feat(#13): Add priority for tasks execution

Closes #13.

README.md

@@ -105,6 +105,16 @@ end).
 
 In this case, the second task won't be executed and the call to `nhooks:do/3` will return `{stopped, {some, data}}`.
 
+All tasks are registered with a default priority of `0` and then executed from lowest to highest priority. That priority can be passed as the last argument of `nhooks:registar_task/4`:
+
+```erl
+
+ok = nhooks:register_task(test_app, init, fun(Term) -> first, -1).
+ok = nhooks:register_task(test_app, init, fun(Term) -> second end).
+ok = nhooks:register_task(test_app, init, fun(Term) -> third end, 5).
+
+```
+
 
 ## Support
 

src/nhooks.erl

@@ -20,13 +20,15 @@
 -export([
     do/3,
     register_task/3,
+    register_task/4,
     deregister_task/2,
     deregister_app_tasks/1,
     consult_tasks/2,
     consult_app_tasks/1
 ]).
 
 %%%% MACROS
+-define(DEFAULT_PRIORITY, 0).
 -define(PERSISTENT_TERM_KEY(AppName, Hook),
     {nhooks, AppName, Hook}
 ).
@@ -38,11 +40,14 @@ do(AppName, Hook, Args) ->
     Hooks = persistent_term:get(?PERSISTENT_TERM_KEY(AppName, Hook), []),
     do(AppName, Hook, Hooks, Args).
 
-register_task(AppName, Hook, Task) when is_function(Task) ->
-    do_register_task(AppName, Hook, Task);
-register_task(AppName, Hook, {M, F} = Task) when is_atom(M), is_atom(F) ->
-    do_register_task(AppName, Hook, Task);
-register_task(_AppName, _Hook, Other) ->
+register_task(AppName, Hook, Task) ->
+    register_task(AppName, Hook, Task, ?DEFAULT_PRIORITY).
+
+register_task(AppName, Hook, Task, Priority) when is_function(Task) ->
+    do_register_task(AppName, Hook, Task, Priority);
+register_task(AppName, Hook, {M, F} = Task, Priority) when is_atom(M), is_atom(F) ->
+    do_register_task(AppName, Hook, Task, Priority);
+register_task(_AppName, _Hook, Other, _Priority) ->
     throw({badarg, Other}).
 
 deregister_task(AppName, Hook) ->
@@ -65,7 +70,7 @@ consult_app_tasks(AppName) ->
 %%%-----------------------------------------------------------------------------
 do(_AppName, _Hook, [], _Args) ->
     ok;
-do(AppName, Hook, [Task | T], Args) ->
+do(AppName, Hook, [{_Priority, Task} | T], Args) ->
     try
         case do_apply(Task, Args) of
             stop -> stopped;
@@ -83,17 +88,15 @@ do_apply({Mod, Fun}, Args) ->
 do_apply(Fun, Args) when is_function(Fun) ->
     erlang:apply(Fun, Args).
 
-do_register_task(AppName, Hook, Task) ->
+do_register_task(AppName, Hook, Task, Priority) ->
     Hooks = AppName:hooks(),
     case lists:member(Hook, Hooks) of
         true ->
-            ExistentTasks = persistent_term:get(
-                ?PERSISTENT_TERM_KEY(AppName, Hook), []
-            ),
-            persistent_term:put(
-                ?PERSISTENT_TERM_KEY(AppName, Hook),
-                ExistentTasks ++ [Task]
-            );
+            Key = ?PERSISTENT_TERM_KEY(AppName, Hook),
+            ExistentTasks = persistent_term:get(Key, []),
+            NewTasks = ExistentTasks ++ [{Priority, Task}],
+            SortedTasks = lists:sort(fun({X,_}, {Y,_}) -> X =< Y end, NewTasks),
+            persistent_term:put(Key, SortedTasks);
         false ->
             erlang:error('non_existent_hook')
     end.

test/nhooks_SUITE.erl

@@ -35,6 +35,7 @@ all() ->
         do_error_no_log,
         do_until_stopped,
         do_until_stopped_with_exceptions,
+        do_until_stopped_with_priority,
         execution_of_one_fun_per_hook,
         execution_of_one_mod_fun_per_hook,
         execution_of_several_tasks_per_hook,
@@ -214,6 +215,25 @@ do_until_stopped_with_exceptions(_Conf) ->
     3 = counters:get(Counter, 1),
     ok.
 
+do_until_stopped_with_priority() ->
+    [{userdata, [{doc, "Tests do until stopped with priority"}]}].
+
+do_until_stopped_with_priority(_Conf) ->
+    Counter = counters:new(1, []),
+    ok = nhooks:register_task(?APP_NAME, init, fun(CounterRef) ->
+        counters:add(CounterRef, 1, 1)
+    end, 0),
+    ok = nhooks:register_task(?APP_NAME, init, fun(CounterRef) ->
+        counters:add(CounterRef, 1, 1),
+        {stop, {some, data}}
+    end, 5),
+    ok = nhooks:register_task(?APP_NAME, init, fun(CounterRef) ->
+        counters:add(CounterRef, 1, 1)
+    end, 3),
+    {stopped, {some, data}} = ?APP_NAME:init(Counter),
+    3 = counters:get(Counter, 1),
+    ok.
+
 execution_of_one_fun_per_hook() ->
     [
         {userdata, [