stack_trace 1.1.2

A package for manipulating stack traces and printing them readably.

This library provides the ability to parse, inspect, and manipulate stack traces produced by the underlying Dart implementation. It also provides functions to produce string representations of stack traces in a more readable format than the native StackTrace implementation.

Traces can be parsed from native StackTraces using Trace.from, or captured using Trace.current. Native StackTraces can also be directly converted to human-readable strings using Trace.format.

Here's an example native stack trace from debugging this library:

#0      Object.noSuchMethod (dart:core-patch:1884:25)
#1      Trace.terse.<anonymous closure> (file:///usr/local/google-old/home/goog/dart/dart/pkg/stack_trace/lib/src/trace.dart:47:21)
#2      IterableMixinWorkaround.reduce (dart:collection:29:29)
#3      List.reduce (dart:core-patch:1247:42)
#4      Trace.terse (file:///usr/local/google-old/home/goog/dart/dart/pkg/stack_trace/lib/src/trace.dart:40:35)
#5      format (file:///usr/local/google-old/home/goog/dart/dart/pkg/stack_trace/lib/stack_trace.dart:24:28)
#6      main.<anonymous closure> (file:///usr/local/google-old/home/goog/dart/dart/test.dart:21:29)
#7      _CatchErrorFuture._sendError (dart:async:525:24)
#8      _FutureImpl._setErrorWithoutAsyncTrace (dart:async:393:26)
#9      _FutureImpl._setError (dart:async:378:31)
#10     _ThenFuture._sendValue (dart:async:490:16)
#11     _FutureImpl._handleValue.<anonymous closure> (dart:async:349:28)
#12     Timer.run.<anonymous closure> (dart:async:2402:21)
#13     Timer.Timer.<anonymous closure> (dart:async-patch:15:15)

and its human-readable representation:

dart:core-patch                             Object.noSuchMethod
pkg/stack_trace/lib/src/trace.dart 47:21    Trace.terse.<fn>
dart:collection                             IterableMixinWorkaround.reduce
dart:core-patch                             List.reduce
pkg/stack_trace/lib/src/trace.dart 40:35    Trace.terse
pkg/stack_trace/lib/stack_trace.dart 24:28  format
test.dart 21:29                             main.<fn>
dart:async                                  _CatchErrorFuture._sendError
dart:async                                  _FutureImpl._setErrorWithoutAsyncTrace
dart:async                                  _FutureImpl._setError
dart:async                                  _ThenFuture._sendValue
dart:async                                  _FutureImpl._handleValue.<fn>
dart:async                                  Timer.run.<fn>
dart:async-patch                            Timer.Timer.<fn>

You can further clean up the stack trace using Trace.terse. This folds together multiple stack frames from the Dart core libraries, so that only the core library method that was directly called from user code is visible. For example:

dart:core                                   Object.noSuchMethod
pkg/stack_trace/lib/src/trace.dart 47:21    Trace.terse.<fn>
dart:core                                   List.reduce
pkg/stack_trace/lib/src/trace.dart 40:35    Trace.terse
pkg/stack_trace/lib/stack_trace.dart 24:28  format
test.dart 21:29                             main.<fn>
dart:async                                  Timer.Timer.<fn>

Stack Chains

This library also provides the ability to capture "stack chains" with the Chain class. When writing asynchronous code, a single stack trace isn't very useful, since the call stack is unwound every time something async happens. A stack chain tracks stack traces through asynchronous calls, so that you can see the full path from main down to the error.

To use stack chains, just wrap the code that you want to track in Chain.capture. This will create a new Zone in which stack traces are recorded and woven into chains every time an asynchronous call occurs. Zones are sticky, too, so any asynchronous operations started in the Chain.capture callback will have their chains tracked, as will asynchronous operations they start and so on.

Here's an example of some code that doesn't capture its stack chains:

import 'dart:async';

void main() {
  scheduleAsync();
}

void scheduleAsync() {
  return new Future.delayed(new Duration(seconds: 1))
      .then((_) => runAsync());
}

void runAsync() {
  throw 'oh no!';
}

If we run this, it prints the following:

Uncaught Error: oh no!
Stack Trace: 
#0      runAsync (file:///usr/local/google-old/home/goog/dart/dart/test.dart:13:3)
#1      scheduleAsync.<anonymous closure> (file:///usr/local/google-old/home/goog/dart/dart/test.dart:9:28)
#2      _rootRunUnary (dart:async/zone.dart:717)
#3      _RootZone.runUnary (dart:async/zone.dart:854)
#4      _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:488)
#5      _Future._propagateToListeners (dart:async/future_impl.dart:571)
#6      _Future._complete (dart:async/future_impl.dart:317)
#7      _SyncCompleter.complete (dart:async/future_impl.dart:44)
#8      Future.Future.delayed.<anonymous closure> (dart:async/future.dart:219)
#9      _createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:11)
#10     _handleTimeout (dart:io/timer_impl.dart:292)
#11     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:115)

Notice how there's no mention of main in that stack trace. All we know is that the error was in runAsync; we don't know why runAsync was called.

