maximum that's really available to the user.

I wonder how come various apps (such as file manager apps, like Total Commander) get the real total device storage.

EDIT: OK got another workaround, which is probably more reliable, based on the storageManager.getStorageVolume(File) function.

So here is the merging of the 2 workarounds:

fun getStorageVolumePath(context: Context, storageVolumeToGetItsPath: StorageVolume): String? {
    //first, try to use reflection
    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP)
        return null
    try {
        val storageVolumeClazz = StorageVolume::class.java
        val getPathMethod = storageVolumeClazz.getMethod("getPath")
        val result = getPathMethod.invoke(storageVolumeToGetItsPath) as String?
         if (!result.isNullOrBlank())
            return result
    } catch (e: Exception) {
        e.printStackTrace()
    }
    //failed to use reflection, so try mapping with app's folders
    val storageVolumeUuidStr = storageVolumeToGetItsPath.uuid
    val externalCacheDirs = ContextCompat.getExternalCacheDirs(context)
    val storageManager = context.getSystemService(Context.STORAGE_SERVICE) as StorageManager
    for (externalCacheDir in externalCacheDirs) {
        val storageVolume = storageManager.getStorageVolume(externalCacheDir) ?: continue
        val uuidStr = storageVolume.uuid
        if (uuidStr == storageVolumeUuidStr) {
            //found storageVolume<->File match
            var resultFile = externalCacheDir
            while (true) {
                val parentFile = resultFile.parentFile ?: return resultFile.absolutePath
                val parentFileStorageVolume = storageManager.getStorageVolume(parentFile)
                        ?: return resultFile.absolutePath
                if (parentFileStorageVolume.uuid != uuidStr)
                    return resultFile.absolutePath
                resultFile = parentFile
            }
        }
    }
    return null
}

And to show the available and total space, we use StatFs as before:

for (storageVolume in storageVolumes) {
    val storageVolumePath = getStorageVolumePath(this@MainActivity, storageVolume) ?: continue
    val statFs = StatFs(storageVolumePath)
    val availableSizeInBytes = statFs.availableBytes
    val totalBytes = statFs.totalBytes
    val formattedResult = "availableSizeInBytes:${android.text.format.Formatter.formatShortFileSize(this, availableSizeInBytes)} totalBytes:${android.text.format.Formatter.formatShortFileSize(this, totalBytes)}"
    Log.d("AppLog", "storageVolume \"${storageVolume.getDescription(this)}\" - storageVolumePath:$storageVolumePath - $formattedResult")
}

EDIT: shorter version, without using the real file-path of the storageVolume:

fun getStatFsForStorageVolume(context: Context, storageVolumeToGetItsPath: StorageVolume): StatFs? {
    //first, try to use reflection
    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.N)
        return null
    try {
        val storageVolumeClazz = StorageVolume::class.java
        val getPathMethod = storageVolumeClazz.getMethod("getPath")
        val resultPath = getPathMethod.invoke(storageVolumeToGetItsPath) as String?
        if (!resultPath.isNullOrBlank())
            return StatFs(resultPath)
    } catch (e: Exception) {
        e.printStackTrace()
    }
    //failed to use reflection, so try mapping with app's folders
    val storageVolumeUuidStr = storageVolumeToGetItsPath.uuid
    val externalCacheDirs = ContextCompat.getExternalCacheDirs(context)
    val storageManager = context.getSystemService(Context.STORAGE_SERVICE) as StorageManager
    for (externalCacheDir in externalCacheDirs) {
        val storageVolume = storageManager.getStorageVolume(externalCacheDir) ?: continue
        val uuidStr = storageVolume.uuid
        if (uuidStr == storageVolumeUuidStr) {
            //found storageVolume<->File match
            return StatFs(externalCacheDir.absolutePath)
        }
    }
    return null
}

Usage:

        for (storageVolume in storageVolumes) {
            val statFs = getStatFsForStorageVolume(this@MainActivity, storageVolume)
                    ?: continue
            val availableSizeInBytes = statFs.availableBytes
            val totalBytes = statFs.totalBytes
            val formattedResult = "availableSizeInBytes:${android.text.format.Formatter.formatShortFileSize(this, availableSizeInBytes)} totalBytes:${android.text.format.Formatter.formatShortFileSize(this, totalBytes)}"
            Log.d("AppLog", "storageVolume \"${storageVolume.getDescription(this)}\" - $formattedResult")
        }

Note that this solution doesn't require any kind of permission.

