How to call iframe controller function from parent

Question

I have a iframe in my parent window. I use angularjs on both the parent and iframe. Normally, when i call a function in iframe I use

document.getElementById('myframe').contentWindow.function_name();

I dont know how to call a angularjs controller function. My controller in the iframe looks like this

app.controller('UserController',['$scope', function($scope){
    this.showOptions = function(type){
    alert(type);
}

how can I call this.showoptions in the iframe from parent.