`TypeMatcher` not working when used with `throwsA`

Question

I have the following code in my app:

expect(() => dataSource.getLastPost(), throwsA(TypeMatcher<CacheException>()));

This results in the following error

Expected: throws <Instance of 'TypeMatcher<CacheException>'>
  Actual: <Closure: () => Future<PostModel>>
   Which: threw <Instance of 'CacheException'>

If I remove the TypeMatcher wrapping CacheException...

expect(() => dataSource.getLastPost(), throwsA(CacheException()));

then it still gives a similar error

Expected: throws <Instance of 'CacheException'>
  Actual: <Closure: () => Future<PostModel>>
   Which: threw <Instance of 'CacheException'>

I was wondering what I am doing wrong? Either way it's clear that we are expecting a CacheException and a CacheException is being thrown. Then why does the expect test not pass?

Answer 1

It is correct to write throwsA(TypeMatcher<CacheException>())

The problem is you're likely facing is, in the context of Flutter and tests, there are two classes with the name TypeMatcher:

• TypeMatcher from the package matcher, for tests
• TypeMatcher from flutter/widgets, to navigate inside BuildContext.

They are both used the same way, aka TypeMatcher<SomeClass>. But only one of them is a Matcher, which tests understand.

Your problem being, you likely used TypeMatcher from flutter/widgets. And since it's not a matcher, then:

throwsA(TypeMatcher<MyClass>())

is interpreted as:

throwsA(equals(TypeMatcher<MyClass>())

---

The solution is to use the correct TypeMatcher, from package:matcher/matcher.dart.

But to begin with, you should not use TypeMatcher directly.

Instead of:

throwsA(TypeMatcher<MyClass>())

you should use the shorthand isA<T> matcher:

throwsA(isA<MyClass>())

This removes the name conflict entirely