Android Scoped Storage: 如何读取外部存储特定文件夹的文件

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.net.Uri;
import android.os.storage.StorageManager;
import android.util.Log;

public class Check {
    Context context;
    private String TAG="SOMNATH";

    public Check(Context context) {
        this.context = context;
    }

    public boolean  ch(){
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.Q) {
            StorageManager sm = (StorageManager) context.getSystemService(Context.STORAGE_SERVICE);

            Intent intent = sm.getPrimaryStorageVolume().createOpenDocumentTreeIntent();
            //String startDir = "Android";
            //String startDir = "Download"; // Not choosable on an Android 11 device
            //String startDir = "DCIM";
            //String startDir = "DCIM/Camera";  // replace "/", "%2F"
            //String startDir = "DCIM%2FCamera";
            String startDir = "Documents";

            Uri uri = intent.getParcelableExtra("android.provider.extra.INITIAL_URI");

            String scheme = uri.toString();

            Log.d(TAG, "INITIAL_URI scheme: " + scheme);

            scheme = scheme.replace("/root/", "/document/");

            scheme += "%3A" + startDir;

            uri = Uri.parse(scheme);

            intent.putExtra("android.provider.extra.INITIAL_URI", uri);

            Log.d(TAG, "uri: " + uri.toString());

            ((Activity) context).startActivityForResult(intent, 12123);

            return true;
        }
        else{
            return false;
        }
    }



}

package com.axanor.saf_sample

import android.app.Activity
import android.content.Intent
import android.net.Uri
import android.provider.DocumentsContract
import android.util.Log
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import androidx.documentfile.provider.DocumentFile
import java.io.FileNotFoundException
import java.io.FileOutputStream
import java.io.IOException

class StorageAccess {
    val activity:Activity;
    val LOGTAG = "SOMNATH"
    val REQUEST_CODE = 12123
    constructor(activity: Activity) {
        this.activity = activity
    }




    public fun openDocumentTree() {
        val check = Check(activity)
        val uriString = SpUtil.getString(SpUtil.FOLDER_URI, "")
        when {
            uriString == "" -> {
                Log.w(LOGTAG, "uri not stored")


                if (!check.ch()){
                    askPermission()
                }

            }
            arePermissionsGranted(uriString) -> {
                makeDoc(Uri.parse(uriString))
            }
            else -> {
                Log.w(LOGTAG, "uri permission not stored")
                if (!check.ch()){
                    askPermission()
                }
            }
        }
    }

    // this will present the user with folder browser to select a folder for our data
    public fun askPermission() {
        val intent = Intent(Intent.ACTION_OPEN_DOCUMENT_TREE)
        activity.startActivityForResult(intent, REQUEST_CODE)
    }



    public fun makeDoc(dirUri: Uri) {
        val dir = DocumentFile.fromTreeUri(activity, dirUri)
        if (dir == null || !dir.exists()) {
            //the folder was probably deleted
            Log.e(LOGTAG, "no Dir")
            //according to Commonsware blog, the number of persisted uri permissions is limited
            //so we should release those we cannot use anymore
            //https://commonsware.com/blog/2020/06/13/count-your-saf-uri-permission-grants.html
            releasePermissions(dirUri)
            //ask user to choose another folder
            Toast.makeText(activity,"Folder deleted, please choose another!", Toast.LENGTH_SHORT).show()
            openDocumentTree()
        } else {
            val file = dir.createFile("*/txt", "test.txt")
            if (file != null && file.canWrite()) {
                Log.d(LOGTAG, "file.uri = ${file.uri.toString()}")
                alterDocument(file.uri)
            } else {
                Log.d(LOGTAG, "no file or cannot write")
                //consider showing some more appropriate error message
                Toast.makeText(activity,"Write error!", Toast.LENGTH_SHORT).show()