Now let's look at the same code with stack chains captured:

import 'dart:async';

import 'package:stack_trace/stack_trace.dart';

void main() {
  Chain.capture(() {
    scheduleAsync();
  });
}

void scheduleAsync() {
  new Future.delayed(new Duration(seconds: 1))
      .then((_) => runAsync());
}

void runAsync() {
  throw 'oh no!';
}

Now if we run it, it prints this:

Uncaught Error: oh no!
Stack Trace: 
test.dart 17:3                                                runAsync
test.dart 13:28                                               scheduleAsync.<fn>
package:stack_trace/src/stack_zone_specification.dart 129:26  registerUnaryCallback.<fn>.<fn>
package:stack_trace/src/stack_zone_specification.dart 174:15  StackZoneSpecification._run
package:stack_trace/src/stack_zone_specification.dart 177:7   StackZoneSpecification._run
package:stack_trace/src/stack_zone_specification.dart 175:7   StackZoneSpecification._run
package:stack_trace/src/stack_zone_specification.dart 129:18  registerUnaryCallback.<fn>
dart:async/zone.dart 717                                      _rootRunUnary
dart:async/zone.dart 449                                      _ZoneDelegate.runUnary
dart:async/zone.dart 654                                      _CustomizedZone.runUnary
dart:async/future_impl.dart 488                               _Future._propagateToListeners.handleValueCallback
dart:async/future_impl.dart 571                               _Future._propagateToListeners
dart:async/future_impl.dart 317                               _Future._complete
dart:async/future_impl.dart 44                                _SyncCompleter.complete
dart:async/future.dart 219                                    Future.Future.delayed.<fn>
package:stack_trace/src/stack_zone_specification.dart 174:15  StackZoneSpecification._run
package:stack_trace/src/stack_zone_specification.dart 119:52  registerCallback.<fn>
dart:async/zone.dart 706                                      _rootRun
dart:async/zone.dart 440                                      _ZoneDelegate.run
dart:async/zone.dart 650                                      _CustomizedZone.run
dart:async/zone.dart 561                                      _BaseZone.runGuarded
dart:async/zone.dart 586                                      _BaseZone.bindCallback.<fn>
package:stack_trace/src/stack_zone_specification.dart 174:15  StackZoneSpecification._run
package:stack_trace/src/stack_zone_specification.dart 119:52  registerCallback.<fn>
dart:async/zone.dart 710                                      _rootRun
dart:async/zone.dart 440                                      _ZoneDelegate.run
dart:async/zone.dart 650                                      _CustomizedZone.run
dart:async/zone.dart 561                                      _BaseZone.runGuarded
dart:async/zone.dart 586                                      _BaseZone.bindCallback.<fn>
dart:async-patch/timer_patch.dart 11                          _createTimer.<fn>
dart:io/timer_impl.dart 292                                   _handleTimeout
dart:isolate-patch/isolate_patch.dart 115                     _RawReceivePortImpl._handleMessage
===== asynchronous gap ===========================
dart:async/zone.dart 476                   _ZoneDelegate.registerUnaryCallback
dart:async/zone.dart 666                   _CustomizedZone.registerUnaryCallback
dart:async/future_impl.dart 164            _Future._Future._then
dart:async/future_impl.dart 187            _Future.then
test.dart 13:12                            scheduleAsync
test.dart 7:18                             main.<fn>
dart:async/zone.dart 710                   _rootRun
dart:async/zone.dart 440                   _ZoneDelegate.run
dart:async/zone.dart 650                   _CustomizedZone.run
dart:async/zone.dart 944                   runZoned
package:stack_trace/src/chain.dart 93:20   Chain.capture
test.dart 6:16                             main
dart:isolate-patch/isolate_patch.dart 216  _startIsolate.isolateStartHandler
dart:isolate-patch/isolate_patch.dart 115  _RawReceivePortImpl._handleMessage

That's a lot of text! If you look closely, though, you can see that main is listed in the first trace in the chain.

Thankfully, you can call Chain.terse just like Trace.terse to get rid of all the frames you don't care about. The terse version of the stack chain above is this:

test.dart 17:3   runAsync
test.dart 13:28  scheduleAsync.<fn>
dart:isolate     _RawReceivePortImpl._handleMessage
===== asynchronous gap ===========================
dart:async                                _Future.then
test.dart 13:12                           scheduleAsync
test.dart 7:18                            main.<fn>
package:stack_trace/src/chain.dart 93:20  Chain.capture
test.dart 6:16                            main
dart:isolate                              _RawReceivePortImpl._handleMessage

That's a lot easier to understand!