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  4. test/
  7. mono_pkg.yaml
  8. pubspec.yaml
  9. pubspec_overrides.yaml

pub package package publisher

package:checks is a library for expressing test expectations and it features a literate API.

package:checks preview

package:checks is in preview; to provide feedback on the API, please file an issue with questions, suggestions, feature requests, or general feedback.

For documentation about migrating from package:matcher to checks, see the migration guide.


  1. Add a dev_dependency on checks: ^0.2.0.

  2. Add an import for package:checks/checks.dart.

  3. Use checks in your test code:

void main() {
  test('sample test', () {
    // test code here


Checking expectations with checks

Expectations start with check. This utility returns a Subject, and expectations can be checked against the subject. Expectations are defined as extension methods, and different expectations will be available for subjects with different value types.

check(someString).contains('expected pattern');

If a failure may not have enough context about the actual or expected values from the expectation calls alone, add a “Reason” in the failure message by passing a because: argument to check().

  because: 'log lines must start with the severity',
).every((l) => l
    (l) => l.startsWith('ERROR'),
    (l) => l.startsWith('WARNING'),
    (l) => l.startsWith('INFO'),

Composing expectations

Multiple expectations can be checked against the same value using cascade syntax. When multiple expectations are checked against a single value, a failure will included descriptions of the expectations that already passed.


Some nested checks may be not be possible to write with cascade syntax. There is a which utility for this use case which takes a Condition.

  // A cascade would not be possible on `length`
  ..length.which((l) => l

Some expectations return a Subject for another value derived from the original value, such as the length extension.


Fields or derived values can be extracted from objects for checking further properties with the has utility.

  .has((value) =>, 'property')

Passing a set of expectations as an argument

Some expectations take arguments which are themselves expectations to apply to other values. These expectations take Condition arguments which have the signature void Function(Subject). The conditions check expectations when they are called with a Subject` argument.

check(someList).any((e) => e.isGreaterThan(0));

Checking asynchronous expectations

Expectation extension methods checking asynchronous behavior return a Future. The future should typically be awaited within the test body, however asynchronous expectations will also ensure that the test is not considered complete before the expectation is complete. Expectations with no concrete end conditions, such as an expectation that a future never completes, cannot be awaited and may cause a failure after the test has already appeared to complete.

Asynchronous expectations do not return a Subject. When an expectation extracts a derived value further expectations can be checked by passing a Condition.

await check(someFuture).completes((r) => r.isGreaterThan(0));

Subjects for Stream instances must first be wrapped into a StreamQueue to allow multiple expectations to test against the stream from the same state. The withQueue extension can be used when a given stream instance only needs to be checked once, or if it is a broadcast stream, but if single subscription stream needs to have multiple expectations checked separately it should be wrapped with a StreamQueue.

await check(someStream).withQueue.inOrder([
  (s) => s.emits((e) => e.equals(1)),
  (s) => s.emits((e) => e.equals(2)),
  (s) => s.emits((e) => e.equals(3)),
  (s) => s.isDone(),

var someQueue = StreamQueue(someOtherStream);
await check(someQueue).emits((e) => e.equals(1));
// do something
await check(someQueue).emits((e) => e.equals(2));
// do something

Writing custom expectations

Expectations are written as extensions on Subject with specific generics. The library package:checks/context.dart gives access to a context getter on Subject which offers capabilities for defining expectations on the subject's value.

The Context allows checking an expectation with expect, expectAsync and expectUnawaited, or extracting a derived value for performing other checks with nest and nestAsync. Failures are reported by returning a Rejection, or an Extracted.rejection, extensions should avoid throwing exceptions.

Descriptions of the clause checked by an expectations are passed through a separate callback from the predicate which checks the value. Nesting calls are made with a label directly. When there are no failures the clause callbacks are not called. When a condition callback is described, the clause callbacks are called, but the predicate callbacks are not called. Conditions can be checked against values without throwing an exception using softCheck or softCheckAsync.

extension CustomChecks on Subject<CustomType> {
  void someExpectation() {
    context.expect(() => ['meets this expectation'], (actual) {
      if (_expectationIsMet(actual)) return null;
      return Rejection(which: ['does not meet this expectation']);

  Subject<Foo> get someDerivedValue =>
      context.nest(() => ['has someDerivedValue'], (actual) {
        if (_cannotReadDerivedValue(actual)) {
          return Extracted.rejection(which: ['cannot read someDerivedValue']);
        return Extracted.value(_readDerivedValue(actual));

  // for field reads that will not get rejected, use `has`
  Subject<Bar> get someField => has((a) => a.someField, 'someField');

Extensions may also compose existing expectations under a single name. When such expectations fail, the test output will refer to the individual expectations that were called.

extension ComposedChecks on Subject<Iterable> {
  void hasLengthInRange(int min, int max) {