gomega

Write correct synchronous Gomega assertions — Expect/Ω notation, the To/NotTo/ToNot/Should/ShouldNot equivalences, the multi-return error idiom, Succeed vs HaveOccurred, the .Error() chaining form, annotating assertions (format-string and func()string), tuning failure output via the format subpackage (MaxLength/MaxDepth/UseStringerRepresentation/GomegaStringer/TruncatedDiff/RegisterCustomFormatter/format.Object), and asserting inside helper functions with GinkgoHelper/WithOffset/ExpectWithOffset, NewWithT(t) for plain testing, and the g Gomega callback. Use when writing or reviewing synchronous (non-polling) Gomega assertions.

Polling assertions in Gomega — Eventually (poll until it passes) and Consistently (must keep passing), the func(g Gomega) callback idiom, WithTimeout/WithPolling/Within/ProbeEvery, WithContext and Ginkgo SpecContext, StopTrying/TryAgainAfter bail-outs, MustPassRepeatedly, and default-interval tuning. Use when an assertion can't be true synchronously — anything involving goroutines, channels, network calls, eventual consistency, or "wait until / stays true".

Build compound Gomega assertions by combining matchers — And/SatisfyAll (all pass), Or/SatisfyAny (any pass), Not (negate), WithTransform to map the actual before matching, Satisfy for an ad-hoc predicate, HaveValue to dereference pointers/interfaces, HaveField for struct fields and method results, HaveEach for every element, plus the matchers-as-arguments idiom that lets you nest matchers inside ContainElement/ConsistOf/HaveKeyWithValue/Receive. Use when one Expect needs several requirements at once, or you want to assert deep into a value without writing a custom matcher.

Writing your own Gomega matchers — the GomegaMatcher interface (Match/FailureMessage/NegatedFailureMessage), gcustom.MakeMatcher with message templates and template data, the format package helpers (format.Message/format.Object), MatchMayChangeInTheFuture and StopTrying for Eventually/Consistently, and how to test and package custom matchers. Use when a built-in or composed matcher can't express your domain assertion and you need to build one.

Testing streaming io buffers with gbytes — gbytes.NewBuffer() (an io.Writer also returned by gexec sessions), the Say(regexp) matcher that forward-scans from a moving read cursor, the canonical Eventually(buffer).Should(Say(...)) streaming pattern, sequential cursor-advancing Say calls, Contents(), BufferWithBytes/BufferReader, buffer.Detect for branching, and TimeoutReader/Writer/Closer for testing blocking io.Reader/Writer/Closer. Use when asserting on streaming or incremental output (process stdout/stderr, API streams, io.Readers) rather than a complete value.