Sketch Plugin Script - Ask user for Sketch file, then copy a layer from it.js

// Name of layer to copy
var sourceLayerName = "gray_rectangle"

// Ask user to select a Sketch file:
var openDialog = NSOpenPanel.openPanel()
openDialog.setCanChooseFiles(true)
openDialog.setAllowedFileTypes(["sketch"])
openDialog.setCanChooseDirectories(false)
openDialog.setAllowsMultipleSelection(false)
openDialog.setCanCreateDirectories(false)
openDialog.setTitle("Select the source Sketch document")

if( openDialog.runModal() == NSOKButton ) {
  
  var sourceDoc = MSDocument.new()
  if(sourceDoc.readFromURL_ofType_error(openDialog.URL(), "com.bohemiancoding.sketch.drawing", nil)) {
    
    var allChildren = sourceDoc.pages().valueForKeyPath("@distinctUnionOfArrays.children")
    
    // Modify the predicate to change search parameters. Here it's by layer name
    var predicate = NSPredicate.predicateWithFormat("name == %@", sourceLayerName);
    
    var layerToCopy = allChildren.filteredArrayUsingPredicate(predicate).firstObject()
    if (layerToCopy) {
      var duplicateLayerToCopy = layerToCopy.duplicate()
      duplicateLayerToCopy.removeFromParent()
      var currentDoc = context.document
      currentDoc.currentPage().currentArtboard().addLayers([duplicateLayerToCopy])
      if(duplicateLayerToCopy.isSymbol()) {
        currentDoc.documentData().layerSymbols().addSymbolWithName_firstInstance(duplicateLayerToCopy.name(), duplicateLayerToCopy)
      }
    }
  }
  sourceDoc.close()
  sourceDoc = nil
}