Editing WoW:API hooksecurefunc (section)

==Notes==
This is the only safe way to hook functions that execute protected functionality.

The ''hookfunc'' is invoked after the original function, and receives the same parameters; return values from hookfunc are discarded. 

If another addon, or the same addon, replaces ''functionName'' after you use this function to hook it, ''hookfunc'' will no longer fire. ... except of course if your new function calls the old original!
 hooksecurefunc("ToggleBackpack", function(...)
     print("ToggleBackpack called.")
 end)
 ToggleBackpack = function(...)
     -- Do something different
 end
 ToggleBackpack() -- "ToggleBackpack called." never prints.

After hooksecurefunc() is used on a function, setfenv() throws an error when attempted on that function

You cannot "unhook" a function that is hooked with this function except by a UI reload. Calling hooksecurefunc() multiple times only ''adds'' more hooks to be called.

  hooksecurefunc("ToggleBackpack", function() print(1) end)
  hooksecurefunc("ToggleBackpack", function() print(2) end)

  ToggleBackpack()
  > 1
  > 2

Also note that using hooksecurefunc() may not always produce the expected result. Keep in mind that it actually replaces the original function with a new one (the function reference changes).

For example, if you try to hook the global function "MacroFrame_OnHide" (after the macro frame has been displayed), your function will not be called when the macro frame is closed.

It's simply because in Blizzard_MacroUI.xml the OnHide function call is registered like this:
 <OnHide function="MacroFrame_OnHide"/>
The function ID called when the frame is hidden is the former one, before you hooked "MacroFrame_OnHide".
The workaround is to hook a function called by "MacroFrame_OnHide":
 hooksecurefunc(MacroPopupFrame, "Hide", MyFunction)
However, when you want to hook a frame's script handler, it's better to use [[API_Frame_HookScript|Frame:HookScript]] like this:
 MacroPopupFrame:HookScript("OnHide", MyFunction) -- the client will call MacroFrame_OnHide(MacroPopupFrame) first followed by MyFunction(MacroPopupFrame)