Call a function when ng-show is triggered?

I think, it would be better to $watch your model, on which ng-show is bound.

Here is example:

<div ng-app="myApp" ng-controller="myCtrl" ng-init="isDisplayed = true">
    <div ng-show="isDisplayed">something</div>
    <button ng-click="isDisplayed = !isDisplayed">Toggle</button>
</div>

var myApp = angular.module('myApp', [])
.controller('myCtrl', function($scope, $log) {
    $scope.$watch('isDisplayed', function(newValue, oldValue) {
        if (newValue !== oldValue) {
            $log.log('Changed!');
        }
    });
});

and fiddle