New to Angular - Computed Variables

Question

I am moving to Angular from Knockout, and I have a few issues. I'm assuming that I must be doing something a non-angular type of way.

http://jsfiddle.net/LostInDaJungle/BxELP/

I linked to jsfiddle so I didn't have to include my code here Stack Overflow will not let me post my question without a code block. 

Here is a very basic fiddle that outlines two of my major problems...

Problem 1: val1 and val2 are initialized as 3 and 4, and add up to 7 properly. However, if you change either of the values in the text boxes, the new value is treated as a string and I get concatenation instead of addition. Change val1 to 4 and you get 44 when it should be 8. What is the best way around this behaviour?

Problem 2: Calculated fields. I can get a calculated field by using the curly brackets like {{val1 + val2}} and have the calculated fields auto update when the underlying model changes, but this is totally unacceptable. In my full fledged app, we generate a "cost" that is used several times throughout and having to put in the cost calculation each and every time is a pain. Not to mention that when this calculation changes, I now have the unenviable task of finding 15 different places that use the cost calculation and updating them all.

In addition, if I try to put an ng-model="cost" on the input with the curly brackets, then the curly brackets don't work. So nothing jumps out at me as a way to bind cost.

http://jsfiddle.net/LostInDaJungle/QNVwe/

This example is more like the structure I desire. However, unlike a ko.observable, the calculated fields do not update when the values that generate them change. The boilerplate solution everyone has foisted on me is to write a bunch of ng-change handlers... But that is awful. If width changes change the cost and change the payback calculations, etc... It quickly becomes a tangled mess.

Both of these methods fail as far as separating logic from presentation. Method one has my business logic embedded in my HTML. Method two puts a whole bunch of ng-change handlers in my code which isn't that much different from having to write a whole mess of onChange handlers in plain ol' HTML. If I HAVE to do a bunch of ng-change handlers, I would just as soon do an onChange handler in Javascript because I can at least declare them outside of my presentation layer.

Here's a knockout version of the same:

http://jsfiddle.net/LostInDaJungle/eka4S/2/

This is more like what I would expect... Nothing but data-binds on my inputs, all program logic nicely contained within the view model. Also, since my computable is a Javascript function, I don't have to scratch my head about how to ensure my two values are numeric.

So....

Computed variables: Is there a way to watch the underlying variables and update the computed amount automatically? Without having to bury my program logic in my HTML?

Is there a good way to keep my numbers from turning into strings?

Thank you for your help.

FYI, also posted to Google Groups: https://groups.google.com/forum/#!topic/angular/0dfnDTaj8tw

Answer 1

For a calculated field, add a method to your controller . . .

$scope.cost = function() { return $scope.val1 + $scope.val2 }; 

and then bind to it directly. It will know when it needs to recalculate as its constituent values change.

<div>{{cost()}}</div> 

Answer 2

Ok,

A few hours later and I think I have my answer.

Using $scope.$watch.

$scope.$watch('(height * width) * 40', function(v) {$scope.cost = v;}); 

or

$scope.$watch('height + width', function() {$scope.cost = (Number(height) * Number(width)) * 40;}); 

This auto-updates any computables for watched variables. And it gives me a way to work with these without having to live inside curly brackets.

Also, the computed values can be reused and tracked for cascading updates:

$scope.$watch('height * width', function(v) {$scope.dim = v;}); $scope.$watch('dim * 40', function(v) {$scope.cost = v;}); 

So if height and/or width change, dim is updated, and since dim has changed, cost is updated.