launch method
Implementation
Future<Null> launch(String path, ViewerParams params, {Rect? rect}) async {
final basicArgs = <String, dynamic>{'path': path};
if (rect != null) {
basicArgs['rect'] = {
'left': rect.left,
'top': rect.top,
'width': rect.width,
'height': rect.height
};
}
final Map<String, dynamic> args = params.toMap(basicArgs);
await _channel.invokeMethod('launch', args);
}