Dart how to drop a breakpoint or debugger

Question

In Python, I can insert a breakpoint() keyword on any line of code, and when I run the script from the command-line, it will stop when it reaches that line, and I'll have an opportunity to interact with or access any previously-defined variables. I was looking for a way to do this in the Dart language but so far without success.

I've seen references to the debugger keyword provided by the dart:developer library, but instead of allowing me to interact, the script just hangs:

// bin/my_script.dart

import "dart:developer"; // source of debugger();

main() {
  var x = 5;
  print("X: ${x}"); //> X: 5

  debugger(); // ... just hangs

  print("END");
}

I've also seen references to the console package, but I'm not seeing it do anything:

// bin/my_script.dart

import "package:console/console.dart"; // source of Console.init()

main() {
  var x = 5;
  print("X: ${x}"); //> X: 5

  Console.init(); // ... nothing happens

  print("END");
}

FYI: I'm running this script via dart bin/my_script.dart, and a command-line solution would be ideal, but a solution using the VS Code text editor would also be sufficient.

Solution

You can't debug without a debugger attached, that's why running from the command line will hang in the breakpoint, since you can step on the next instruction.

If you use VS Code or Intellij and use the debugger() while in debug mode Shift+Cmd+R, it will trigger a breakpoint there and you can analyze your variables and step forward to the next instructions.

Comments

[s2t2]

Shift Command R does bring up an opportunity for me to "play", but when I press play, it plays right through the debugger() without stopping, and I don't see any output from my script at all.

[Miguel Ruivo]

There must be something wrong. I've just tried and when I use debugger() while in debug mode (play button) it stops there and let me step over the next instructions.

[s2t2]

It does seem to do something when the script is lib/main.dart but not differently-named files. When I do it with the lib/main.dart script, it does stop, and I see information in the left pane, but how do I find the console where I can type variable names and statements / interpret dart expressions?

[Miguel Ruivo]

You can write expressions directly on the debug console itself (View > Debug console).