API 26+: WRITE_EXTERNAL_STORAGE permission is always denied

问题

I have switched my app to target API 27 and now it can't be granted WRITE_EXTERNAL_STORAGE permission -- grantResult is always -1.

My app needs this permission since it doesn't use apps private external storage space (which doesn't require WRITE_EXTERNAL_STORAGE starting from API 19).

I know that in API 26 there have been behavior changes for permissions. However this doesn't explain my problem.

I'm requesting both READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE permissions in a standard way:

ActivityCompat.requestPermissions(activity, new String[] {
        Manifest.permission.READ_EXTERNAL_STORAGE,
        Manifest.permission.WRITE_EXTERNAL_STORAGE
    }, requestCode);

(both permissions are declared via <uses-permission in manifest).

The dialog appears and I click "Allow":

However inside onRequestPermissionsResult callback I'm getting a -1(denied) for WRITE_EXTERNAL_STORAGE (and 0 (granted) for READ_EXTERNAL_STORAGE).

Shouldn't the result be 0 for both since I have requested and, presumably, granted both?

I have tried to request WRITE_EXTERNAL_STORAGE alone, but in this case the dialog doesn't appear at all.

One more detail: I have just checked the merged manifest in build/intermediates/manifests/full/debug and noticed that WRITE_EXTERNAL_STORAGE permission has attribute android:maxSdkVersion="18" (there is no such attribute in my manifest). This could be happening because my app has minApiVersion=21, but I'm not sure.

回答1:

Somewhere along the line, you are picking up that android:maxSdkVersion="18" attribute. My guess is that it is coming from a library. Check the "Merged Manifest" tab in Android Studio, when you are editing your own manifest. It will have details of what is contributing the various elements and attributes.

android:maxSdkVersion has the effect of removing your <uses-permission> element on higher Android SDK versions, at least in terms of how runtime permissions work.

Since you need this permission for all versions, adding tools:remove="android:maxSdkVersion" on the <uses-permission> element should revert the android:maxSdkVersion="18" and give you what you expect.

来源：https://stackoverflow.com/questions/49709463/api-26-write-external-storage-permission-is-always-denied