BlockNSInvocationTest.m

- (void)applicationDidEnterBackground:(UIApplication *)application {    
    @weakify(self);
    self.backgroundTaskIdentifier = [application beginBackgroundTaskWithExpirationHandler:^{
        @strongify(self);
        [application endBackgroundTask:self.backgroundTaskIdentifier];
        self.backgroundTaskIdentifier = UIBackgroundTaskInvalid;
    }];
}

- (void)testApplicationDidEnterBackgroundExpirationHandler {
    //Given:
    NSUInteger identifier = 1;
    __block void (^expirationHandler)(void);
    OCMStub([self.application beginBackgroundTaskWithExpirationHandler:OCMOCK_ANY]).andDo(^(NSInvocation *invocation) {
        void (^handler)(void);
        [invocation getArgument:&handler atIndex:2];
        expirationHandler = [handler copy];
        
//        NSValue *returnValue = [NSValue valueWithBytes:&identifier objCType:@encode(NSUInteger)];
        void *value = (void *)identifier;
        [invocation setReturnValue:&value];
    });
    
    //When:
    [self.appDelegate applicationDidEnterBackground:self.application];
    expirationHandler();

    //Then:
    OCMVerify([self.application endBackgroundTask:identifier]);
    XCTAssertEqual(self.appDelegate.backgroundTaskIdentifier, UIBackgroundTaskInvalid);
    XCTAssertNotEqual(self.appDelegate.backgroundTaskIdentifier, identifier);
}