            }
        }
    }


    public fun releasePermissions(uri: Uri) {
        val flags: Int = Intent.FLAG_GRANT_READ_URI_PERMISSION or
                Intent.FLAG_GRANT_WRITE_URI_PERMISSION
        activity.contentResolver.releasePersistableUriPermission(uri,flags)
        //we should remove this uri from our shared prefs, so we can start over again next time
        SpUtil.storeString(SpUtil.FOLDER_URI, "")
    }


    //Just a test function to write something into a file, from https://developer.android.com
    //Please note, that all file IO MUST be done on a background thread. It is not so in this
    //sample - for the sake of brevity.
    public fun alterDocument(uri: Uri) {
        try {

            activity.contentResolver.openFileDescriptor(uri, "w")?.use { parcelFileDescriptor ->
                FileOutputStream(parcelFileDescriptor.fileDescriptor).use {
                    it.write(
                        ("String written at ${System.currentTimeMillis()}\n")
                            .toByteArray()
                    )
                    Toast.makeText(activity,"File Write OK!", Toast.LENGTH_SHORT).show()


                    val intent = Intent(Intent.ACTION_OPEN_DOCUMENT)
                    intent.addCategory(Intent.CATEGORY_OPENABLE)
                    intent.type = "application/pdf"

                    // Optionally, specify a URI for the file that should appear in the
                    // system file picker when it loads.

                    // Optionally, specify a URI for the file that should appear in the
                    // system file picker when it loads.
                    intent.putExtra(DocumentsContract.EXTRA_INITIAL_URI, uri)

                    activity.startActivityForResult(intent, 2)



                }
            }
        } catch (e: FileNotFoundException) {
            e.printStackTrace()
        } catch (e: IOException) {
            e.printStackTrace()
        }
    }


    public fun arePermissionsGranted(uriString: String): Boolean {
        // list of all persisted permissions for our app
        val list = activity.contentResolver.persistedUriPermissions
        for (i in list.indices) {
            val persistedUriString = list[i].uri.toString()
            //Log.d(LOGTAG, "comparing $persistedUriString and $uriString")
            if (persistedUriString == uriString && list[i].isWritePermission && list[i].isReadPermission) {
                //Log.d(LOGTAG, "permission ok")
                return true
            }
        }
        return false
    }







}

StorageAccess access = new StorageAccess(this);
access.openDocumentTree();

    public fun makeDoc(dirUri: Uri) {
        val dir = DocumentFile.fromTreeUri(activity, dirUri)
        if (dir == null || !dir.exists()) {
            //the folder was probably deleted
            Log.e(LOGTAG, "no Dir")
            //according to Commonsware blog, the number of persisted uri permissions is limited
            //so we should release those we cannot use anymore
            //https://commonsware.com/blog/2020/06/13/count-your-saf-uri-permission-grants.html
            releasePermissions(dirUri)
            //ask user to choose another folder
            Toast.makeText(activity,"Folder deleted, please choose another!", Toast.LENGTH_SHORT).show()
            openDocumentTree()
        } else {
            val file = dir.createFile("*/txt", "test.txt")
            if (file != null && file.canWrite()) {
                Log.d(LOGTAG, "file.uri = ${file.uri.toString()}")
                alterDocument(file.uri)
            } else {
                Log.d(LOGTAG, "no file or cannot write")
                //consider showing some more appropriate error message
                Toast.makeText(activity,"Write error!", Toast.LENGTH_SHORT).show()

            }
        }
    }

    public fun arePermissionsGranted(uriString: String): Boolean {
        // list of all persisted permissions for our app
        val list = activity.contentResolver.persistedUriPermissions
        for (i in list.indices) {
            val persistedUriString = list[i].uri.toString()
            //Log.d(LOGTAG, "comparing $persistedUriString and $uriString")
            if (persistedUriString == uriString && list[i].isWritePermission && list[i].isReadPermission) {
                //Log.d(LOGTAG, "permission ok")
                return true
            }
        }
        return false
    }