$scope.$watch listener expressions can access $newVal & $oldVal

[flazz]

a string listener expression can evaluate $newVal and $oldVal

example:

$scope.$watch('foo', 'foo = $newVal');

[pkozlowski-opensource]

@flazz what is the motivation behind this change? If I understand correctly it just makes it easier for people to write JavaScript functions as strings (expressions). I've got my private opinion on this but before sharing it I would love to hear more about your use-case here.

[petebacondarwin]

So there is the question of whether it is a good idea to allow strings as listeners at all!
If AngularJS is going to continue to support string listeners in the future then I think this is a good addition.

[flazz]

My biggest practical use case (to date) is sample above.

Also the case where the watchExpression is a function and not a simple expression, returning a value not in scope.

I like @peterbacondarwin's reasoning, I feel without access to old and new there is a half feature. How much value does the full feature provide?.

Also is $newVal and $oldVal the best naming aesthetics? would $newValue or $new suffice better?

[pkozlowski-opensource]

@flazz I'm not sure I understand your example (please bear with me...). Are you talking about this one:

$scope.$watch('foo', 'foo = $newVal');

If so, it seems to be assigning a new value value to a variable... that its changing its value to a new one anyway (this is why the watcher is executing in the first place). I'm sure that I'm missing something here so if you could provide a practical use-case it would be awesome!

[petebacondarwin]

This is effectively a no-op!

$scope.$watch('foo', 'foo = $newVal');

I could imagine calling a function...

$scope.$watch('foo', 'saveModel($newVal, $oldVal)');

Also, I think that there should also be a unit test where $oldVal was not undefined.

[jtymes]

@petebacondarwin Wouldn't your second example already be taken care of by the $watch function?

$scope.$watch('foo', saveModel);

Where saveModel is defined as saveModel(newVal, oldVal)

[pkozlowski-opensource]

@petebacondarwin This is exactly my point, the first example does nothing... unless I'm mistaken.

While I could also imagine this call $scope.$watch('foo', 'saveModel($newVal, $oldVal)'); I'm not sure what is the added value of this as compared to simply writing:

$scope.$watch('foo', $scope.saveModel);

There must be something I'm missing here...

[petebacondarwin]

To some extent I agree, hence my thought that string based listeners could be dropped altogether. But in defence of my example, you may be passed a string for some reason - perhaps as an attribute on a directive.

[flazz]

@petebacondarwin the below case illustrates a non-noop case

$scope.watch(function(_scope) { return myService.val() }, 'scope.x = $newVal');

This could easily be written with the listener as a function. Is there a use to string expression listeners beyond a shorthand sugar?

[pkozlowski-opensource]

@flazz for me it just moves to string something that could be written as a normal function. So it would be -1 from me but I might be missing a good use-case here...

@IgorMinar could you have a quick look and have a final word here?

[IgorMinar]

-1

we should not be encouraging people to put code into strings within javascript files.

closing...

[petebacondarwin]

Then should we not deprecate the use of strings as listeners altogether?

On 8 January 2013 21:53, Igor Minar notifications@github.com wrote:

-1

we should not be encouraging people to put code into strings within
javascript files.

closing...

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