Stop execution of a function via hook

[Nico Knoll]

Hi,

is there a way to stop the execution of a function via hook?

/ Nico

[Pete]

Hi Nico

Do you have a more detailed description or a code example of what you're trying to do? Specifically which function you're trying to stop?

[ryan]

Nico, this is something that is technically possible with the way the hook system is designed, but I've not implemented it yet. The idea behind it was that there would be a 'replace' hook, where you can have your own function completely replace the functionality of a core function, so that the core one is never called. I haven't implemented this because it's not so friendly in an environment where there may be multiple modules hooked to a given function. If one module completely takes over a core function, it alters program flow and the behavior that other modules may be expecting. So I've left it out because I think it's a bit too unpredictable, though may still add it in if there's interest. However, you can achieve the same effect by using an 'after' hook and completely replacing the $event->return value with your own. Not as efficient as replacing the execution too, but much more predictable.

[Nico Knoll]

@Ryan: I'm using the $event->return way right now, but the problem is that the original function throws a 404 header. (It's because I'm changing the URLs with my module).

[ryan]

I understand -- I think that's a good reason for us to add a replace hook. For my own testing when implementing this, can you tell me which function it is that you are hooking into?

[Nico Knoll]

$this->addHook('ProcessPageView::execute', $this, 'getPage');

(See the rest of my code at GitHub if you need it )

[ryan]

Thanks, I'll add this capability to the hooks API.

[ryan]

Nico, this has been added in the latest commit of 2.2. To make use of it, have your hooked function (getPage in your case) do this:

$event->replace = true; // now original function won't be called
$event->return = 'you are now providing the return value';

Also your hook obviously must be a 'before' hook in order to replace the original. So you'd init the hook like this:

$this->addHookBefore('ProcessPageView::execute', $this, 'getPage');

Unrelated to the above, but another cool addition: I upgraded the hook system so that it will let you set or get hooked function arguments by name rather than just by position. So if you'd hooked into Pages::save, and you wanted to refer to the 'page' argument, you can now do this:

$page = $event->arguments('page');

Before you could only do this, having to know what position the argument was in:

$page = $event->arguments[0]; 

You can also set arguments back to the event by name if you want to, i.e.

$event->arguments('page', $mypage); 

[Soma]

That's really great to have the arguments by name and not position! Thanks Ryan for the update.

But I'm not quite getting the last one. Maybe a little example could make it more clear?

[apeisa]

Yes, great additions.

Soma, in the example Ryan is changing original page with different/modified one.

[ryan]

But I'm not quite getting the last one. Maybe a little example could make it more clear?

Antti's right about what that example is doing, but let me do another. Lets say we've hooked in before $session->redirect(). If I go look at the function definition in /wire/core/Session.php, I can see it has two arguments: $url and $http301. So lets say I want to hook into that and automatically add a GET variable to every redirected URL, like "url.com/path/?is_redirected=1", I'd do this:

public function hookSessionRedirect(HookEvent $event) {
   $url = $event->arguments('url'); // get the existing value 
   $url .= (strpos($url, '?') === false ? "?" : "&") . "is_redirected=1";
   $event->arguments('url', $url); // set the new value that gets sent to $session->redirect
}

Btw, there is some overhead in using the argument names as opposed to the position (it involves using Reflection), so in most cases I think it's still better to use the argument position rather than the name. But have wanted to make both options available because I think some people will find it more straightforward to to use the argument names... and it no doubt makes code examples more readable.

[Nico Knoll]

Wow, looks nice. Have to try PW 2.2 in the next days. I didn't do it until now because I thought I could wait for the final release