Stack trace

Learn stack trace and debugging.

Stack trace

When an exception is thrown Dart records the current call stack. This is called a stack trace.

It is captured as the second parameter in a catch block: try { ... } catch (e, s) { print(s); }. Here s is a StackTrace object.

The stack trace shows where the error originated (file, line, function). It is the main debugging tool; in production it is sent to logging tools.

A stack trace example

Typical usage: try { riskyOperation(); } catch (e, s) { print('Error: $e'); print(s); }.

s is a StackTrace object; it is turned into text with toString(). print(s) does this automatically.

In development you print it; in production you send it to a logging service. Without a stack trace an error stays mysterious.

Exercise Catch the error and the trace

Write void risky() { throw StateError('bad state'); }. In main call it with try { risky(); } catch (e, s) { print(e); }. The second parameter s of catch (e, s) is the StackTrace; here we print e.

Show example solution

void risky() {
  throw StateError('bad state');
}

void main() {
  try {
    risky();
  } catch (e, s) {
    print(e);
  }
}

Test yourself

Stack trace parts

catch (e, s)

Captures StackTrace
s

StackTrace variable
print(s)

Prints the trace
throw

Throws the exception
How is a StackTrace captured in a catch block?
- catch (e, s)
- catch (s, e)
- catch (e)
- catch (s)
Show answer

catch (e, s) — catch (e, s) takes the StackTrace as the second parameter.

StackTrace parameter

try { risky(); } catch (e, ____) { print(s); }

Show answer

try { risky(); } catch (e, s) { print(s); }

Next lesson: Result pattern

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