How to Test Functions that Throw in Flutter

When writing test expectations for functions that throw, we need a bit careful.

For example, consider this simple test:

test('function throws', () { expect( someFunctionThatThrows(), throwsStateError, ); });

In this case, the test will fail, because someFunctionThatThrows() is evaluated before the expect function is called.

This is equivalent to writing the test like this:

test('function throws', () { // if the function throws, the test will fail immediately final result = someFunctionThatThrows(); // and this line will never be exectuted expect( result, throwsStateError, ); });

To fix this, we can pass an anonymous function as an argument to the expect method:

test('function throws', () { expect( // function argument, will be evaluated *inside* the expect function () => someFunctionThatThrows(), throwsStateError, ); });

Or even better, we can use a tear-off and pass someFunctionThatThrows directly as an argument (without calling it):

test('function throws', () { expect( someFunctionThatThrows, throwsStateError, ); });

This is safer, and more concise too! 👌

For more info about tear-offs, see this rule about effective Dart usage.

Happy coding!

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