ACF mvc.lua reference

The Model Viewer Controller (mvc.lua) Module

mvc.lua is a proposed framework for building acf, the alpine configuration framework. This page demonstrates how to build a very simple application using mvc.lua and haserl. This is the "Haserl Light Rail" framework

Architecture

mvc.lua has a :new constructor that creates a mvc "object". Through the use of metatable ._index methods, these objects are nested to construct the MVC based application:

ACF's MVC architecture dia source

Each of these objects are the same - they are built from the core mvc:new function, but with "lua-style" oop. (They inherit some properties from the parent.)

When created with the :new constructor, a table is returned with the following subtables.

worker	The controller (A table of controller methods), as well as an optional initializer.
model	The model (A table of model methods)
conf	A table of configuration parameters - application name, config file, application directory, selected prefix, controller, action, etc. Note this table is automatically created only if it does not exist in any parent.
clientdata	A table for user supplied data. Note this table is automatically created only if it does not exist in any parent.
view_resolver	A function that finds the view, and returns it as a function
exception_handler	A function that gets called if an error occurs
other local methods	as needed

The inheritance for these tables is shown in the diagram below. The diagram should be read from the "bottom" up:

ACF's inheritance dia source

This may appear slightly less than intuitive, but follows the way most applications naturally "want" things to resolve.

The above diagram is created with this code:

 require("mvc")
 MVC=mvc:new()
   APP=MVC:new()
    controller=APP:new()
      subcontroller=controller:new()

if you try to run subcontroller.model.somefunction(), and it does not exist, the inheritance will look for somefunction() in ...

1. subcontroller.worker
2. controller
3. controller.worker
4. APP
5. APP.worker
6. MVC
7. MVC.worker

This allows, for instance, the application to set a default method that is available to all child controllers. The reason the model looks to its parent worker table first is that controller methods are usually in the worker table, and models do not normally inherit from each other.

The framework is not an application

mvc.lua provides everything to build a web-based mvc application, but it is not a full application. Normally, the application will call :dispatch(), which performs the following steps:

1. Determine the application name
2. Load the configuration file if found
3. Parse the prefix, controller and action
4. Create an application object
5. Run the application's worker.init function (if found)
6. Create a controller object
7. Run the controller's worker.init function (if found)
8. Run the contoller's worker.[action] method
9. Run the view, based on the controller's view_resolver function

The application must supply a view_resolver function. It is assumed the worker.[action] method will return a table. This table is passed to the view_resolver and the view render functions. The format of the table is up to the application.

Building a Sample Application

For this demo, we will use alpine, busybox httpd, and haserl to create a simple "hostname" setting application. The Application name will be "helloworld", and it will have one controller, "hostname". The application code will reside at /var/lib/app.

Step 1 - Get the web interface going

• Edit /etc/inetd.conf and add the following line:

www stream tcp nowait root /bin/busybox httpd -h /var/lib/www

• kill -HUP $( pidof inetd )
• mkdir -p /var/lib/www/cgi-bin
• mkdir -p /etc/helloworld
• Use your browser to make sure that http://<hostname>/cgi-bin/helloword results in a 404 error.
• copy mvc.lua to /var/lib/www/cgi-bin 'or to /usr/local/lib/lua/5.1
• create /var/lib/www/cgi-bin/helloworld:

#!/usr/bin/haserl --shell=lua
<?
require("mvc")
mvc.dispatch(mvc:new())
?>

• chmod +x /var/lib/www/cgi-bin/helloworld
• Use your browser to go to http://<hostname>/cgi-bin/helloworld/hostname/get

If you see this, everything is working!

The following unhandled application error occured:


./mvc.lua:62: attempt to call field '?' (a nil value)
stack traceback:
	./mvc.lua:62: in function <./mvc.lua:52>
	[C]: in function 'xpcall'
	./mvc.lua:52: in function 'dispatch'
	[string "..."]:6: in main chunk

What's happened: We created the basic application, and the mvc object ran, caught and reported the error.

Step 2. Configure the Application

• Edit /etc/helloworld/helloworld.conf and add:

# The helloworld configuration parameters

appdir = /var/lib/app/

(The trailing / is important)

What's happened: We told the application where it will reside. The :new method looks in several places for a configuration file named with the appname (helloworld)

Step 3. Create the model and controller

• Create /var/lib/app/hostname-controller.lua

-- The "hostname" controller
module( ... , package.seeall)


-- Initialization goes here, if there were any.  This controller has no specific
-- initialization steps
init = function (self, parent)
end

-- Public methods.  Any of these methods can be called directly from the client
-- <prefix>/hostname/get
get = function (self)
        return ( { hostname = self.model:get() } )
end
 
-- <prefix>/hostname/set
set = function (self)
        return ( { hostname = self.model:set(self.clientdata.hostname) } )
end

• Create /var/lib/app/hostname-model.lua

-- the hostname model functions
module( ... , package.seeall)
 
-- The model does not have an initializer.  Use the controller (worker) for that.
 
