Breakpoints set in a Dart web app with vscode are not being before the app runs

[nshahan]

Repro:

1. Open the example directory (https://github.com/dart-lang/webdev/tree/main/example) in vscode.
2. Set a breakpoint in main.dart on line 13.

   webdev/example/web/main.dart

   Line 13 in 5b7299f

   print('Initial Print :)');

3. Click the debug link vscode inserts above the main method.

You should see the app build and start running, chrome pops up and there will be prints appearing in the vscode debug console.

Initial Print :)
Counter is now: 1
Counter is now: 2
Counter is now: 3
Counter is now: 4
...

4. Now add a breakpoint on line 23.

   webdev/example/web/main.dart

   Line 23 in 5b7299f

   print('Counter is now: ${++count}');

You should see execution will pause.

5. Click the "Restart" button in the vscode debugging UI to hot restart the app.

You should see that the breakpoint on line 13 is skipped again but the breakpoint on line 23 still works.

[nshahan]

@Markzipan @jyameo
Did some of our recent changes allow the app to start before breakpoints get set?

[nshahan]

Setting breakpoints on the first line of a main method in a flutter application does work as expected. It seems like the integration there properly waits to set breakpoints before running the main method.

[jyameo]

@Markzipan @jyameo Did some of our recent changes allow the app to start before breakpoints get set?

I'm not aware of any recent changes that would have caused that.

[Markzipan]

I'm not aware of any changes either; most of my changes shouldn't be user visible with AMD modules. Do we know when this started breaking?

[Markzipan]

I'm curious how Flutter Web handles this. I notice that wrapping main in any asynchronous logic allows the initial breakpoint to be hit. Is Flutter doing something similar - or is it all in a handshake that defers main?

[nshahan]

IIUC the flutter_tool integration dwds handles this by waiting for all scripts to load, then sets all existing breakpoints, and once that is complete finally executes the main method.

[DanTup]

Yep, this is quite a long-standing issue - see #830

VM and Flutter web start paused to allow the IDE to send breakpoints before main runs.