Freeze or manually/conditionally re-evaluate a scope

[dgieselaar]

Hi all,

I would love to see more control over whether a Scope's watchers should be re-evaluated. Currently, there's no way to 'freeze' a Scope as far as I can tell (without writing really weird brittle workarounds w/ private APIs). React has a shouldComponentUpdate method which is pretty nice and perhaps something could be of use to Angular.

Suppose a Scope has a $shouldEvaluate method, which accepts one parameter, which could either be:

• 'inherit' the default: re-evaluates if the Scope's parent is re-evaluated.
• true: always re-evaluate a Scope in a $digest loop.
• false: never re-evaluate (effectively freezes the Scope).
• getEvalIdentifier: a function which is called before re-evaluating the Scope's watchers. Its result is compared to the previous result. If they're not equal, the Scope is re-evaluated. This, for example, could be an id of an object.

Additionaly, a Scope would have a $evaluateNow method, which (obviously) would immediately re-evaluate all watchers.

This approach does mean child directives could for example freeze parent scopes, which is pretty bad, so an alternative would perhaps be to set this through a DDO (in conjuction with a child or isolated scope).

[petebacondarwin]

@Plestik - Have you taken a look at @shahata's idea here #10658

[dgieselaar]

@petebacondarwin aye, thanks, didn't see that. That looks good. It's much simpler. My suggestion is perhaps a little too complex, but I think having a function that returns a value to compare is pretty useful. Without it you'd have to suspend/resume manually (in response to events? not sure).

I'd be happy to put in the work for a PR for this, but would like to gather some feedback on whether this is actually not a terrible idea first.

[joshrtay]

+1 for shouldevaluate method on scope

[Narretz]

We are planning to add #10658 which obsoletes this feature proposed here