Why does adding additional AngularJS validation directives cause $asyncValidators to run multiple times?

Question

Why does adding additional AngularJS validation directives cause $asyncValidators to run multiple times on page load?

I created a custom directive which

Answer 1

This is because validation directives like ngMaxlength, ngPattern invoke an initial validation cycle with a call to ngModelController.$validate().

This causes all the validation directive to run their validation logic, including the async validators.

One way to prevent the redundant $http calls, and in fact it is a good practice anyway, is to cache the validation result for each input.