js/libs/ember-1.5.1.js
function(startIdx, removeAmt, addAmt) {

    // if no args are passed assume everything changes
    if (startIdx===undefined) {
      startIdx = 0;
      removeAmt = addAmt = -1;
    } else {
      if (removeAmt === undefined) removeAmt=-1;
      if (addAmt    === undefined) addAmt=-1;
    }

    // Make sure the @each proxy is set up if anyone is observing @each
    if (Ember.isWatching(this, '@each')) { get(this, '@each'); }

    Ember.sendEvent(this, '@array:before', [this, startIdx, removeAmt, addAmt]);

    var removing, lim;
    if (startIdx>=0 && removeAmt>=0 && get(this, 'hasEnumerableObservers')) {
      removing = [];
      lim = startIdx+removeAmt;
      for(var idx=startIdx;idx<lim;idx++) removing.push(this.objectAt(idx));
    } else {
      removing = removeAmt;
    }

    this.enumerableContentWillChange(removing, addAmt);

    return this;
  }
function(startIdx, removeAmt, addAmt) {

    // if no args are passed assume everything changes
    if (startIdx===undefined) {
      startIdx = 0;
      removeAmt = addAmt = -1;
    } else {
      if (removeAmt === undefined) removeAmt=-1;
      if (addAmt    === undefined) addAmt=-1;
    }

    // Make sure the @each proxy is set up if anyone is observing @each
    if (Ember.isWatching(this, '@each')) { get(this, '@each'); }

    Ember.sendEvent(this, '@array:before', [this, startIdx, removeAmt, addAmt]);

    var removing, lim;
    if (startIdx>=0 && removeAmt>=0 && get(this, 'hasEnumerableObservers')) {
      removing = [];
      lim = startIdx+removeAmt;
      for(var idx=startIdx;idx<lim;idx++) removing.push(this.objectAt(idx));
    } else {
      removing = removeAmt;
    }

    this.enumerableContentWillChange(removing, addAmt);

    return this;
  }
If you are implementing an object that supports `Ember.Array`, call this
method just before the array content changes to notify any observers and
invalidate any related properties. Pass the starting index of the change
as well as a delta of the amounts to change.

@method arrayContentWillChange
@param {Number} startIdx The starting index in the array that will change.
@param {Number} removeAmt The number of items that will be removed. If you
pass `null` assumes 0
@param {Number} addAmt The number of items that will be added. If you
pass `null` assumes 0.
@return {Ember.Array} receiver