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### Dynamic Pages

So far we only have a static page with not much functionality. Just having a hello world example is not very useful, so let's go beyond that and add some real functionality to our application.

Our first feature will be dynamic pages generated from [markdown](http://en.wikipedia.org/wiki/Markdown) files.

Create a `Page` controller with the following content:

```php
<?php

namespace Example\Controllers;

class Page
{
    public function show($params)
    {
        var_dump($params);
    }
}
```
Once you have done that, add a new route:

```php
['GET', '/{slug}', ['Example\Controllers\Page', 'show']],
```

Now try and visit a few urls, for example `http://localhost:8000/test` and `http://localhost:8000/hello`. As you can see, the `Page` controller is called every time and the `$params` array receives the slug of the page.

So let's create a few pages to get started. We won't use a database yet, so create a new folder `pages` in the root folder of your project. In there add a few files with the file extensions `.md` and add some text to them. For example `page-one.md` with the content:

```
This is a page.
```

Now we will have to write some code to read the proper file and display the content. It might seem tempting to just put all that code into the `Page` controller. But remember [Separation of Concerns](http://en.wikipedia.org/wiki/Separation_of_concerns). There is a good chance that we will need to read the pages in other places in the application as well (for example in an admin area).

So let's put that functionality into a separate class. There is a good chance that we might switch from files to a database later, so let's use an interface again to make our page reader decoupled from the actual implementation.

In your 'src' folder, create a new folder `Page`. In there we will put all the page related classes. Add a new file in there called `PageReader.php` with this content:

```php
<?php

namespace Example\Page;

interface PageReader
{
    public function readBySlug($slug);
}
```

For the implementation, create a `FilePageReader.php` file. The file will looks like this:

```php
<?php

namespace Example\Page;

use InvalidArgumentException;

class FilePageReader implements PageReader
{
    private $pageFolder;

    public function __construct($pageFolder)
    {
        if (!is_string($pageFolder)) {
            throw new InvalidArgumentException('pageFolder must be a string');
        }
        $this->pageFolder = $pageFolder;
    }

    public function readBySlug($slug)
    {
        return 'I am a placeholder';
    }
}
```

As you can see we are requiring the page folder path as a constructor argument. This makes the class flexible and if we decide to move files or write unit tests for the class, we can easily change the location with the constructor argument.

You could also put the page related things into it's own package and reuse it in different applications. Because we are not tightly coupling things, things are very flexible.

Because PHP does not have the ability to type hint for scalar values (things like strings and integers), we have to manually check that `$pageFolder` is a string. If we don't do that, there might be a bug in the future that is hard to find if a wrong type is injected. By throwing an exception, this can be caught and debugged immediately.

This will do for now. Let's create a template file for our pages with the name `Page.mustache` in the `templates` folder. For now just add `{{ content }}` in there.

Add the following to your `Dependencies.php` file so that the application know which implementation to inject for our new interface. We also define the the `pageFolder` there.

```php
$injector->define('Example\Page\FilePageReader', [
    ':pageFolder' => __DIR__ . '/../pages',
]);

$injector->alias('Example\Page\PageReader', 'Example\Page\FilePageReader');
$injector->share('Example\Page\FilePageReader');
```


Now go back to the `Page` controller and change the `show` method to the following:

```php
public function show($params)
{
    $slug = $params['slug'];
    $data['content'] = $this->pageReader->readBySlug($slug);
    $html = $this->renderer->render('Page', $data);
    $this->response->setContent($html);
}
```

To make this work, we will need to inject a `Response`, `Renderer` and a `PageReader`. I will leave this to you as an exercise. Remember to `use` all the proper namespaces. Use the `Homepage` controller as a reference.

Did you get everything to work?

If not, this is how the beginning of your controller should look now:

```php
<?php

namespace Example\Controllers;

use Http\Response;
use Example\Template\Renderer;
use Example\Page\PageReader;

class Page
{
    private $response;
    private $renderer;
    private $pageReader;

    public function __construct(
        Response $response,
        Renderer $renderer,
        PageReader $pageReader
    ) {
        $this->response = $response;
        $this->renderer = $renderer;
        $this->pageReader = $pageReader;
    }
...
```

So far so good, now let's make our `FilePageReader` actually do some work.

Again, let's check first that the proper type was passed into the method:

```php
public function readBySlug($slug)
{
    if (!is_string($slug)) {
        throw new InvalidArgumentException('slug must be a string');
    }
}
```

We also need to be able to communicate that a page was not found. For this we can create a custom exception that we can catch later. In your `src/Page` folder, create a `InvalidPageException.php` file with this content:

```php
<?php

namespace Example\Page;

use Exception;

class InvalidPageException extends Exception
{
    public function __construct($slug, $code = 0, Exception $previous = null)
    {
        $message = "No page with the slug `$slug` was found";
        parent::__construct($message, $code, $previous);
    }
}
```

Then in the `FilePageReader` file add this code at the end of your `readBySlug` method:

```
$path = "$this->pageFolder/$slug.md";

if(!file_exists($path)) {
    throw new InvalidPageException($slug);
}

return file_get_contents($path);
```

Now if you navigate to a page that does not exist, you should see an `InvalidPageException`. If a file exists, you should see the content.

Of course showing the user an exception for an invalid URL does not make sense. So let's catch the exception and show a 404 error instead.

Go to your `Page` controller and refactor the `show` method so that it looks like this:

```php
public function show($params)
{
    $slug = $params['slug'];

    try {
        $data['content'] = $this->pageReader->readBySlug($slug);
    } catch (InvalidPageException $e) {
        $this->response->setStatusCode(404);
        return $this->response->setContent('404 - Page not found');
    }
    
    $html = $this->renderer->render('Page', $data);
    $this->response->setContent($html);
}
```

Add this at the top of your file:
```php
use Example\Page\InvalidPageException;
```

It is important that you don't forget this step, otherwise it will try to catch the wrong exception (it's looking in the wrong namespace) and thus will never catch it. 

Try a few different URLs to check that everything is working as it should. If something is wrong, go back and debug it until it works.

And as always, don't forget to commit your changes.

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