EDIT: I actually found out that I tried to do it in the past, but for some reason it crashed for me on the SD-card StoraveVolume on the emulator:

        val storageStatsManager = getSystemService(Context.STORAGE_STATS_SERVICE) as StorageStatsManager
        for (storageVolume in storageVolumes) {
            val uuidStr = storageVolume.uuid
            val uuid = if (uuidStr == null) StorageManager.UUID_DEFAULT else UUID.fromString(uuidStr)
            val availableSizeInBytes = storageStatsManager.getFreeBytes(uuid)
            val totalBytes = storageStatsManager.getTotalBytes(uuid)
            val formattedResult = "availableSizeInBytes:${android.text.format.Formatter.formatShortFileSize(this, availableSizeInBytes)} totalBytes:${android.text.format.Formatter.formatShortFileSize(this, totalBytes)}"
            Log.d("AppLog", "storageVolume \"${storageVolume.getDescription(this)}\" - $formattedResult")
        }

The good news is that for the primary storageVolume, you get the real total space of it.

On a real device it also crashes for the SD-card, but not for the primary one.

So here's the latest solution for this, gathering the above:

        for (storageVolume in storageVolumes) {
            val availableSizeInBytes: Long
            val totalBytes: Long
            if (storageVolume.isPrimary) {
                val storageStatsManager = getSystemService(Context.STORAGE_STATS_SERVICE) as StorageStatsManager
                val uuidStr = storageVolume.uuid
                val uuid = if (uuidStr == null) StorageManager.UUID_DEFAULT else UUID.fromString(uuidStr)
                availableSizeInBytes = storageStatsManager.getFreeBytes(uuid)
                totalBytes = storageStatsManager.getTotalBytes(uuid)
            } else {
                val statFs = getStatFsForStorageVolume(this@MainActivity, storageVolume)
                        ?: continue
                availableSizeInBytes = statFs.availableBytes
                totalBytes = statFs.totalBytes
            }
            val formattedResult = "availableSizeInBytes:${android.text.format.Formatter.formatShortFileSize(this, availableSizeInBytes)} totalBytes:${android.text.format.Formatter.formatShortFileSize(this, totalBytes)}"
            Log.d("AppLog", "storageVolume \"${storageVolume.getDescription(this)}\" - $formattedResult")
        }

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叼着烟拽天下

4楼-- · 2020-02-09 07:56

Is getAllocatableBytes indeed the way to get the free space?

Android 8.0 Features and APIs states that getAllocatableBytes(UUID):

Finally, when you need to allocate disk space for large files, consider using the new allocateBytes(FileDescriptor, long) API, which will automatically clear cached files belonging to other apps (as needed) to meet your request. When deciding if the device has enough disk space to hold your new data, call getAllocatableBytes(UUID) instead of using getUsableSpace(), since the former will consider any cached data that the system is willing to clear on your behalf.

So, getAllocatableBytes() reports how many bytes could be free for a new file by clearing cache for other apps but may not be currently free. This does not seem to be the right call for a general-purpose file utility.

In any case, getAllocatableBytes(UUID) doesn't seem to work for any volume other than the primary volume due to the inability to get acceptable UUIDs from StorageManager for storage volumes other than the primary volume. See Invalid UUID of storage gained from Android StorageManager? and Bug report #62982912. (Mentioned here for completeness; I realize that you already know about these.) The bug report is now over two years old with no resolution or hint at a work-around, so no love there.

