Xamarin iOS DocumentPicker: How to import large files? - ios

Question

In the DidPickDocument event of UIDocumentPickerViewController I try to import/write the selected file into the app's local Documents directory.
This works fine with "small" files (e.g < 100MB) using a subclassed UIDocument class with overriden
public override bool LoadFromContents(NSObject contents, string typeName, out NSError outError)
{
outError = null;
if (contents != null)
{
Content = ((NSData)contents).ToArray();
}
...
...and by calling
MySubclassedDoc mySubclassedDoc = new MySubclassedDoc (nsurl);
bool success = await mySubclassedDoc.OpenAsync();
File.WriteAllBytes("targetFile.xyz", mySubclassedDoc.Content);
But if the file is larger (eg. 400MB) the app crashes before LoadFromContents is called because of insufficent memory (RAM).
So there need to be a way to stream the selected file directly to a file.
How can you do this using the given NSUrl ?

Answer 1

Since you have got the url, there's no need to convert it to NSData then store the data to a file. We can just use this url with NSFileManager like:
url.StartAccessingSecurityScopedResource();
string docPath = NSSearchPath.GetDirectories(NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomain.User)[0];
string filePath = Path.Combine(docPath, "fileName.type");
NSError error;
NSFileManager fileManager = new NSFileManager();
fileManager.Copy(url.Path, filePath, out error);
url.StopAccessingSecurityScopedResource();
In this way we can store the file to our app's own document directory.

Answer 2

why don’t you just clone the file over to your Documents directory before reading it instead of deserializing the contents and reserializing them?
your code seems really inefficient. If your files can be 400MB you should clearly not load all the contents into memory. I guess you must have very large binary objects in the files if they can be 400MB ; try mmapping the file and storing pointers to the individual objects instead?