Ember.computed.defaultTo behaves in an unexpected way

[simonexmachina]

Ember.computed.defaultTo defaults to the value of the other property only once, and doesn't update when the other property changes. This seems very unexpected to me. Maybe we could add the ability to indicate that the default should update when the other property changes eg. defautlTo(defaultPath, bound).

Here's a JSBin that shows the unexpected behaviour.

[rwjblue]

Yep, that is one of the reasons it is now deprecated (deprecation added in #4979).

[rwjblue]

Use Ember.computed.oneWay for scenarios where you want the properties value to be in sync with another property until you set it (at which point it will diverge from the upstream).

[simonexmachina]

Okay, for anyone who's interested here's my implementation of defaultTo that has a binding to the default value:

function defaultTo(property, defaultPath) {
  return Ember.computed(defaultPath, function(key, val) {
    var value = this['_' + property];
    if (arguments.length == 1) {
      return Ember.isNone(value) ? this.get(defaultPath) : value;
    }
    else {
      return this['_' + property] = val;
    }
  });
}

[rwjblue]

The Ghost folks have a great implementation:

/**
 * Defines a property similarly to `Ember.computed.oneway`,
 * save that while a `oneway` loses its binding upon being set,
 * the `BoundOneWay` will continue to listen for upstream changes.
 *
 * This is an ideal tool for working with values inside of {{input}}
 * elements.
 * @param transform: a function to transform the **upstream** value.
 */
var BoundOneWay = function (upstream, transform) {
    if (typeof transform !== 'function') {
        //default to the identity function
        transform = function (value) { return value; };
    }
    return Ember.computed(upstream, function (key, value) {
        return arguments.length > 1 ? value : transform(this.get(upstream));
    });
};

export default BoundOneWay;

[simonexmachina]

Great, thanks.​