If you want the type of free space reported by "Files by Google" or other file managers, then you will want to approach free space in a different way as explained below.

How can I get the free and real total space (in some cases I got lower values for some reason) of each StorageVolume, without requesting any permission, just like on Google's app?

Here is a procedure to get free and total space for available volumes:

Identify external directories: Use getExternalFilesDirs(null) to discover available external locations. What is returned is a File[]. These are directories that our app is permitted to use.

extDirs = {File2@9489
0 = {File@9509} "/storage/emulated/0/Android/data/com.example.storagevolumes/files"
1 = {File@9510} "/storage/14E4-120B/Android/data/com.example.storagevolumes/files"

(N.B. According to the documentation, this call returns what are considered to be stable devices such as SD cards. This does not return attached USB drives.)

Identify storage volumes: For each directory returned above, use StorageManager#getStorageVolume(File) to identify the storage volume that contains the directory. We don't need to identify the top-level directory to get the storage volume, just a file from the storage volume, so these directories will do.

Calculate total and used space: Determine the space on the storage volumes. The primary volume is treated differently from an SD card.

For the primary volume: Using StorageStatsManager#getTotalBytes(UUID get the nominal total bytes of storage on the primary device using StorageManager#UUID_DEFAULT . The value returned treats a kilobyte as 1,000 bytes (rather than 1,024) and a gigabyte as 1,000,000,000 bytes instead of 2³⁰. On my SamSung Galaxy S7 the value reported is 32,000,000,000 bytes. On my Pixel 3 emulator running API 29 with 16 MB of storage, the value reported is 16,000,000,000.

Here is the trick: If you want the numbers reported by "Files by Google", use 10³ for a kilobyte, 10⁶ for a megabyte and 10⁹ for a gigabyte. For other file managers 2¹⁰, 2²⁰ and 2³⁰ is what works. (This is demonstrated below.) See this for more information on these units.

To get free bytes, use StorageStatsManager#getFreeBytes(uuid). Used bytes is the difference between total bytes and free bytes.

For non-primary volumes: Space calculations for non-primary volumes is straightforward: For total space used File#getTotalSpace and File#getFreeSpace for the free space.

Here are a couple of screens shots that display volume stats. The first image shows the output of the StorageVolumeStats app (included below the images) and "Files by Google." The toggle button at the top of the top section switches the app between using 1,000 and 1,024 for kilobytes. As you can see, the figures agree. (This is a screen shot from a device running Oreo. I was unable to get the beta version of "Files by Google" loaded onto an Android Q emulator.)

The following image shows the StorageVolumeStats app at the top and output from "EZ File Explorer" on the bottom. Here 1,024 is used for kilobytes and the two apps agree on the total and free space available except for rounding.

MainActivity.kt

This small app is just the main activity. The manifest is generic, compileSdkVersion and targetSdkVersion are set to 29. minSdkVersion is 26.

class MainActivity : AppCompatActivity() {
    private lateinit var mStorageManager: StorageManager
    private val mStorageVolumesByExtDir = mutableListOf<VolumeStats>()
    private lateinit var mVolumeStats: TextView
    private lateinit var mUnitsToggle: ToggleButton
    private var mKbToggleValue = true
    private var kbToUse = KB
    private var mbToUse = MB
    private var gbToUse = GB

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        if (savedInstanceState != null) {
            mKbToggleValue = savedInstanceState.getBoolean("KbToggleValue", true)
            selectKbValue()
        }
        setContentView(statsLayout())

        mStorageManager = getSystemService(Context.STORAGE_SERVICE) as StorageManager

        getVolumeStats()
        showVolumeStats()
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putBoolean("KbToggleValue", mKbToggleValue)
    }

    private fun getVolumeStats() {
        // We will get our volumes from the external files directory list. There will be one
        // entry per external volume.
        val extDirs = getExternalFilesDirs(null)

