# Framework and Application Hooks

Hooks allow the developers to tie into different pieces of the application. A hook can be defined anywhere, be it internally in the application, or in a plugin. Once a hook is defined, functions can be registered to that hook so that when the hook is called, all functions registered to that hook will be run. By defining a hook, you are saying that you are going to honor that hook somewhere in your application. Using descriptive hook names are good for clarity. For example, `pre_database_connect` is obviously a hook that will be run before a database connection is attempted.

The most important thing to remember when defining hooks for your application is to properly document them. Include whether anything is expected in return or what, if any, arguments will be passed to the hook functions when called.

API Reference:

> - Cement Hook Module

## Defining a Hook

A hook can be defined anywhere, however it is generally recommended to define the hook as early as possible. A hook definition simply gives a label to the hook, and allows the developer (or third-party plugin developers) to register functions to that hook. It’s label is arbitrary.

The most convenient way to define a hook is via `CementApp.Meta.define_hooks`:

    from cement.core.foundation import CementApp

    class MyApp(CementApp):
        class Meta:
            label = 'myapp'
            define_hooks = ['my_example_hook']

Alternatively, if you need more control you might do it in `CementApp.setup()`:

    from cement.core.foundation import CementApp

    class MyApp(CementApp):
        class Meta:
            label = 'myapp'

        def setup(self):
            # always run core setup first
            super(MyApp, self).setup()

            # define application hooks here
            self.hook.define('my_example_hook')

## Registering Functions to a Hook

A hook is just an identifier, but the functions registered to that hook are what get run when the hook is called. Registering a hook function should also be done early on in the bootstrap process, any time after the application has been created, after the hook is defined, and before the hook is run. Note that every hook is different, and therefore should be clearly documented by the ‘owner’ of the hook (application developer, plugin developer, etc).

The most convenient way to register a hook function is with `CementApp.Meta.hooks`:

    from cement.core.foundation import CementApp

    def my_hook1(app):
        pass

    def my_hook2(app):
        pass

    class MyApp(CementApp):
        class Meta:
            hooks = [
                ('post_argument_parsing', my_hook1),
                ('pre_close', my_hook2),
                ]

    with MyApp() as app:
        app.run()

Where `CementApp.Meta.hooks` is a list of tuples that define the hook label, and the function to register to that hook.

Alternatively, if you need more control you might use:

    from cement.core.foundation import CementApp

    def my_hook1(app):
        pass

    with CementApp('myapp') as app:
        app.hook.register('post_argument_parsing', my_hook1)
        app.run()

Or, for a third-party plugin:

    def my_hook1(app):
        pass

    def load(app):
        app.hook.register('post_argument_parsing', my_hook1)

What you return depends on what the developer defining the hook is expecting. Each hook is different, and the nature of the hook determines whether you need to return anything or not. That is up to the developer. Also, the `args` and `kwargs` coming in depend on the developer. You have to be familiar with the purpose of the defined hook in order to know whether you are receiving any `args` or `kwargs`.

## Running a hook

Now that a hook is defined, and functions have been registered to that hook all that is left is to run it. Keep in mind, you don’t want to run a hook until after the application load process... meaning, after all plugins and other code are loaded. If you receive an error that the hook doesn’t exist, then you are trying to register a hook too soon before the hook is defined. Likewise, if it doesn’t seem like your hook is running and you don’t see it mentioned in `--debug` output, you might be registering your hook after the hook has already run.

That said, this is how you run a hook:

    from cement.core.foundation import CementApp

    with CementApp('myapp') as app:
        for res in app.hook.run('my_example_hook'):
            # do something with res?
            pass

As you can see we iterate over the hook, rather than just calling `app.hook.run()` by itself. This is necessary because `app.hook.run()` yields the results from each hook function as they are run. Hooks can be run anywhere after the hook is defined, and hooks are registered to that hook.

## Controlling Hook Run Order

Sometimes you might have a very specific purpose in mind for a hook, and need it to run before or after other functions in the same hook. For that reason there is an optional `weight` parameter that can be passed when registering a hook function.

The following is an example application that defines, registers, and runs a custom application hook:

    from cement.core.foundation import CementApp

    class MyApp(CementApp):
        class Meta:
            label = 'myapp'

        def setup(self):
            # always run core setup
            super(MyApp, self).setup()

            # define hooks in setup
            self.hook.define('my_hook')

    # the following are the function that will run when ``my_hook`` is called
    def func1(app):
        print 'Inside hook func1'

    def func2(app):
        print 'Inside hook func2'

    def func3(app):
        print 'Inside hook func3'

    with MyApp() as app:
        # register all hook functions *after* the hook is defined (setup) but
        # also *before* the hook is called (different for every hook)
        app.hook.register('my_hook', func1, weight=0)
        app.hook.register('my_hook', func2, weight=100)
        app.hook.register('my_hook', func3, weight=-99)

        # run the application
        app.run()

        # run our custom hook
        for res in self.hook.run('my_hook', app):
            pass

And the result is:

    $ python my_hook_test.py
    Inside hook func3
    Inside hook func1
    Inside hook func2

As you can see, it doesn’t matter what order we register the hook, the weight runs then in order from lowest to highest.

## Cement Framework Hooks

Cement has a number of hooks that tie into the framework.

### pre_setup

Run first when `CementApp.setup()` is called. The application object is passed as an argument. Nothing is expected in return.

### post_setup

Run last when CementApp.setup() is called. The application object is passed as an argument. Nothing is expected in return.

### pre_run

Run first when CementApp.run() is called. The application object is passed as an argument. Nothing is expected in return.

### post_run

Run last when CementApp.run() is called. The application object is passed as an argument. Nothing is expected in return.

### pre_argument_parsing

Run after CementApp.run() is called, just before argument parsing happens. The application object is passed as an argument to these hook functions. Nothing is expected in return.

### post_argument_parsing

Run after CementApp.run() is called, just after argument parsing happens. The application object is passed as an argument to these hook functions. Nothing is expected in return.

This hook is generally useful where the developer needs to perform actions based on the arguments that were passed at command line, but before the logic of app.run() happens.

### pre_render

Run first when CementApp.render() is called. The application object, and data dictionary are passed as arguments. Must return either the original data dictionary, or a modified one.

Note: This does not affect anything that is ‘printed’ to console.

### post_render

Run last when CementApp.render() is called. The application object, and rendered output text are passed as arguments. Must return either the original output text, or a modified version.

### pre_close

Run first when app.close() is called. This hook should be used by plugins and extensions to do any ‘cleanup’ at the end of program execution. Nothing is expected in return.

### post_close

Run last when app.close() is called. Most use cases need pre_close(), however this hook is available should one need to do anything after all other ‘close’ operations.

### signal

Run when signal handling is enabled, and the defined signal handler callback is executed. This hook should be used by the application, plugins, and extensions to perform any actions when a specific signal is caught. Nothing is expected in return.