$watchCollection newValue always == oldValue

[mattlanders]

I trying to use the $watchCollection to watch an array, and I noticed that the newvalue is always equal to the oldvalue. After looking at the code, it looks like the values are copied to oldValue prior to firing the listener. It should be an easy fix.

[Soviut]

I noticed this too when trying to iterate over the oldCollection to remove Google Maps markers.

[appsforartists]

Reduction:

http://jsfiddle.net/Y8yEZ/6/

[Soviut]

I think you mean "reproduction". I legitimately thought maybe you were talking about a "reduce" function at first ;)

[appsforartists]

@Soviut http://www.webkit.org/quality/reduction.html

[Soviut]

Ah I see. I've always just called them test cases, examples or even proofs. I still stand by the fact that the language is ambiguous given that "reduce" is a common enough term general programming.

Thanks for the test case/example/proof though! ;)

[appsforartists]

$watchCollection seems to be exceptionally flaky. Not only is there this unbelievably glaring issue (really, I'm not sure how a bug this big makes it to release at a place like Google), but in my limited testing, it seems to be behaving more like a deep watch than a shallow one. It's not my codebase, so I don't yet know if there's something particularly funky in the app, but I'm getting watchCollection calls just by changing the details of objects nested within the array I'm watching.

[Soviut]

There isn't much documentation on $watchCollection so the intent is a bit ambiguous. On first thought, I'd say that bubbling child events through the parent collection makes sense. That said, how do you only get it to watch the collection itself?

[appsforartists]

$watchCollection: Shallow watches the properties of an object and fires
whenever any of the properties change (for arrays this implies watching the
array items, for object maps this implies watching the properties).

-- docs

If you want to deep watch, you can $scope.$watch(..., true).

For instance:

cart = []

$scope.$watch(cart, listenerA, false);     // standard watch
$scope.$watch(cart, listenerB, true);      // deep watch
$scope.$watchCollection(cart, listenerC);  // shallow watch

cart = [{'item': 'blackberries', 'quantity': 3}, {'item': 'pears',

'quantity': 1}];
// The value of cart is now different and all three listeners fire.

cart.push({'item': 'apples', 'quantity': 5});
// The value of cart is the same, but it contains a new value.

listenerB and listenerC fire.

cart[0].quantity++;
// The value of cart is the same and it has the same contents.  A

property on one of the items in the cart has changed. Only listenerB fires.

On Fri, May 31, 2013 at 1:12 AM, Soviut notifications@github.com wrote:
There isn't much documentation on $watchCollection so the intent is a bit
ambiguous. On first thought, I'd say that bubbling child events through the
parent collection makes sense. That said, how do you only get it to watch
the collection itself?

—
Reply to this email directly or view it on GitHub.

[appsforartists]

Here's a new test for this:

Versal@2c83c08?w=0

[appsforartists]

@mattlanders I followed your advice and moved all the side-effects from watchCollectionWatch into watchCollectionAction, leaving only the change detecting logic in watchCollectionWatch. Unfortunately, that doesn't seem to work. (The watchCollection tests are all failing now.)

I'd have to spend some more time reading and debugging this before I'd be able to submit a solid pull request.

[appsforartists]

Alright. I'm getting close. Here's the work so far.

All the watchCollection tests are passing. Here are the failures:

FAILED ngRepeat stability should throw error on adding existing duplicates and recover 
FAILED ngRepeat stability should throw error on new duplicates and recover 

And here is the error causing those failures:

[NgErr27] 10 $digest() iterations reached. Aborting!  Watchers fired in the last 5 iterations: []

The fixed version of watchCollection seems to be triggering more digests than the old version. I had to add this line to keep duplicate lines from showing up in the watchCollection tests. I imagine that whatever was causing watchCollectionAction to be called multiple times in digest is the same thing causing these failures.

[appsforartists]

Also, I added a note to the docs about newCollection and oldCollection being of different types. It would be a better choice if newCollection and oldCollection were always the same type (in this case, a POJO or an Array), but that has the potential to be a breaking change.