DarkRoom/apps/darkroom-android/android/java/paris/tourolle/darkroom/Cards.java

package paris.tourolle.darkroom;

import android.Manifest;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Build;
import android.os.Environment;
import android.os.storage.StorageManager;
import android.os.storage.StorageVolume;
import android.provider.Settings;
import android.util.Log;

import java.io.File;
import java.util.ArrayList;
import java.util.List;

/**
 * Finding a camera card, and the permission that makes it readable (FR-CAT-10).
 *
 * <p>An import reads the card as files: the survey walks it, the probe reads
 * each header and the copy streams each original, all through the same
 * {@code std::fs} code the desktop uses. Android hands out such paths —
 * {@code /storage/9C33-6BBD/DCIM} — to an app holding "all files access"
 * ({@code MANAGE_EXTERNAL_STORAGE}, API 30), which covers the root of an SD
 * card and of a USB card reader. Below API 30 the same paths are readable
 * with {@code READ_EXTERNAL_STORAGE}.
 *
 * <p>Not the folder picker {@link FolderPicker} uses for albums. A tree
 * granted through SAF is {@code content://} URIs, not paths, and since API 30
 * the picker refuses the root of a card outright; reading a card through it
 * would mean a second storage implementation under the importer, where this
 * needs none.
 *
 * <p>Google Play restricts this permission to file managers and the like.
 * DarkRoom is not distributed through Play, so the restriction does not
 * apply; it would need revisiting if that changed.
 */
public final class Cards {
    private static final String TAG = "DarkRoom";

    private Cards() {
    }

    /** Whether this app may read a card's files by path. */
    public static boolean hasAccess(Context context) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
            return Environment.isExternalStorageManager();
        }
        return context.checkSelfPermission(Manifest.permission.READ_EXTERNAL_STORAGE)
                == PackageManager.PERMISSION_GRANTED;
    }

    /**
     * Open the system page where the user grants it.
     *
     * <p>A settings page rather than a permission dialog because there is no
     * dialog for this one on API 30 and up: the user flips "Allow access to
     * manage all files" for this app. Below 30 the context is the application
     * context, which cannot raise a runtime permission request (that needs an
     * Activity's result), so the app's own settings page is the route there
     * too. Either way the app learns of the grant by asking
     * {@link #hasAccess} again.
     */
    public static void requestAccess(Context context) {
        Uri self = Uri.parse("package:" + context.getPackageName());
        Intent intent;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
            intent = new Intent(Settings.ACTION_MANAGE_APP_ALL_FILES_ACCESS_PERMISSION, self);
        } else {
            intent = new Intent(Settings.ACTION_APPLICATION_DETAILS_SETTINGS, self);
        }
        // The context is not an Activity; see FolderPicker.start.
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        try {
            context.startActivity(intent);
        } catch (RuntimeException e) {
            // Some builds ship without the per-app page; the list of every
            // app holding the permission is the fallback that always exists.
            Log.w(TAG, "no per-app all-files page; opening the list", e);
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
                Intent list = new Intent(Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION);
                list.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                context.startActivity(list);
            }
        }
    }

    /**
     * Every mounted volume other than the device's own storage.
     *
     * <p>One string per volume, {@code path \t description \t removable},
     * where removable is {@code 1} or {@code 0}: the reason {@link Intents}
     * gives for keeping the JNI surface to strings. The primary volume is left
     * out — it is the device's internal storage, never a card — and so is
     * anything not mounted, which is a card being ejected or one the system
     * could not read.
     */
    public static String[] volumes(Context context) {
        List<String> out = new ArrayList<String>();
        StorageManager manager = (StorageManager) context.getSystemService(Context.STORAGE_SERVICE);
        if (manager == null) {
            return new String[0];
        }
        for (StorageVolume volume : manager.getStorageVolumes()) {
            if (volume.isPrimary()) {
                continue;
            }
            String state = volume.getState();
            if (!Environment.MEDIA_MOUNTED.equals(state)
                    && !Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
                continue;
            }
            String path = path(volume);
            if (path == null) {
                Log.w(TAG, "a mounted volume with no path: " + volume);
                continue;
            }
            String description = volume.getDescription(context);
            if (description == null) {
                description = new File(path).getName();
            }
            out.add(path + "\t" + description.replace('\t', ' ') + "\t"
                    + (volume.isRemovable() ? "1" : "0"));
        }
        return out.toArray(new String[0]);
    }

    /**
     * Where the volume is mounted.
     *
     * <p>{@code getDirectory} is API 30. Below it the same answer is the
     * hidden {@code getPath}, which every release from 24 to 29 has, reached by
     * reflection because android.jar does not declare it.
     */
    private static String path(StorageVolume volume) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
            File dir = volume.getDirectory();
            return dir == null ? null : dir.getPath();
        }
        try {
            Object path = StorageVolume.class.getMethod("getPath").invoke(volume);
            return path == null ? null : path.toString();
        } catch (ReflectiveOperationException e) {
            Log.w(TAG, "StorageVolume.getPath", e);
            return null;
        }
    }
}