How to modify the `print` function in Dart?

April 13, 2023

The print function is defined in both these files:

flutter/bin/cache/pkg/sky_engine/lib/core/print.dart

flutter/bin/cache/dart-sdk/lib/core/print.dart

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I opened each of them and modified the function to add this line at the start

if (kDebugMode) return;

I restarted the IDE, run the app with this code

print(kDebugMode); // Prints true

The print function shouldn’t have printed anything. If I go and check the implementation of those print function, my if condition line is still there.

So, what am I doing wrong?

>Solution :

You probably don’t want to modify the source code anyway.
Instead, you can use Zone to override the print behavior in your code without modifying the SDK.

You can do:

import 'dart:async';

void main() {
  runZoned(
    () => print('Hello world'),
    zoneSpecification: ZoneSpecification(
      print: (self, parent, zone, line) {
        // Your logic here:
        // if (kDebugMode) return;
        // Now use the original print implementation:
        parent.print(zone, 'This is a custom print: $line');
      },
    ),
  );
}

This example overrides print to customize the output. In this example, even though we called print('Hello world'), the output will be This is a custom print: Hello world