dart:developer library

Interact with developer tools such as the debugger and inspector.

This is a specialized library intended for interacting with the Dart runtime programmatically for debugging and inspection. Sample uses include advanced debugging, and creating developer tools.

This library has platform specific implementations for Dart web and Dart Native (VM). A specific platform may not support all operations.

The functionality provided by this library is generally only available to Dart code run in development mode, e.g., dart run, and not in production mode, e.g., the output of dart compile exe.

Debugging

The debugger function can be used to stop the program as if a breakpoint was hit. The breakpoint will be placed right after the call to debugger. This functionality can be useful for triggering breakpoints based on logic in the code.

Example:

var counter = 0;
final someInterestingValue = 1000;
while (true) {
  if (counter == someInterestingValue) {
    // Trigger a breakpoint in the the debugger.
    debugger();
  }
  counter++;
}

When executed with dart run --observe, and opened in DevTools, the debugger will be stopped with counter at the value 1000.

Inspection

Developer tools, such as Dart DevTools, connected to the runtime system may allow for inspecting execution timing in a "timeline" view. The static methods of Timeline can add extra information and timing events to this view.

Example:

void main() {
  Timeline.timeSync('Calculation loop', () {
    for (var i = 30; i < 50; i++) {
      Timeline.timeSync('fib($i)', () {
        fibonacci(i);
      });
    }
  });
}

int fibonacci(int n) => (n < 2) ? n : fibonacci(n - 2) + fibonacci(n - 1);

When executed with dart run --observe, and opened in DevTools, the Performance tab will display a timeline containing the annotations passed to timeSync.

Developer tools

A developer tool, like the debugger built into the dart command, may access information about the running application exposed by the runtime system, using the Service and ServiceProtocolInfo classes.