Added ifCached function

[Tommatheussen]

As discussed, the .ifCached method uses the same callback method as the .then method.

Right now nothing happens when useLegacyPromiseExtensions is set to false, the application will just throw a general error saying the .cached function does not exist for the method (when it is used). I noticed in the angular code itself they throw the legacy errors like this:

var $httpMinErr = minErr('$http');
var $httpMinErrLegacyFn = function(method) {
  return function() {
    throw $httpMinErr('legacy', 'The method `{0}` on the promise returned from `$http` has been disabled.', method);
  };
};

(https://github.com/angular/angular.js/blob/v1.5.x/src/ng/http.js#L12)

Which they invoke using:

promise.success = $httpMinErrLegacyFn('success');

(https://github.com/angular/angular.js/blob/v1.5.x/src/ng/http.js#L1006)

I will try and see if it's possible to import this method, but it doesn't seem like it. If not, do you think it's worth making it the same kind of error, or should a generic thrown error be sufficient?

[shaungrady]

I'd say just do it like below. I'll add info about it to the docs and add a link in the error.

throw new Error('The method `cached` on the promise returned from `$http` has been disabled.')

Can you write a couple tests please? One testing ifCached and one testing useLegacyPromiseExtensions set to false and cached throwing the error. You can see how I wrote another test about that here: https://github.com/shaungrady/angular-http-etag/blob/master/test/httpDecorator.js#L286-L296

[Tommatheussen]

I'll try to add tests this weekend, as well as the error being thrown.

[shaungrady]

Awesome, thank you so much!