Adding description of how to test a command which expects user input

Author: Sgoettschkes

components/console/helpers/dialoghelper.rst

@@ -97,4 +97,49 @@ You can set the max number of times to ask in the ``$attempts`` argument.
 If you reach this max number it will use the default value, which is given
 in the last argument. Using ``false`` means the amount of attempts is infinite.
 The user will be asked as long as he provides an invalid answer and will only
-be able to proceed if her input is valid.
+be able to proceed if her input is valid.
+
+Testing a command which expects input
+-------------------------------------
+
+If you want to write a unit test for a command which expects some kind of input
+from the command line, you need to overwrite the HelperSet used by the command::
+
+    use Symfony\Component\Console\Helper\DialogHelper;
+    use Symfony\Component\Console\Helper\HelperSet;
+    
+    // ...
+    
+    public function testExecute()
+    {
+    
+        // ..
+
+        $commandTester = new CommandTester($command);
+        
+        $dialog = new DialogHelper();
+        $dialog->setInputStream($this->getInputStream('Test\n')); 
+        // Equals to a user inputing "Test" and hitting ENTER
+        // If you need to enter a confirmation, "yes\n" will work
+        
+        $command->setHelperSet(new HelperSet(array($dialog)));
+        
+        $commandTester->execute(array('command' => $command->getName()));
+    
+        // assert
+        
+    }
+    
+    protected function getInputStream($input)
+    {
+        $stream = fopen('php://memory', 'r+', false);
+        fputs($stream, $input);
+        rewind($stream);
+
+        return $stream;
+    }
+    
+By setting the inputStream of the `DialogHelper`, you do the same the
+console would do internally with all user input through the cli. This way
+you can test any user interaction (even complex ones) by passing an appropriate
+input stream.