File saver

Let the user choose where to save a file, then write your data to it

Open the native save dialog. FileKit returns the location the user picked as a PlatformFile, and you write your data to it. The file saver is not available on the web, so call it from a non-web source set such as nonWebMain (see shared source sets).

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Usage

Compose

val scope = rememberCoroutineScope()
val launcher = rememberFileSaverLauncher(
    dialogSettings = FileKitDialogSettings.createDefault(),
    onError = { failure -> showError(failure.message) },
) { file ->
    // onError covers the dialog only: handle write errors (IOException) here
    scope.launch { file?.writeString("Hello FileKit!") }
}

Button(onClick = { launcher.launch(suggestedName = "notes", defaultExtension = "txt") }) {
    Text("Save")
}

The result is null when the user cancels. Failures go to onError in Compose, or are thrown as a FileKitDialogException by the suspend API. See Results and errors.

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Options

suggestedName · String · required · launch()

File name without the extension.

defaultExtension · String? · default: "null" · launch()

Extension without the dot, appended to suggestedName.

allowedExtensions · Set<String>? · default: "null" · launch()

Save types offered by the dialog, without dots. A hint, not a validation.

directory · PlatformFile? · default: "null" · launch()

Initial folder. Ignored on Android.

dialogSettings · FileKitDialogSettings · default: "createDefault()" · rememberFileSaverLauncher

Required by the Compose launcher. In a Compose Desktop Window, WindowScope.rememberFileSaverLauncher makes it optional and parents the dialog to the window. See Dialog settings.

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Offer several file types

Pass allowedExtensions to let the user pick a format. defaultExtension stays the preselected one:

val file = FileKit.openFileSaver(
    suggestedName = "report",
    defaultExtension = "pdf",
    allowedExtensions = setOf("pdf", "txt"),
)

Not every platform enforces the filter, so check file.extension if your app needs a specific format.

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Download file on the web

Browsers don’t let you choose a save location: use FileKit.download to send bytes to the user’s downloads folder instead. See Download files.

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Platform notes

Platform	Behavior
Android	Opens the system “Create document” screen, which creates an empty file at the chosen location. With several allowedExtensions, the MIME type falls back to */*.
iOS	Exports a temporary placeholder file through the Files picker, then removes it so you can write to the returned location. allowedExtensions is ignored.
Desktop	The save dialog shows dialogSettings.title on Linux native, Windows native, and JVM on macOS. The macOS native saver and the JVM savers on Windows and Linux ignore it.
Linux native	Needs a running XDG Desktop Portal. See Dialogs setup.
Web	Not available. Use FileKit.download.

Upgrading from older versions

The extension parameter of openFileSaver and launch() is deprecated. Rename it to defaultExtension; the behavior is the same.

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Related

• Writing files for streaming large content.
• Dialog settings for titles and parent windows.