针对 Android 10的 WRITE_EXTERNAL_STORAGE

Try to add this in your Manifest:

<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:requestLegacyExternalStorage="true" //Add this Line
android:label="@string/app_name">


---------<Activity, Sevices or Recivers>----------


</application>

and remove the READ_EXTERNAL_STORAGE permission again:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

A workaround is to actually ignore the warning, as it is just informational and therefore harmless. By setting maxSdkVersion to 28 no need to worry anymore.

<uses-permission
android:name="android.permission.WRITE_EXTERNAL_STORAGE"
android:maxSdkVersion="28"
tools:ignore="ScopedStorage" />

Note that using the android:requestLegacyExternalStorage flag as stated in other answers is not a solution, is just a temporary patch which will no longer work at all in Android 11 (API 30), and future versions

UPDATE, to clarify the doubts and confusions shown by some developers in the comments:

• If using the requestLegacyExternalStorage flag in Android 10 (API 29) then request the WRITE_EXTERNAL_STORAGE permission as usual.

• The flag requestLegacyExternalStorage does nothing in Android 11 (API 30), it is completely ignored, and there is not workaround for it.

• WRITE_EXTERNAL_STORAGE does not give any privileges in Android 11 (API 30), it does nothing at all, therefore in API 11 you need to set the maxSdkVersion to 29.

• If in Android 10 (API 29) you are also not using requestLegacyExternalStorage then set maxSdkVersion to 28 instead of 29.

• Starting in Android 11 (API 30), the older File API can again be used but "only" when accessing the public "shared storage" folders (DCIM, Music, etc.), or your app "private" directory. For other locations the DocumentFile API is required.

• Consider that the File API is now much slower in Android 11 (API 30), because has been refactored becoming essentially a wrapper. This is to enforce its usage just to the allowed locations. So, is no longer a fast system file API, is just a wrapper that internally delegates the work to the MediaStore. When using the File API in Android 11 or above you should consider the performance penalty hit, as according to the Android team it will be 2 to 3 times slower than if accessing directly the MediaStore.

As answered @PerracoLabs the warning is just informational. It will be better if at first you'll read his answer. https://stackoverflow.com/a/65477206/6055194

I use WRITE_EXTERNAL_STORAGE permission only for downloading of .pdf-files into Downloads and app "private" directories by using of DownloadManager (that's important and you can understand why if you will read discussion under @PerracoLabs answer). So I've just added max level of SDK for this permission and ignore tag for scoped storage.

<uses-permission
android:name="android.permission.WRITE_EXTERNAL_STORAGE"
android:maxSdkVersion="28"
tools:ignore="ScopedStorage" />

P.S. if tools tag not recognized, than you must declare it by adding xmlns:tools="http://schemas.android.com/tools" to your AndroidManifest.xml like this.

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
package="com.app.my">

But you must note that after it onRequestPermissionsResult function will not be called for Build.VERSION_CODES.Q and higher (API 29+). So you must call request permission WRITE_EXTERNAL_STORAGE only for API lower than 29.

So you can add check write external storage function like this (Kotlin).

   private fun hasWriteStoragePermission(): Boolean {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
return true
} else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
if (ActivityCompat.checkSelfPermission(activity!!, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
requestPermissions(
arrayOf(Manifest.permission.WRITE_EXTERNAL_STORAGE),
REQUEST_PERMISSIONS_CODE_WRITE_STORAGE
)


return false
}
}


return true
}

Add the line:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" tools:remove="android:maxSdkVersion"/>

and add the below code to to AndroidManifest.xml:

 android:requestLegacyExternalStorage="true"

Using File-apis ?

Based on @PerracoLabs reply the following changes will make your app work on the devices running

1. Android-9 and less
2. Android-10
3. Android-11 and above

File: "AndroidManifest.xml"

<!-- Without this folders will be inaccessible in Android-11 and above devices -->
<uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE" />


<!-- Without this entry storage-permission entry will not be visible under app-info permissions list Android-10 and below -->
<uses-permission
android:name="android.permission.WRITE_EXTERNAL_STORAGE"
android:maxSdkVersion="29"
tools:ignore="ScopedStorage"/>


<!-- Without this entry the folders will remain in-accessible in Android-10, even if WRITE_EXTERNAL_STORAGE as above is present. -->
<application
android:requestLegacyExternalStorage="true"/>

Java Source:

1. Don't forget to give Runtime-permissions, before doing file operation. - Needed for Android-10 and below. DONT request for runtime permissions for Android-11, they are to be requested for Android-10 and below.

2. Navigate the user to app-permission page to allow him to enable file-permissions using the following code (needed for Android-11 and above support):

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R && false == Environment.isExternalStorageManager()) {
   Uri uri = Uri.parse("package:" + BuildConfig.APPLICATION_ID);
   startActivity(new Intent(Settings.ACTION_MANAGE_APP_ALL_FILES_ACCESS_PERMISSION, uri));
   }