Mock Object – Returning a Mock from Another Mock

designmockingprogramming practicestddunit testing

I'm trying to return an object when mocking a parser class. This is the test code using PHPUnit 3.7

    //set up the result object that I want to be returned from the call to parse method
    $parserResult= new ParserResult();
    $parserResult->setSegment('some string');        

    //set up the stub Parser object
    $stubParser=$this->getMock('Parser');
    $stubParser->expects($this->any())
               ->method('parse')
               ->will($this->returnValue($parserResult));

    //injecting the stub to my client class
    $fileWriter= new FileWriter($stubParser);
    $output=$fileWriter->writeStringToFile();

Inside my writeStringToFile() method I'm using $parserResult like this:

writeStringToFile(){
   //Some code...
    $parserResult=$parser->parse();
    $segment=$parserResult->getSegment();//that's why I set the segment in the test.
}

Should I mock ParserResult in the first place, so that the mock returns a mock?
Is it good design for mocks to return mocks?

Is there a better approach to do this all?!

Best Answer

In general having a chain of mocks returning other mocks is considered a code smell in tests.

That being said, the answer of whether this is good idea is of course "it depends".

About your ParserResult object - if it is a small value object then constructing it and passing it in is probably a good idea.

If it is a large object with dependancies of its own, then mocking it would be a better option. Not mocking it would increase the coupling in your tests.

Another question to ask about the design is what is the result of ParserResult.getSegment used for? Perhaps a Tell Don't Ask style might be better, can you tell ParserResult or Parser to do something with the segment information instead of extracting it and then operating on it?

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