        mStorageVolumesByExtDir.clear()
        extDirs.forEach { file ->
            val storageVolume: StorageVolume? = mStorageManager.getStorageVolume(file)
            if (storageVolume == null) {
                Log.d(TAG, "Could not determinate StorageVolume for ${file.path}")
            } else {
                val totalSpace: Long
                val usedSpace: Long
                if (storageVolume.isPrimary) {
                    // Special processing for primary volume. "Total" should equal size advertised
                    // on retail packaging and we get that from StorageStatsManager. Total space
                    // from File will be lower than we want to show.
                    val uuid = StorageManager.UUID_DEFAULT
                    val storageStatsManager =
                        getSystemService(Context.STORAGE_STATS_SERVICE) as StorageStatsManager
                    // Total space is reported in round numbers. For example, storage on a
                    // SamSung Galaxy S7 with 32GB is reported here as 32_000_000_000. If
                    // true GB is needed, then this number needs to be adjusted. The constant
                    // "KB" also need to be changed to reflect KiB (1024).
//                    totalSpace = storageStatsManager.getTotalBytes(uuid)
                    totalSpace = (storageStatsManager.getTotalBytes(uuid) / 1_000_000_000) * gbToUse
                    usedSpace = totalSpace - storageStatsManager.getFreeBytes(uuid)
                } else {
                    // StorageStatsManager doesn't work for volumes other than the primary volume
                    // since the "UUID" available for non-primary volumes is not acceptable to
                    // StorageStatsManager. We must revert to File for non-primary volumes. These
                    // figures are the same as returned by statvfs().
                    totalSpace = file.totalSpace
                    usedSpace = totalSpace - file.freeSpace
                }
                mStorageVolumesByExtDir.add(
                    VolumeStats(storageVolume, totalSpace, usedSpace)
                )
            }
        }
    }

    private fun showVolumeStats() {
        val sb = StringBuilder()
        mStorageVolumesByExtDir.forEach { volumeStats ->
            val (usedToShift, usedSizeUnits) = getShiftUnits(volumeStats.mUsedSpace)
            val usedSpace = (100f * volumeStats.mUsedSpace / usedToShift).roundToLong() / 100f
            val (totalToShift, totalSizeUnits) = getShiftUnits(volumeStats.mTotalSpace)
            val totalSpace = (100f * volumeStats.mTotalSpace / totalToShift).roundToLong() / 100f
            val uuidToDisplay: String?
            val volumeDescription =
                if (volumeStats.mStorageVolume.isPrimary) {
                    uuidToDisplay = ""
                    PRIMARY_STORAGE_LABEL
                } else {
                    uuidToDisplay = " (${volumeStats.mStorageVolume.uuid})"
                    volumeStats.mStorageVolume.getDescription(this)
                }
            sb
                .appendln("$volumeDescription$uuidToDisplay")
                .appendln(" Used space: ${usedSpace.nice()} $usedSizeUnits")
                .appendln("Total space: ${totalSpace.nice()} $totalSizeUnits")
                .appendln("----------------")
        }
        mVolumeStats.text = sb.toString()
    }

    private fun getShiftUnits(x: Long): Pair<Long, String> {
        val usedSpaceUnits: String
        val shift =
            when {
                x < kbToUse -> {
                    usedSpaceUnits = "Bytes"; 1L
                }
                x < mbToUse -> {
                    usedSpaceUnits = "KB"; kbToUse
                }
                x < gbToUse -> {
                    usedSpaceUnits = "MB"; mbToUse
                }
                else -> {
                    usedSpaceUnits = "GB"; gbToUse
                }
            }
        return Pair(shift, usedSpaceUnits)
    }

