Xamarin iOS DocumentPicker: How to import large files?

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 ?

score 0 · Answer 1 · answered Apr 23 '18 at 08:06

0

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?

answered Apr 23 '18 at 08:06

Thomas Deniau

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I was fixated on UiDocument (which is recommended when accessing files returned from the document picker on various web sites, eg the xamarin ones), so I did not see the simple solution using StartAccessingSecurityScopedResource and simple copy from a to b. – fritz Apr 24 '18 at 09:16

score 0 · Accepted Answer · answered Apr 23 '18 at 16:06

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.

Xamarin iOS DocumentPicker: How to import large files?

2 Answers2