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MediaStore, Storage Access Framework, and FileProvider

These APIs solve different sharing problems:

  • MediaStore: publish/query user media in shared collections.
  • Storage Access Framework (SAF): let the user choose a document/provider destination.
  • FileProvider: temporarily share an app-owned file with another app.

Write user-visible media atomically​

val values = ContentValues().apply {
    put(MediaStore.Images.Media.DISPLAY_NAME, "receipt-$receiptId.jpg")
    put(MediaStore.Images.Media.MIME_TYPE, "image/jpeg")
    put(MediaStore.Images.Media.RELATIVE_PATH, "Pictures/ExpenseBook")
    put(MediaStore.Images.Media.IS_PENDING, 1)
}

val uri = resolver.insert(
    MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
    values,
) ?: error("Cannot create media row")

try {
    resolver.openOutputStream(uri, "w")!!.use { output ->
        source.inputStream().use { input -> input.copyTo(output) }
    }
    values.clear()
    values.put(MediaStore.Images.Media.IS_PENDING, 0)
    resolver.update(uri, values, null, null)
} catch (t: Throwable) {
    resolver.delete(uri, null, null)
    throw t
}

IS_PENDING prevents other apps from observing a half-written media item on supported versions.

User-selected documents with SAF​

val openDocument = registerForActivityResult(
    ActivityResultContracts.OpenDocument()
) { uri ->
    if (uri != null) importDocument(uri)
}

openDocument.launch(arrayOf("application/pdf"))

If access must survive process restart, request persistable URI permission when the contract/provider permits it. Persist the URI string, not an invented path. Access can still disappear if the provider removes the document or grant.

Share an app-private file​

<provider
    android:name="androidx.core.content.FileProvider"
    android:authorities="${applicationId}.files"
    android:exported="false"
    android:grantUriPermissions="true">
    <meta-data
        android:name="android.support.FILE_PROVIDER_PATHS"
        android:resource="@xml/file_paths" />
</provider>
val uri = FileProvider.getUriForFile(
    context,
    "${context.packageName}.files",
    reportFile,
)
val share = Intent(Intent.ACTION_SEND).apply {
    type = "application/pdf"
    putExtra(Intent.EXTRA_STREAM, uri)
    addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
}
context.startActivity(Intent.createChooser(share, "Share report"))

Configure only the narrow directories that should ever be shareable. Never expose the filesystem root or broad sensitive directory.

A URI grant is capability access

Validate incoming URI scheme and MIME/content; grant only read or write access required; avoid long-lived grants unless the product needs them; never assume a display name is a safe filename.

Corner cases​

  • Provider reports no size or misleading MIME type.
  • Input is non-seekable and cannot be reopened.
  • User revokes a persisted grant.
  • Target app ignores URI permissions or lacks a handler.
  • Insert succeeds but writing fails due to storage pressure.
  • Malicious document is a decompression bomb or oversized despite extension.

Sources​

Exercise​

Implement “Export monthly report” in two modes: save to a user-chosen location and share a temporary PDF. Define ownership, grants, cancellation, cleanup, and error recovery.