-- Public methods.  Use a local table for private methods
get  = function (self)
        local f = io.popen("/bin/hostname")
        local n = f:read("*a") or "unknown"
        f:close()
        return ( n )
end
 
 
set  = function (self, name)
        local f = io.open("/etc/hostname", "w")
        if f then
                f:write(name)
                f:close()
        end
        f = io.popen ("/bin/hostname -F /etc/hostname")
        f:close()
        return get(self)
end

• Use your browser to go to http://<hostname>/cgi-bin/helloworld/hostname/get

Your controller and application did not specify a view resolver.
The MVC framework has no view available. sorry.

What Happened: The mvc framework ran your controller (which ran your model code)!!! Success! Unfortunately, the framework doesn't know how to render the results, so we get the generic mvc:view_resolver results.

Step 4. Create an Application Controller

We could fix the problem by creating a view_resolver in hostname-controller.lua, but that would only work for that one controller. Instead, we will create a method in the application controller that will apply to all controllers.

• Create /var/lib/app/helloworld-controller.lua

-- The APPLICATION worker table.  This gets loaded once for the application
module( ... , package.seeall)

-- A table for private methods
local private = {}

-- This function gets run when this module is loaded.  Note that in this case,
-- "self" refers to the object as a whole.  In the other cases, "self" will
-- refer to the "self.worker" table objects
init = function (self, parent)
        -- do some final fixup on ourselves
        -- the framework doesn't provide a view_resolver, so we do
                self.view_resolver = private.view_resolver
        end

-- Returns the function to render the view.  We make it private so that nobody calls it as a method.
private.view_resolver = function(self)
        local filename, file
        filename  = self.conf.appdir .. self.conf.prefix .. self.conf.controller .. "-view.lsp"
        file = io.open(filename)
        if file then
                file:close()
                return haserl.loadfile(filename)
        else
                return function() end -- return a blank screen
        end
end

• Create a /var/lib/app/hostname-view.lsp

<? local view = ... ?>
Content-type: text/html

<html>
<body>
<h1>Hello World!</h1>
<p>The hostname is <?= view.hostname ?></p>
</body>
</html>

• Use your browser to go to http://<hostname>/cgi-bin/helloworld/hostname/get

Hello World!

The hostname is Alpine 

What Happened: All functions in a module are normally public methods. The init function in the Application controller is actually <sometable>.worker.init. Since we don't want the view_resolver to be a public method (e.g. http://hostname/cgi-bin/helloworld/hostname/view_resover) we put it in a private table. We then override the Application's view_resolver method with our new method. Note that the active view_resolver is a method of the object itself, not of the object's worker table, so we must set self.view_resolver. (The init function is the only function where self is the the object as a whole. All other functions receive self as object.worker)

The view resolver uses haserl's loadfile function to process the "lsp" style page. We make the function generic so that it will work with any controller. We then define a simple .lsp file.

When the application runs, it runs all the way through, looks for a hostname.view_resolver function, and when it can't find one, uses the application.view_resolver function instead. Life is good!

Step 5. Get User Input

• Set the hostname with: http://<hostname>/cgi-bin/helloworld/hostname/set?hostname=Desert

The following unhandled application error occured:


/var/lib/app//hostname-model.lua:18: bad argument #1 to 'write' (string expected, got nil)
stack traceback:
	[C]: in function 'write'
	/var/lib/app//hostname-model.lua:18: in function 'set'
	/var/lib/app//hostname-controller.lua:19: in function '?'
	./mvc.lua:62: in function <./mvc.lua:52>
	[C]: in function 'xpcall'
	./mvc.lua:52: in function 'dispatch'
	[string "..."]:6: in main chunk

The mvc framework does not know how to get user input. That is the application's responsibility. So let's edit the application controller and add clientdata:

• Edit /var/lib/app/helloworld-controller.lua and add this to the init function:

       -- The application must populate the clientdata table
                self.clientdata = FORM

• Set the hostname with: http://<hostname>/cgi-bin/helloworld/hostname/set?hostname=Desert

Hello World!

The hostname is Desert 

What Happened: Since the application could be CLI, Web, or GUI, the framework makes no assumptions on how to get user input to the controller. So the Application controller must do that. In this case, we just accept FORM from haserl.

Step 6. Move the Controller and Application

This step is just to show that the framework is flexible. We're going to change the name of the application and where the controller is, at the same time.

• mv /etc/helloworld /etc/acf
• mv /etc/acf/helloworld.conf /etc/acf/acf.conf
• mv /var/lib/www/cgi-bin/helloworld /var/lib/www/cgi-bin/acf
• mkdir /var/lib/app/alpine-base
• mv /var/lib/app/hostname* /var/lib/app/alpine-base
• mv /var/lib/app/helloworld-controller.lua /var/lib/app/acf-controller.lua
• Go to http://<hostname>/cgi-bin/acf/alpine-base/hostname/get

Summary

To build an application, you must provide:

1. The config file
2. A <application>-controller.lua - with a view_resolver (and exception_handler, if desired)
3. <controller>-controller.lua and optionally <controller>-model.lua
4. A function to produce the view output