    @SuppressLint("SetTextI18n")
    private fun statsLayout(): SwipeRefreshLayout {
        val swipeToRefresh = SwipeRefreshLayout(this)
        swipeToRefresh.setOnRefreshListener {
            getVolumeStats()
            showVolumeStats()
            swipeToRefresh.isRefreshing = false
        }

        val scrollView = ScrollView(this)
        swipeToRefresh.addView(scrollView)
        val linearLayout = LinearLayout(this)
        linearLayout.orientation = LinearLayout.VERTICAL
        scrollView.addView(
            linearLayout, ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.WRAP_CONTENT
        )

        val instructions = TextView(this)
        instructions.text = "Swipe down to refresh."
        linearLayout.addView(
            instructions, ViewGroup.LayoutParams.WRAP_CONTENT,
            ViewGroup.LayoutParams.WRAP_CONTENT
        )
        (instructions.layoutParams as LinearLayout.LayoutParams).gravity = Gravity.CENTER

        mUnitsToggle = ToggleButton(this)
        mUnitsToggle.textOn = "KB = 1,000"
        mUnitsToggle.textOff = "KB = 1,024"
        mUnitsToggle.isChecked = mKbToggleValue
        linearLayout.addView(
            mUnitsToggle, ViewGroup.LayoutParams.WRAP_CONTENT,
            ViewGroup.LayoutParams.WRAP_CONTENT
        )
        mUnitsToggle.setOnClickListener { v ->
            val toggleButton = v as ToggleButton
            mKbToggleValue = toggleButton.isChecked
            selectKbValue()
            getVolumeStats()
            showVolumeStats()
        }

        mVolumeStats = TextView(this)
        mVolumeStats.typeface = Typeface.MONOSPACE
        val padding =
            16 * (resources.displayMetrics.densityDpi.toFloat() / DisplayMetrics.DENSITY_DEFAULT).toInt()
        mVolumeStats.setPadding(padding, padding, padding, padding)

        val lp = LinearLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, 0)
        lp.weight = 1f
        linearLayout.addView(mVolumeStats, lp)

        return swipeToRefresh
    }

    private fun selectKbValue() {
        if (mKbToggleValue) {
            kbToUse = KB
            mbToUse = MB
            gbToUse = GB
        } else {
            kbToUse = KiB
            mbToUse = MiB
            gbToUse = GiB
        }
    }

    companion object {
        fun Float.nice(fieldLength: Int = 6): String =
            String.format(Locale.US, "%$fieldLength.2f", this)

        // StorageVolume should have an accessible "getPath()" method that will do
        // the following so we don't have to resort to reflection.
        @Suppress("unused")
        fun StorageVolume.getStorageVolumePath(): String {
            return try {
                javaClass
                    .getMethod("getPath")
                    .invoke(this) as String
            } catch (e: Exception) {
                e.printStackTrace()
                ""
            }
        }

        // See https://en.wikipedia.org/wiki/Kibibyte for description
        // of these units.

        // These values seems to work for "Files by Google"...
        const val KB = 1_000L
        const val MB = KB * KB
        const val GB = KB * KB * KB

        // ... and these values seems to work for other file manager apps.
        const val KiB = 1_024L
        const val MiB = KiB * KiB
        const val GiB = KiB * KiB * KiB

        const val PRIMARY_STORAGE_LABEL = "Internal Storage"

        const val TAG = "MainActivity"
    }

    data class VolumeStats(
        val mStorageVolume: StorageVolume,
        var mTotalSpace: Long = 0,
        var mUsedSpace: Long = 0
    )
}

Addendum

Let's get more comfortable with using getExternalFilesDirs():

We call Context#getExternalFilesDirs() in the code. Within this method a call is made to Environment#buildExternalStorageAppFilesDirs() which calls Environment#getExternalDirs() to obtain the volume list from StorageManager. This storage list is used to create the paths we see returned from Context#getExternalFilesDirs() by appending some static path segments to the path identified by each storage volume.

We would really want access to Environment#getExternalDirs() so we can immediately determine space utilization, but we are restricted. Since the call we make depends upon a file list that is generated from the volume list, we can be comfortable that all volumes are covered by out code and we can get the space utilization information we need.

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