What is simplest way to prevent frame drop in flutter application on heavy task?

Issue

I’ve ran into problem getting UI lags when this line is running:

var keys = crypt.generateKeys();

Here is the full function:

void createKeys(_) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    var keys = crypt.generateKeys();  // laggy line
    prefs.setString('persPriv', keys[0]);
    prefs.setString('persPub', keys[1]);
    prefs.setString('mesPriv', keys[2]);
    prefs.setString('mesPub', keys[3]);
  }

I was trying to wrap this function with Isolate.

Isolate.spawn(createKeys, null);

But I’ve got an error:

[VERBOSE-2:ui_dart_state.cc(199)] Unhandled Exception: Invalid argument(s): Isolate.spawn expects to be passed a static or top-level function

What is the correct way to start isolates to prevent frame drops and UI lagging?

Solution

I don’t believe you can use SharedPreferences within an Isolate without support for MethodChannel / accessing platform-specific underlying OS frameworks on iOS / Android.

You would need to use FlutterIsolate or a similar package to provide that support.

chunhunghan has a good answer detailing this.

Alternatively, you could run the crypt.generateKeys() by itself in your Isolate.spawn() call and use the results after in a separate method accessing SharedPreferences. (Assuming that crypt package is also not relying on platform-specific code.)

Answered By – Baker

Answer Checked By – David Goodson (FlutterFixes Volunteer)