XCTestExpectation:如何避免在等待上下文结束后调用履行方法? [英] XCTestExpectation: how to avoid calling the fulfill method after the wait context has ended?

问题描述

我正在使用 Xcode 6 的新异步测试功能.当异步任务在超时之前结束时，一切正常.但如果任务花费的时间超过超时时间，事情就会变得更加复杂.

I’m using the new asynchronous testing capabilities of Xcode 6. Everything works fine when the asynchronous task ends before the timeout. But if the task takes longer than the timeout, things get more complicated.

以下是我的测试方式:

@interface AsyncTestCase : XCTestCase @end

@implementation AsyncTestCase

// The asynchronous task would obviously be more complex in a real world scenario.
- (void) startAsynchronousTaskWithDuration:(NSTimeInterval)duration completionHandler:(void (^)(id result, NSError *error))completionHandler
{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(duration * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        completionHandler([NSObject new], nil);
    });
}

- (void) test1TaskLongerThanTimeout
{
    XCTestExpectation *expectation = [self expectationWithDescription:@"Test 1: task longer than timeout"];
    [self startAsynchronousTaskWithDuration:4 completionHandler:^(id result, NSError *error) {
        XCTAssertNotNil(result);
        XCTAssertNil(error);
        [expectation fulfill];
    }];
    [self waitForExpectationsWithTimeout:2 handler:nil];
}

- (void) test2TaskShorterThanTimeout
{
    XCTestExpectation *expectation = [self expectationWithDescription:@"Test 2: task shorter than timeout"];
    [self startAsynchronousTaskWithDuration:5 completionHandler:^(id result, NSError *error) {
        XCTAssertNotNil(result);
        XCTAssertNil(error);
        [expectation fulfill];
    }];
    [self waitForExpectationsWithTimeout:10 handler:nil];
}

@end

不幸的是，在超时到期后调用 fulfill 方法会导致测试套件崩溃并出现以下错误:

Unfortunately, calling the fulfill method after the timeout has expired crashes the test suite with this error:

API 违规 - 在等待上下文结束后调用 -[XCTestExpectation meet].

API violation - called -[XCTestExpectation fulfill] after the wait context has ended.

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'API violation - called -[XCTestExpectation fulfill] after the wait context has ended.'
*** First throw call stack:
(
  0   CoreFoundation   0x000000010c3a6f35 __exceptionPreprocess + 165
  1   libobjc.A.dylib  0x000000010a760bb7 objc_exception_throw + 45
  2   CoreFoundation   0x000000010c3a6d9a +[NSException raise:format:arguments:] + 106
  3   Foundation       0x000000010a37d5df -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 195
  4   XCTest           0x0000000115c48ee1 -[XCTestExpectation fulfill] + 264
  ...
)
libc++abi.dylib: terminating with uncaught exception of type NSException

当然我可以在调用 fulfill 方法之前检查测试是否完成，如下所示:

Of course I can check if the test is finished before calling the fulfill method like this:

- (void) test1TaskLongerThanTimeout
{
    XCTestExpectation *expectation = [self expectationWithDescription:@"Test 1: task longer than timeout"];

    __block BOOL testIsFinished = NO;
    [self startAsynchronousTaskWithDuration:4 completionHandler:^(id result, NSError *error) {
        if (testIsFinished) {
            return;
        }
        XCTAssertNotNil(result);
        XCTAssertNil(error);
        [expectation fulfill];
    }];

    [self waitForExpectationsWithTimeout:2 handler:^(NSError *error) {
        testIsFinished = YES;
    }];
}

但这似乎过于复杂，并且使测试更难阅读.我错过了什么吗?有没有更简单的方法来解决这个问题?

But this seems overly complicated and makes the test much harder to read. Am I missing something? Is there a simpler way to solve this problem?

推荐答案

是的，有一种更简单的方法可以避免这种 API 违规问题:只需将您的期望变量声明为 __weak.虽然没有明确记录，但预期会在超时到期时释放.因此，如果任务花费的时间超过超时时间，则在调用任务完成处理程序时期望变量将为 nil.因此 fulfill 方法将在 nil 上被调用，什么都不做.

Yes, there is a much simpler way to avoid this API violation issue: just declare your expectation variable as __weak. Although not clearly documented, the expectation will be released when the timeout expires. So if the task takes longer than the timeout, the expectation variable will be nil when the task completion handler is called. Thus the fulfill method will be called on nil, doing nothing.

- (void) test1TaskLongerThanTimeout
{
    __weak XCTestExpectation *expectation = [self expectationWithDescription:@"Test 1: task longer than timeout"];
    [self startAsynchronousTaskWithDuration:4 completionHandler:^(id result, NSError *error) {
        XCTAssertNotNil(result);
        XCTAssertNil(error);
        [expectation fulfill];
    }];
    [self waitForExpectationsWithTimeout:2 handler:nil];
}

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