How to roll back changes when there is an error in a promise chain

问题

In my angular app I want to make changes to several locations in my firebase with a mix of transactions and set. I have written a promise chain with a little help. Now I need to handle any errors that may occur.

In the event of an error on any of the promises I would want to roll back any changes made in firebase (the successful promises) and alert the user to the failure.

Current code below

$scope.addNewPost = function() {
  var refPosts = new Firebase(FBURL).child('/posts').push();
  // Get tags into array for incrementing counters
  var tags = $scope.post.tags.split(', ');
  var allPromises = [];
  // Iterate through tags and set promises for transactions to increment tag count
  angular.forEach(tags, function(value, index){
    var dfd = $q.defer();
    var refTag = new Firebase(FBURL).child('/tags/' + value);
    refTag.transaction( function (current_value) {
      return current_value + 1;
    }, function(error, committed, snapshot) {
      if (committed) {
        dfd.resolve( snapshot );
      } else {
        dfd.reject( error );
      }
    });
    allPromises.push( dfd.promise );
  });

  // Add promise for setting the post data
  var dfd = $q.defer();
  refPosts.set( $scope.post, function (error) {
    if (error) {
      dfd.reject(error);
    } else {
      dfd.resolve('post recorded');
    }
  });
  allPromises.push( dfd.promise );

  $q.all( allPromises ).then(
    function () {
      $scope.reset(); // or redirect to post
    },
    function (error) {
      // error handling goes here how would I
      // roll back any data written to firebase
      alert('Error: something went wrong your post has not been created.');
    }
  );
};

So what I need to know is how do I roll back any changes that happen to my firebase data in the event that one of these promises fail. There could be any number of updates happening in firebase. (for example: 3 tags being incremented via transaction and the post data being set)

How would I write the failure function to calculate what was successful and undo it? If this is this even possible.

--------------- sub question from original post has been solved ---------------

Also how do you force errors? I've tried setting a variable like below but it doesn't seem to work, is there something wrong with my .then?

refPosts.set( $scope.post, function (error) {
  var forceError = true;
  if (forceError) {
    dfd.reject(forceError);
  } else {
    dfd.resolve('post recorded');
  }
  allPromises.push( dfd.promise );
});

回答1:

There are two instances of this line, and they are both in the wrong place:

allPromises.push( dfd.promise );

In the first block, it should be in the last statement in the forEach callback, not in the transaction callback.
In the second block, it should be after the call to set(), not in the callback.

The way your code is written now, $q.all() is getting an empty array of promises. That could also be what's interfering with the forceError test you're attempting.

来源：https://stackoverflow.com/questions/20858242/how-to-roll-back-changes-when-there-is-an-error-in-a-promise-chain

标签

angularjs

error-handling

firebase

promise

angularfire