Frequently, no wonder they write that this is a difficult topic for understanding. I did not understand how most.
Where to use this thing?

Example:

. Directive ('Clickable', Function () {
Return {
 RESTRICT: "E",
 Link: Function ($ Scope, Element, Attrs) {
  element.bind ('Click', Function () {
 $ scope. $ Apply (Function () {
   $ scope.user ++;
   $ scope.bar ++;
 });
  Console.log ('1')
  });
 }
}
});

without wrapping $ scope. $ Apply () Example will not work.

I do not know, I found such a note:

Important: Any browser events are called out of the visibility of Angularjs, so within your handlers such events you need to call $ scope. $ Apply

Answer 1, Authority 100%

So, what exactly is not clear? Functions $ Apply / $ Digest launch Dirty-Check, and if the data in $ scope has changed, then update the view, that’s all the magic. When you change the data inside Angular, for example, on the NG-Click, it calls the $ Apply method if your code is not performed via Angular, then you need to run Dirty-Check hands. Here are very approximate examples:

ng-click

& lt; button ng-click = "counter ++" & gt; counter & lt; / button & gt;

inside yourself angular, replaced this design to:

jqlite ('buck'). ON ('Click', Function (EVT) {
    EVT.PreventDefault ();
    $ scope.counter ++;
    $ scope. $ apply ();
});

or

$ timeout (function () {$ scope.foo = "bar";}, 100);
// Equivalent
SetTimeout (Function () {
  $ scope.foo = "Bar";
  $ scope. $ apply ();
}, 100);

All this is very about, but the point is that when you use binding or Angular methods, he causes $ Apply / $ Digest for you, that’s all, nothing complicated here.