Angular lazy tree - angularjs

Question

I'm studying angular and trying to write treeview with dynamic load. It has some initial pre-loaded nodes, but many of them have to be loaded by demand.
I write initial tree view here jsfiddle but cannot understand how to implement lazy load.
when I click NodeLazy I want to update treeService.Nodes and this update have to refresh view
when I click any Node I want select this node by bold font and be able get selected from treeService
Sure, I could use one of existing angular tree, but I want to understand how to do this.
var app = angular.module('demo', []);
app.directive("treeView", function() {
return {
restrict: "E",
replace: true,
scope: {
t: "=src"
},
template: "<ul class='ul-node'><tree-branch ng-repeat='c in t.children' src='c'></tree-branch></ul>"
};
});
app.directive("treeBranch", function($compile) {
return {
restrict: "E",
replace: true,
scope: {
b: "=src"
},
template: "<li class='{{b.class_li}}'>{{b.name}}</li>",
controller: function($scope) {
var has_children = angular.isArray($scope.b.children);
$scope.b.class_li = has_children ? "li-node" : "li-leaf";
},
link: function(scope, element, attrs) {
var has_children = angular.isArray(scope.b.children);
if (has_children) {
element.append('<tree-view src="b"></tree-view>');
$compile(element.contents())(scope);
}
element.on("click", function(event) {
console.log("click|" + scope.b.name);
event.stopPropagation();
if (has_children) {
element.toggleClass("ul-collapsed");
}
});
}
};
});
app.service("treeService", function() {
var nodes = {
children: [
{ name: "Node_1",
children: [
{ name: "Node_1.1",
children: [{ name: "Leaf_1" }, { name: "Leaf_2" }] },
{ name: "NodeLazy_1.2", }] },
{ name: "NodeLazy_2", }]
};
return {
Nodes : nodes
};
});
app.controller("ctrl", function($scope, treeService) {
$scope.nodes = treeService.Nodes;
});