Observer on computed property is firing when the computed value is unchanged

[adamlogic]

Here is a jsfiddle to demonstrate the issue: http://jsfiddle.net/NfEgM/3/

When "set" is called on the "prop" property, the observer only fires if it is set with a new value. This is expected behavior.

The "computedProp" observer is firing every time, though, even when the computed value is the same. I believe this is unexpected behavior. The value is being recalculated because one of it's dependencies changes, but I think the observer should only fire if the actual value changes.

Is this a bug or intended behavior?

[wagenet]

@adamlogic There's no way around it. We don't compute CPs until get is called, which happens after the observer is fired. If we checked right away, we'd have to compute much more frequently and performance would suffer.

[adamlogic]

@wagenet Understood. Would you be open to a docs patch that clarifies this behavior?

[wagenet]

@adamlogic Certainly. It would definitely be worth highlighting the fact that CPs are lazily computed. It's totally worth it, but sometimes it catches people by surprise, especially if they are trying to use CPs when they should use observers.

[adamlogic]

@wagenet Added a pull request. Curious if you have any examples handy of people using CPs when they should use observers.

[wagenet]

I once saw someone trying to do responsive style behavior in a CP, definitely an odd idea. The main thing you'll notice is that observers fire instantly, whereas bindings wait until the end of the runloop and CPs don't fire until requested.