Why does DocumentFile.getParentFile() return null after it is created with a Uri?

Question

Code:

DocumentFile dfNew = dfDirectory.createFile("video/mp4", "foo.mp4");
//dfNew.getParentFile() is NOT null.  
DocumentFile dfNewCopy = DocumentFile.fromSingleUri(activity, dfNew.getUri());
//dfNewCopy.getParentFile() IS null. Otherwise, dfNewCopy works fine

Why is a DocumentFile created from a DocumentFile's Uri different from the original in this regard?

Thank you for the explanation. It makes a lot of sense. Could you turn your comment into an answer for me to accept? — Hong, Feb 22 '18 at 04:26

score 4 · Accepted Answer · answered Feb 22 '18 at 04:43

Storage Access Framework has been designed in days of API 16 for a very narrow use case (implementing the first official Android file picker). DocumentsProvider lacked a lot of useful functionality back then. Many more methods were added in following versions of platform.

The method, required to check whether one Uri is parent of another Uri (isChildDocument) has been added in API 21.

The method for getting parent of Uri (findDocumentPath) has been added in API 26 (Oreo).

DocumentFile can not return the parent Uri on older platforms, because the system itself does not provide it there.

Why does DocumentFile.getParentFile() return null after it is created with a Uri?

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