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.


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.


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

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


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.


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


A class to represent Flow events.
Functionality available on the native runtime.
Access information about the service protocol and control the web server that provides access to the services provided by the Dart VM for debugging and inspecting Dart programs.
A response to a service protocol extension RPC.
Service protocol is the protocol that a client like the Observatory could use to access the services provided by the Dart VM for debugging and inspecting Dart programs. This class encapsulates the version number and Uri for accessing this service.
Add to the timeline.
An asynchronous task on the timeline. An asynchronous task can have many (nested) synchronous operations. Synchronous operations can live longer than the current isolate event. To pass a TimelineTask to another isolate, you must first call pass to get the task id and then construct a new TimelineTask in the other isolate.
A UserTag can be used to group samples in the DevTools CPU profiler.


extensionStreamHasListener bool
Whether the "Extension" stream currently has at least one listener.
no setter
reachabilityBarrier int
Current reachability barrier state.
no setter


addHttpClientProfilingData(Map<String, dynamic> requestProfile) → void
Records the data associated with an HTTP request for profiling purposes.
debugger({bool when = true, String? message}) bool
If when is true, stop the program as if a breakpoint were hit at the following statement.
getCurrentTag() UserTag
Returns the current UserTag for the isolate.
getHttpClientProfilingData() List<Map<String, dynamic>>
Returns the data added through addHttpClientProfilingData.
inspect(Object? object) Object?
Send a reference to object to any attached debuggers.
log(String message, {DateTime? time, int? sequenceNumber, int level = 0, String name = '', Zone? zone, Object? error, StackTrace? stackTrace}) → void
Emit a log event.
postEvent(String eventKind, Map eventData, {String stream = 'Extension'}) → void
Post an event of eventKind with payload of eventData to the "Extension" event stream.
registerExtension(String method, ServiceExtensionHandler handler) → void
Register a ServiceExtensionHandler that will be invoked in this isolate for method. NOTE: Service protocol extensions must be registered in each isolate.


ServiceExtensionHandler = Future<ServiceExtensionResponse> Function(String method, Map<String, String> parameters)
A service protocol extension handler. Registered with registerExtension.
TimelineAsyncFunction = Future Function()
TimelineSyncFunction<T> = T Function()
A typedef for the function argument to Timeline.timeSync.