单元测试是我们保障代码质量的重要手段， Apple 对此也十分重视，这点可以从 Xcode 新建工程时会自动创建单元测试的 Target 看出来。单元测试牵涉的内容很多，这篇文章是目前我对单元测试的理解。

既然是单元测试，那么什么是单元呢？我没有去考证，但我是这么理解的：面向对象编程范式里一切皆对象，而对象是由实例变量和方法组成，对象之间通过方法互相作用，我们的应用可以看作是一个对象图，对象图上的对象相互作用来实现我们的需求。这么看来我们的单元应该是对象的方法。

对象的方法在工作的时候可能要依赖其他对象，为了去除依赖对象对测试的影响，我们引入 Mock.

Mocks are ‘fake’ objects with pre-defined behavior to stand-in for concrete objects during testing. – HackaZach

引用 Mock 之后，我们在单元测试中如何使用它呢？

The general recipe for using mocks in unit-tests is:

  1. Create the mock object
  2. Specify the expected invocations and return values
  3. Associate the mock object with the code under test
  4. Execute the code under test
  5. Validate that your assertions are correct

    Create the mock object

OCMock 中创建 Mock 对象的方法如下：

Factory Method Description
+mockForClass: Create a mock based on the given class
+mockForProtocol: Create a mock based on the given protocol
+niceMockForClass: Create a “nice” mock based on the given class
+niceMockForProtocol: Create a “nice” mock based on the given protocol
+partialMockForObject: Create a mock based on the given object
+observerMock: Create a notification observer (more on this later)

Mock 对象是只是一个空壳，只能调用预先定义的方法，调用没有预先定义的方法会抛出异常； Nice Mock 对象也是一个空壳，只是它会忽略没有预先定义的方法不会抛出异常； Partial Mock 对象是把一个已有的对象转成 Mock，调用没有预先定义的方法，它会把方法传给已存在的对象；
Observer Mock 对象是用来观察通知的。

Specify the expected invocations and return values

Calling either the -expect or -stub method will return an object that you can use to setup your expectations.

  • Specify the expected invocations

    1. 不带参数的方法

- (void)testInit {
  id mockService = [OCMockObject mockForProtocol:@protocol(AVQuoteService)];
  [[mockService expect] initiateConnection];
  
  AVStockPortfolio *portfolio = [[AVStockPortfolio alloc] initWithService:mockService];
  
  [mockService verify];
}

</br> 2. 带参数的方法

You can use any of the following OCMArg class methods in place of a real argument when setting up your method expectations:

OCMArg method Description
+any Any argument is accepted.
+anyPointer Accepts any pointer
+isNil The given argument must be nil
+isNotNil The given argument must not be nil
+isNotEqual: Given argument is not object-equivalent with expectation
+checkWithSelector: onObject: Check the argument with the given action/target pair
+checkWithBlock: Check the argument with the given block (OS X 10.6 or iOS 4)

OCMock also provides a few handy macros for argument matching:

Macro Description
OCMOCK_ANY() Equivalent to [OCMArg any]
OCMOCK_VALUE(value) A quick way to match a non-object argument
CONSTRAINT(selector) Validate with a given selector on self
CONSTRAINTV(selector,value) Validate with a given selector on self and an additional argument 
id classMock = OCMClassMock([SomeClass class]);
OCMExpect([classMock someMethodWithArgument:[OCMArg isNotNil]]);

/* run code under test, which is assumed to call someMethod */

OCMVerifyAll(classMock)

</br> 3. 通知

- (void)testSellSharesInStock {
  id mock = [OCMockObject observerMock];
  // OCMock adds a custom methods to NSNotificationCenter via a category
  [[NSNotificationCenter defaultCenter] addMockObserver:mock
                                                   name:AVStockSoldNotification
                                                 object:nil];
                                               
  [[mock expect] notificationWithName:AVStockSoldNotification object:[OCMArg any]];

  AVPortfolio *portfolio = [self createPortfolio]; // made-up factory method
  [portfolio sellShares:100 inStock:@"AAPL"];

  [mock verify];
}
  • Specify the return values

If methods on your mocks need to return values, you have a variety of methods to call on the object returned by -expect or -stub.

Method Explanation
-andReturn: Return the given object
-andReturnValue: Return a non-object value (wrapped in a NSValue)
-andThrow: Throw the given exception
-andPost: Post the given notification
-andCall:onObject: Call the selector on the given object
-andDo: Invoke the given block (only on OS X 10.6 or iOS 4)

Execution & Validation

  • Associate the mock object with the code under test
  • Execute the code under test
  • Validate that your assertions are correct
- (void)testSellSharesInStock {
  id quoteService = [[OCMockObject] mockForProtocol:@protocol(AVQuoteService)];
  [[[quoteService expect] andDo:^(NSInvocation *invocation) {
    // validate arguments, set return value on the invocation object
  }] priceForStock:@"AAPL"];
  
  // Associate the mock object with the code under test
  AVStockPortfolio *portfolio = [[AVStockPortfolio alloc] initWithService:quoteService];
  // Execute the code under test
  [portfolio sellShares:100 inStock:@"AAPL"];
  
  // Validate that your assertions are correct
  
  [quoteService verify];
}

Q:How to mocking singleton? A:

// we can replace it with a mock object
id mockManager = [OCMockObject mockForClass:[ArticleManager class]];
[ArticleManager setSharedInstance:mockManager];
// we can reset it so that it returns the actual ArticleManager
[ArticleManager setSharedInstance:nil];

// Objective-C Singleton Pattern Updated For Testability
@implementation ArticleManager

static ArticleManager *_sharedInstance = nil;
static dispatch_once_t once_token = 0;

+(instancetype)sharedInstance {
    dispatch_once(&once_token, ^{
        if (_sharedInstance == nil) {
            _sharedInstance = [[ArticleManager alloc] init];
        }
    });
    return _sharedInstance;
}

+(void)setSharedInstance:(ArticleManager *)instance {
    once_token = 0; // resets the once_token so dispatch_once will run again
    _sharedInstance = instance;
}

@end

Reference:Objective-C Singleton Pattern Updated For Testability

Reference:

OCMock
Making Fun of Things with OCMock
OCMock Test Origami
IMPROVING IOS UNIT TESTS WITH OCMOCK