gistfile1.txt

function getSomeData() {
  // perform some asynchronous operation, resolve or reject the promise when appropriate.
  return $q(function(resolve, reject) {
    $http({
      method: 'GET',
      url: '/someUrl'
    })
    .then(function successCallback(response) {
        resolve(respose);
      }, function errorCallback(response) {
        reject(respose);
    });
  });
}

function doStuffBeforeGettingData(stuff) {
  return $q(function(resolve, reject) {
    
    // Validate stuff.
    
    if (stuffnotvalid) {
      // Create error reasons
      reject(reasons) 
    } else {
      getSomeData().then(resolve, reject)
    }
    
  });
}


// In a Controller or something...

$myService.doStuffBeforeGettingData(things)
.then(function(results){
    // do stuff with results  
  }, 
  function(error){
    // show error to user
  }
)