Add a visible error message for invalid documents

* Also enables concurrent reading, which should be safe

SilicaViewer/Document.swift

@@ -2,6 +2,7 @@ import Cocoa
 import ZIPFoundation
 import CoreFoundation
 import Accelerate
+import CoreMedia
 
 struct SilicaChunk {
     var x: Int = 0
@@ -86,6 +87,10 @@ class Document: NSDocument {
         self.addWindowController(windowController)
     }
     
+    override class func canConcurrentlyReadDocuments(ofType: String) -> Bool {
+        return ofType == "com.procreate"
+    }
+    
     /*
      Pass in an object from the $object array, which always contains a $class key.
      */
@@ -478,24 +483,50 @@ class Document: NSDocument {
             parseSilicaDocument(archive: archive, dict: topObjectClass)
         }
     }
+    
+    struct SilicaParsingError: Error, LocalizedError {
+        enum Kind {
+            case invalid
+        }
+        
+        let kind: Kind
+        let filename: URL?
+        
+        public var errorDescription: String? {
+            switch self.kind {
+            case .invalid:
+                return filename!.lastPathComponent + " is an invalid Silica Document."
+            }
+        }
+    }
+    
+    func throwError(_ error: SilicaParsingError.Kind) {
+        DispatchQueue.main.sync {
+            let _ = presentError(SilicaParsingError(kind: error, filename: fileURL))
+        }
+    }
 
     override func read(from data: Data, ofType typeName: String) throws {
         self.data = data
         
-        guard let archive = Archive(data: data, accessMode: Archive.AccessMode.read) else  {
+        guard let archive = Archive(data: data, accessMode: Archive.AccessMode.read) else {
+            throwError(.invalid)
             return
         }
     
         guard let documentEntry = archive[DocumentArchivePath] else {
+            throwError(.invalid)
             return
         }
         
         guard let documentData = readData(archive: archive, entry: documentEntry) else {
+            throwError(.invalid)
             return
         }
         
         var plistFormat = PropertyListSerialization.PropertyListFormat.binary
         guard let propertyList = try? PropertyListSerialization.propertyList(from: documentData, options: [], format: &plistFormat) else {
+            throwError(.invalid)
             return
         }