How can I detect a change to an entity's EntityState?

Question

I want to put a "Delete" button and a "Cancel" button on each row of a list of Customers. The "Cancel" button is disabled when a customer is "Unchanged". But when a customer transitions to a changed state ("Added", "Modified", "Deleted"), I want to enable the "Cancel" button so the user can reverse the changes -- whatever they are -- before saving.

I can almost do this by subscribing to customer.entityAspect.propertyChanged. A property change signals a potential change in the EntityState. I can subscribe to that event and have my handler update an isChanged observable that I've added to my Customer entities. Then I bind the "Cancel" button enable to the isChanged and I'm good to go.

But the propertyChanged event is only raised when a data property changes, e.g., customer.Name("New Co.");. It isn't raised when the user clicks the "Delete" button. "Delete" triggers customer.entityAspect.setDelete(); which doesn't touch a data property; it simply changes the customer's EntityState.

(1) Why doesn't a change to the customer's EntityState raise propertyChanged and (2) how can I listen for a change to the EntityState so I can control the "Cancel" button?

P.S.: I'm using Knockout.

P.P.S: This question was inspired by a previous SO question "entityAspect.setDeleted doesn't fire the subscribed propertyChanged event".

Answer 1

You are correct that Breeze does not raise propertyChanged whent the EntityState changes. Maybe it should. We will consider that.

Nor does Breeze have a separate event on the entity - no entityStateChanged event - to notify you when the EntityState changes. We've considered that several times. We keep talking ourselves out of it.

There is a perfectly good solution that performs better than a dedicated entityStateChanged event. Right now you'll have to code it yourself.

The trick is to listen to the EntityManager, not to the entity. You'll find one variant of this solution in the DocCode "Teach Tests" sample; look for "can control custom ko entityState property via entityManager.entityChanged" in the entityTest.js module.

I'll tweak that to fit your example. The essence of it is as follows:

1. Subscribe to the entityManager.entityChanged event; when it is raised and the cause is that an entity's EntityState changed, you update that entity's isChanged boolean KO observable (if that property exists).

2. Add the isChanged observable to entity types that should be watched in this way.

Here's an example of step #1: listening for state changes

// subscribe with handler watching for EntityState changes
addEntityStateChangeTracking(manager);

function addEntityStateChangeTracking(entityManager) {

    if (entityManager._entityStateChangeTrackingToken) { return; } // already tracking it

    // remember the change tracking subscription with a token; 
    // might unsubscribe with that token in future
    entityManager._entityStateChangeTrackingToken =
        entityManager.entityChanged.subscribe(entityChanged);

    var entityStateChangeAction = breeze.EntityAction.EntityStateChange;

    function entityChanged(changeArgs) {            
        if (changeArgs.entityAction === entityStateChangeAction) {
            var entity = changeArgs.entity;
            if (entity && entity.isChanged) { // entity has the observable
                var isUnchanged = entity.entityAspect.entityState.isUnchanged();
                entity.isChanged(!isUnchanged);
            }
        }
    }
}

Let's talk about step #2: adding the isChanged observable to the type. You seem to have tackled that one but I'm not sure how. Perhaps the best place to do add it to the type is in the type's initializer so you can be sure the property will be there, whether the entity is created or materialized by a query. Here's an example:

var store = manager.metadataStore;

function customerInit(entity) {
    var isUnchanged = entity.entityAspect.entityState.isUnchanged();
    entity.isChanged = ko.observable(!isUnchanged);
}

store.registerEntityTypeCtor('Customer', null, customerInit);

This all seems like a lot of work. It would be easier if Breeze raised the propertyChanged event when the EntityState changes. We'll give that more consideration ... there may be some good counter arguments. Meanwhile, I think what you see here is the best approach.