# CMS preview

## Overview

With the addition of side-by-side editing, the preview has the ability to appear 
within the CMS window when editing content in the _Pages_ section of the CMS. 
The site is rendered into an iframe. It will update itself whenever the content 
is saved, and relevant pages will be loaded for editing when the user navigates 
around in the preview.

The root element for preview is `.cms-preview` which maintains the internal 
states necessary for rendering within the entwine properties. It provides 
function calls for transitioning between these states and has the ability to 
update the appearance of the option selectors.

In terms of backend support, it relies on `SilverStripeNavigator` to be rendered 
into the `.cms-edit-form`. _LeftAndMain_ will automatically take care of 
generating it as long as the `*_SilverStripeNavigator` template is found -
first segment has to match current _LeftAndMain_-derived class (e.g. 
`LeftAndMain_SilverStripeNavigator`).

We use `ss.preview` entwine namespace for all preview-related entwines.

<div class="notice" markdown='1'>
Caveat: `SilverStripeNavigator` and `CMSPreviewable` interface currently only 
support SiteTree objects that are _Versioned_.  They are not general enough for 
using on any other DataObject. That pretty much limits the extendability of the 
feature.
</div>

## Configuration and Defaults

Like most of the CMS, the preview UI is powered by 
[jQuery entwine](https://github.com/hafriedlander/jquery.entwine). This means 
its defaults are configured through JavaScript, by setting entwine properties.
In order to achieve this, create a new file `mysite/javascript/MyLeftAndMain.Preview.js`.

In the following example we configure three aspects:
 
 * Set the default mode from "split view" to a full "edit view"
 * Make a wider mobile preview
 * Increase minimum space required by preview before auto-hiding

Note how the configuration happens in different entwine namespaces
("ss.preview" and "ss"), as well as applies to different selectors
(".cms-preview" and ".cms-container").

	:::js
	(function($) {
		$.entwine('ss.preview', function($){
			$('.cms-preview').entwine({
				DefaultMode: 'content',
				getSizes: function() {
					var sizes = this._super();
					sizes.mobile.width = '400px';
					return sizes;
				}
			});
		});
		$.entwine('ss', function($){
			$('.cms-container').entwine({
				getLayoutOptions: function() {
					var opts = this._super();
					opts.minPreviewWidth = 600;
					return opts;
				}
			});
		});
	}(jQuery));

Load the file in the CMS via setting adding 'mysite/javascript/MyLeftAndMain.Preview.js'
to the `LeftAndMain.extra_requirements_javascript` [configuration value](/topics/configuration)

	:::yml
	LeftAndMain
	  extra_requirements_javascript:
	    - mysite/javascript/MyLeftAndMain.Preview.js:

In order to find out which configuration values are available, the source code
is your best reference at the moment - have a look in `framework/admin/javascript/LeftAndMain.Preview.js`.
To understand how layouts are handled in the CMS UI, have a look at the
[CMS Architecture](/reference/cms-architecture) guide.

## Enabling preview

The frontend decides on the preview being enabled or disabled based on the 
presence of the `.cms-previewable` class. If this class is not found the preview 
will remain hidden, and the layout will stay in the _content_ mode.

If the class is found, frontend looks for the `SilverStripeNavigator` structure 
and moves it to the `.cms-preview-control` panel at the bottom of the preview. 
This structure supplies preview options such as state selector.

If the navigator is not found, the preview appears in the GUI, but is shown as 
"blocked" - i.e. displaying the "preview unavailable" overlay.

The preview can be affected by calling `enablePreview` and `disablePreview`. You 
can check if the preview is active by inspecting the `IsPreviewEnabled` entwine 
property.

## Preview states

States are the site stages: _live_, _stage_ etc. Preview states are picked up 
from the `SilverStripeNavigator`. You can invoke the state change by calling:

	```js
	$('.cms-preview').entwine('.ss.preview').changeState('StageLink');
	```

Note the state names come from `SilverStripeNavigatorItems` class names - thus 
the _Link_ in their names. This call will also redraw the state selector to fit 
with the internal state. See `AllowedStates` in `.cms-preview` entwine for the
list of supported states.

You can get the current state by calling:

	```js
	$('.cms-preview').entwine('.ss.preview').getCurrentStateName();
	```

## Preview sizes

This selector defines how the preview iframe is rendered, and try to emulate 
different device sizes. The options are hardcoded. The option names map directly 
to CSS classes applied to the `.cms-preview` and are as follows:

* _auto_: responsive layout
* _desktop_
* _tablet_
* _mobile_

You can switch between different types of display sizes programmatically, which 
has the benefit of redrawing the related selector and maintaining a consistent 
internal state:

	```js
	$('.cms-preview').entwine('.ss.preview').changeSize('auto');
	```

You can find out current size by calling:

	```js
	$('.cms-preview').entwine('.ss.preview').getCurrentSizeName();
	```

## Preview modes

Preview modes map to the modes supported by the _threeColumnCompressor_ layout 
algorithm, see [layout reference](../reference/layout) for more details. You 
can change modes by calling: 

	```js
	$('.cms-preview').entwine('.ss.preview').changeMode('preview');
	```

Currently active mode is stored on the `.cms-container` along with related 
internal states of the layout. You can reach it by calling:

	```js
	$('.cms-container').entwine('.ss').getLayoutOptions().mode;
	```

<div class="notice" markdown='1'>
Caveat: the `.preview-mode-selector` appears twice, once in the preview and 
second time in the CMS actions area as `#preview-mode-dropdown-in-cms`. This is 
done because the user should still have access to the mode selector even if
preview is not visible. Currently CMS Actions are a separate area to the preview 
option selectors, even if they try to appear as one horizontal bar.
</div>

## Preview API

Namespace `ss.preview`, selector `.cms-preview`:

* **getCurrentStateName**: get the name of the current state (e.g. _LiveLink_ or _StageLink_).
* **getCurrentSizeName**: get the name of the current device size.
* **getIsPreviewEnabled**: check if the preview is enabled.
* **changeState**: one of the `AllowedStates`.
* **changeSize**: one of _auto_, _desktop_, _tablet_, _mobile_.
* **changeMode**: maps to _threeColumnLayout_ modes - _split_, _preview_, _content_.
* **enablePreview**: activate the preview and switch to the _split_ mode. Try to load the relevant URL from the content.
* **disablePreview**: deactivate the preview and switch to the _content_ mode. Preview will re-enable itself when new
previewable content is loaded.

## Related

 * [Reference: Layout](../reference/layout)