Use

With batching, it's possible for a piece of code to read some observable, and listen to changes in that observable, but have events fired that it should ignore.

For example, consider a list widget that creates <li>'s for each item in the list and listens to updates in that list and adds or removes <li>s:

var makeLi = function(){
  return document.createElement("li")
};

var listWidget = function(list){
  var lis = list.map(makeLi);
  list.on("add", function(ev, added, index){
    var newLis = added.map(makeLi);
    lis.splice.apply(lis, [index, 0].concat(newLis) );
  }).on("remove", function(ev, removed, index){
    lis.splice(index, removed.length);
  });

  return lis;
}

The problem with this is if someone calls listWidget within a batch:

var list = new DefineList([]);

canBatch.start();
list.push("can-event","can-event/batch/");
listWidget(list);
canBatch.stop();

The problem is that list will immediately create an li for both can-event and can-event/batch/, and then, when canBatch.stop() is called, the add event listener will create duplicate lis.

The solution, is to use afterPreviousEvents:

var makeLi = function(){
  return document.createElement("li")
};

var listWidget = function(list){
  var lis = list.map(makeLi);
  canBatch.afterPreviousEvents(function(){
    list.on("add", function(ev, added, index){
      var newLis = added.map(makeLi);
      lis.splice.apply(lis, [index, 0].concat(newLis) );
    }).on("remove", function(ev, removed, index){
      lis.splice(index, removed.length);
    });
  });

  return lis;
}