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| // Creates an event log. You can write to this during testing and then assert | |
| // on the result. | |
| // | |
| // The main use case is for asynchronous e2e tests. It provides a `waitFor` | |
| // method that resolves when the log matches some expected asynchronous sequence | |
| // of events. This is an alternative to setting up a timer loop. It helps catch | |
| // subtle mistakes where the order of events is not expected, or the same | |
| // event happens more than it should. | |
| // | |
| // Based on the Scheduler.log pattern used in the React repo. | |
| export function createTestLog() { | |
| let events: unknown[] = [] | |
| // Represents a pending waitFor call. | |
| let pendingExpectation: null | { | |
| resolve: () => void | |
| reject: (error: Error) => void | |
| expectedEvents: Array<any> | |
| error: Error | |
| } = null | |
| function log(value: unknown) { | |
| // Add to the event log. | |
| events.push(value) | |
| // Check if we've reached the end of the expected log. If there's a | |
| // pending waitFor, and we've reached the last of the expected events, this | |
| // will resolve the promise. | |
| pingExpectation() | |
| } | |
| function assert(expectedEvents: any[]) { | |
| if (pendingExpectation !== null) { | |
| const error = new Error('Cannot assert while a waitFor() is pending.') | |
| Error.captureStackTrace(error, assert) | |
| throw error | |
| } | |
| const actualEvents = events | |
| events = [] | |
| if (!areLogsEqual(expectedEvents, actualEvents)) { | |
| // Capture the stack trace of `assert` so that Jest will report the | |
| // error as originating from the `assert` call instead of here. | |
| const error = new Error( | |
| 'Expected sequence of events did not occur.\n\n' + | |
| createDiff(expectedEvents, actualEvents) | |
| ) | |
| Error.captureStackTrace(error, assert) | |
| throw error | |
| } | |
| } | |
| function waitFor(expectedEvents: any[], timeout: number = 5000) { | |
| // Returns a promise that resolves when the event log matches the | |
| // expected sequence. | |
| // Capture the stack trace of `waitFor` so that if an inner assertion fails, | |
| // Jest will report the error as originating from the `waitFor` call instead | |
| // of inside this module's implementation. | |
| const error = new Error() | |
| Error.captureStackTrace(error, waitFor) | |
| if (pendingExpectation !== null) { | |
| error.message = 'A previous waitFor() is still pending.' | |
| throw error | |
| } | |
| let resolve | |
| let reject | |
| const promise = new Promise<void>((res, rej) => { | |
| resolve = res | |
| reject = rej | |
| }) | |
| const thisExpectation = { | |
| resolve, | |
| reject, | |
| expectedEvents, | |
| error, | |
| } | |
| pendingExpectation = thisExpectation | |
| setTimeout(() => { | |
| if (pendingExpectation === thisExpectation) { | |
| error.message = `waitFor timed out after ${timeout}ms` | |
| reject(error) | |
| } | |
| }, timeout) | |
| pingExpectation() | |
| return promise | |
| } | |
| function pingExpectation() { | |
| if (pendingExpectation !== null) { | |
| const expectedEvents = pendingExpectation.expectedEvents | |
| if (events.length < expectedEvents.length) { | |
| return | |
| } | |
| if (areLogsEqual(expectedEvents, events)) { | |
| // We've reached the end of the expected log. Resolve the promise and | |
| // reset the log. | |
| events = [] | |
| pendingExpectation.resolve() | |
| pendingExpectation = null | |
| } else { | |
| // The log does not match what was expected by the test. Reject the | |
| // promise and reset the log. | |
| // Use the error object that we captured at the start of the `waitFor` | |
| // call. Jest will show that the error originated from `waitFor` call | |
| // instead of inside this internal function. | |
| const error = pendingExpectation.error | |
| error.message = | |
| 'Expected sequence of events did not occur.\n\n' + | |
| createDiff(expectedEvents, events) | |
| events = [] | |
| pendingExpectation.reject(error) | |
| pendingExpectation = null | |
| } | |
| } | |
| } | |
| function createDiff(expected, actual) { | |
| // TODO: Jest exposes the diffing utility that it uses for `expect`. | |
| // We could use that here for nicer output. | |
| return ` | |
| Expected: ${JSON.stringify(expected)} | |
| Actual: ${JSON.stringify(actual)} | |
| ` | |
| } | |
| function areLogsEqual(a, b) { | |
| if (a.length !== b.length) { | |
| return false | |
| } | |
| for (let i = 0; i < a.length; i++) { | |
| if (a[i] !== b[i]) { | |
| return false | |
| } | |
| } | |
| return true | |
| } | |
| return { | |
| log, | |
| waitFor, | |
| assert, | |
| } | |
| } | |