How to implement cross-platform folder picker in tauri for Desktop and Android?

[pan-grayza]

I want user to pick folder so I can get path of this folder. For desktop Im using rfd, for andoid I made my own implementation.
There is no need to concentrate on logic for now, but code looks like this:

#[tauri::command]
pub fn select_directory() -> Result<PathBuf, Error> {
    if cfg!(target_os = "android") {
        select_directory_android()
    } else {
        select_directory_desktop()
    }
}

#[cfg(target_os = "android")]
fn select_directory_android() -> Result<PathBuf, Error> {
    use jni::objects::{JClass, JString};
    use jni::JNIEnv;
    use ndk_glue::native_activity;
    use std::ffi::CString;
    use std::sync::Once;

    static INIT: Once = Once::new();
    static mut SELECTED_DIRECTORY: Option<PathBuf> = None;

    #[no_mangle]
    pub extern "system" fn Java_com_tauri_dev_DirectoryPickerActivity_sendDirectoryPathToRust(
        env: JNIEnv,
        _class: JClass,
        directory_path: JString,
    ) {
        let path: String = env
            .get_string(directory_path)
            .expect("Couldn't get directory path")
            .into();

        unsafe {
            SELECTED_DIRECTORY = Some(PathBuf::from(path));
        }
    }

    // Invoke Android activity to pick a directory
    let activity = native_activity();
    let vm = activity.vm();
    let _env = vm.attach_current_thread().unwrap();

    let class_name = CString::new("com/example/myapp/DirectoryPickerActivity").unwrap();
    let activity_class = _env.find_class(class_name.as_c_str()).unwrap();
    let intent_method_id = _env
        .get_method_id(activity_class, "startActivity", "()V")
        .unwrap();

    _env.call_method(activity_class, intent_method_id, "()V", &[])
        .unwrap();

    // Return the selected directory path or an error if not selected
    unsafe {
        match &SELECTED_DIRECTORY {
            Some(path) => Ok(path.clone()),
            None => Err(Error::from("No directory selected")),
        }
    }
}

#[cfg(not(target_os = "android"))]
fn select_directory_desktop() -> Result<PathBuf, Error> {
    use rfd::FileDialog;

    let selected_dir: Option<PathBuf> = FileDialog::new().set_directory(".").pick_folder();

    match selected_dir {
        Some(path) => Ok(path),
        None => Err(Error::from("No directory selected")),
    }
}

Desktop Function works fine, I tested. But after I added ndk crate Im getting this error:

error: android-ndk-sys only supports compiling for Android  
  --> C:\Users\pasna\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ndk-sys-0.4.1+23.1.7779620\src\lib.rs:22:1
   |
22 | compile_error!("android-ndk-sys only supports compiling for Android");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: ndk-sys only supports compiling for Android
  --> C:\Users\pasna\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ndk-sys-0.6.0+11769913\src\lib.rs:22:1
   |
22 | compile_error!("ndk-sys only supports compiling for Android");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

And if I use pnpm tauri android dev, Im getting errors from rfd for some reason. Errors look like this:

error[E0277]: the trait bound `MessageDialog: MessageDialogImpl` is not satisfied
  --> C:\Users\pasna\.cargo\registry\src\index.crates.io-6f17d22bba15001f\rfd-0.14.1\src\message_dialog.rs:76:33
   |
76 |         MessageDialogImpl::show(self)
   |         ----------------------- ^^^^ the trait `MessageDialogImpl` is not implemented for `MessageDialog`
   |         |
   |         required by a bound introduced by this call
   |
help: this trait has no implementations, consider adding one
  --> C:\Users\pasna\.cargo\registry\src\index.crates.io-6f17d22bba15001f\rfd-0.14.1\src\backend.rs:69:1
   |
69 | pub trait MessageDialogImpl {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rfd` (lib) due to 12 previous errors
warning: build failed, waiting for other jobs to finish...
 ELIFECYCLE  Command failed with exit code 4294967295.
    Error `Failed to run `cargo build`: command ["cargo", "build", "--package", "file-linking", "--manifest-path", "C:\\Users\\pasna\\Stepan\\Code\\mobile_file_share_svelte\\src-tauri\\Cargo.toml", "--target", "x86_64-linux-android", "--lib"] exited with code 101
 ELIFECYCLE  Command failed with exit code 1.

There was overall 12 errors about rfd code. This errors don't exist if Im using pnpm tauri dev.
Before that I thought that writing my own implementation for folder picker on andoroid will solve the problem as rfd doesn't support mobile platforms. But now I have problems with desktop and android code conflicting.

[FabianLars]

Sounds like you didn't add the dependencies in a platform dependent way, could that be? It would look something like this

[target."cfg(not(any(target_os = \"android\", target_os = \"ios\")))".dependencies]
rfd = "123"

[target."cfg(any(target_os = \"android\", target_os = \"ios\"))".dependencies]
ndk = "123"

Also did you know about our dialog plugin? It should work on desktop and mobile now.

[pan-grayza]

Thank you for recommendation, dialog plugin works perfectly on desktop. But when I compile it for android Im getting this error:

error[E0599]: no method named `pick_folder` found for struct `FileDialogBuilder` in the current scope
  --> src\local_dir_man.rs:35:25
   |
35 |     app.dialog().file().pick_folder(move |folder_path| {
   |     --------------------^^^^^^^^^^^
   |
help: there is a method `pick_file` with a similar name
   |
35 |     app.dialog().file().pick_file(move |folder_path| {
   |                         ~~~~~~~~~

I need to pick folder also on mobile. Is it possible?

[FabianLars]

Ah no, that's not implemented yet: tauri-apps/plugins-workspace#933 - I can't give you any ETA on that.

[pan-grayza]

What ETA means? Is somebody already working on this? If not, can I work on this? I need this feature.

[FabianLars]

ETA = Estimated Time of Arrival == An estimation on when this will be implemented.

No, nobody is working on it right now. Contributions are always much appreciated! (I'm not too familiar with this part of the plugin so i can't really help get you started though)

[pan-grayza]

I understand. Thank you