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swiftxcodecocoansdocumentnsdocumentcontroller

NSCocoaErrorDomain Code=256 Cannot open files in the “md” format

I'm developing a macOS application for editing files, but am getting a rather annoying error when trying to use NSDocumentController.shared.makeDocument to create a new NSDocument instance from a file URL.

Below is a simple example of how I am calling makeDocument. The file test.md exists on disk.

let url = URL(fileURLWithPath: "/Users/me/Desktop/test.md"

do {
  let newDocument = try NSDocumentController.shared.makeDocument(withContentsOf: url, ofType: url.pathExtension)
  print("Created \(newDocument)")
} catch {
  print("Error: \(error)")
}

The problem is that this try call fails and it reaches the catch block. The error I get is:

Error: Error Domain=NSCocoaErrorDomain Code=256 "“test.md” could not be handled because MyApp cannot open files in the “md” format." UserInfo={NSLocalizedDescription=“test.md” could not be handled because MyApp cannot open files in the “md” format., NSLocalizedFailureReason= MyApp cannot open files in the “md” format.}

I believe i've correctly set my app's Document Types for markdown files as shown below:

info

I've tried cleaning the build , removing derived data and also adding an 'Imported UTI' type for markdown files but nothing seems to work.

The strange thing is that via File > Open, I am able to open .md files, just not programatically via makeDocument.

Solution

  • makeDocument(withContentsOf:ofType:) expects a type as second parameter, not an extension. Have a look at typeForContents(of url: URL) on how to derive the type from an URL.

    See Figure 6-3 in https://developer.apple.com/library/archive/documentation/DataManagement/Conceptual/DocBasedAppProgrammingGuideForOSX/AdvancedTopics/AdvancedTopics.html

    And as Marek H pointed out in his answer, in the info.plist there should be an UTI (identifier) for the document type.