Pro Android with Kotlin

99
CHAPTER 6: Content Providers
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fun openDocument(documentId: String?, mode: String?, signal: 
CancellationSignal?): ParcelFileDescriptor:
Open and return the requested document. This should return a reliable 
ParcelFileDescriptor to detect when the remote caller has finished reading or writing 
the document. If you block while downloading content, you should periodically check 
CancellationSignal.isCanceled() to abort abandoned open requests. The parameters 
are documentId for the document to return. The mode specifies the “open” mode, 
such as r, w, or rw. Mode r should always be supported. The provider should throw 
UnsupportedOperationException if the passing mode is not supported. You may return a 
pipe or socket pair if the mode is exclusively r or w, but complex modes like rw imply a 
normal file on disk that supports seeking. The signal may be used from the caller if the 
request should be canceled. May be null.
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override fun queryDocument(documentId: String?, projection: 
Array?): Cursor:
Return metadata for a single requested document. The parameters are documentId for the ID 
of the document to return and projection for a list of columns to put into the cursor. Use the 
constants from DocumentsContract.Document. For a list, see the description of the method 
queryChildDocuments(). If you use null here, all columns are to be returned.
Inside the file AndroidManifest.xml, you register the documents provider almost like any 
other provider.
android:name="com.example.YourDocumentProvider"
android:authorities="com.example.documents"
android:exported="true"
android:grantUriPermissions="true"
android:permission=
"android.permission.MANAGE_DOCUMENTS">

"android.content.action.DOCUMENTS_PROVIDER"/>

In the previous queries, we have seen that the Cursor object returns flags to indicate that 
recent documents and searching inside the tree should be supported. For this to work, you 
must implement one or two more methods in your DocumentsProvider implementation.
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override fun queryRecentDocuments(rootId: String, projection: 
Array): Cursor:
This is supposed to return recently modified documents under the 
requested root. The returned documents should be sorted by COLUMN_LAST_
MODIFIED in descending order, and at most 64 entries should be shown. 
Recent documents do not support change notifications.

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