VisualEditor/API/Data Model/Surface

A surface contains a document, selection and history of transactions that have been processed over time. While the user interface provides a familiar affordance for working with a document, everything that is being changed is done through the surface model.

Documents
A document is a combination of a linear data model and a model tree. To modify a document, transactions are created and processed, changing the linear data model. When processing is complete, the model tree is then synchronized, and events are then emitted from the tree so that the user interface can update itself accordingly.

Before working with a document, it's important to understand the nature of the linear model. The linear model itself is generated from the HTML representation of Wikitext documents that Parsoid provides. While HTML is much easier than Wikitext for software to work with it is still not ideal for transactional editing – where each change is differential to a previous state. VisualEditor solves this problem by converting HTML into a linear data model for editing, and then back to HTML when done. The linear data model is essentially an HTML token stream with the exception that elements are not used for text formatting. Instead, annotations are composed onto formatted characters, allowing arbitrary slices of data to be as self-descriptive as possible with minimal processing. Upon saving a page, the linear data model is converted back to HTML and then subsequently Wikitext.

Conversion
To better understand how conversion works, let's take a look at how a simple page looks at each stage in the process.

If the page contains this wikitext: Hello world  !

Parsoid produces this HTML: &lt;p&gt; Hello &lt;b&gt;World&lt;/b&gt; ! &lt;/p&gt;

VisualEditor will convert that into this data structure: // bold === { type:’textStyle/bold’ } [    { type: ’paragraph’ } , ’H’ , ’e’ , ’l’ , ’l’ , ’o’ , ’ ’,    [ ’W’, { ’{type:”textStyle/bold”}’: bold  }, [ bold ] ] , [ ’o’, { ’{type:”textStyle/bold”}’: bold }, [ bold ] ] , [ ’r’, { ’{type:”textStyle/bold”}’: bold }, [ bold ] ] , [ ’l’, { ’{type:”textStyle/bold”}’: bold }, [ bold ] ] , [ ’d’, { ’{type:”textStyle/bold”}’: bold }, [ bold ] ] , ’!’,    { type: ’/paragraph’ } ]

Which, more succinctly, can be represented like this:

Transactions
Transactions describe how to get from one version of a document to another. An important feature of transactions is that they provide a way to return the document to a prior state as well. This reversible processing is what powers undo and redo, and lays the groundwork for history playback and realtime collaboration.

Let's take a look at how a simple transaction can be used to change a document, and then restore it to it's original state.

Our original document contains a single paragraph with the word "Hello" inside, and the text "Hel" is selected.

If we want to remove the selection we need to build a transaction. First we need to keep everything up to the selection; for this we will use retain, which leaves existing items in place. Next we need to remove content inside the selection; replace takes 2 arguments, remove and insert. The transaction looks something like this: retain 1 replace ["h", "e", "l"] [] retain 3

The document now looks like this:

Each operation in a transaction is fully reversible. In the case of retain no modification is necessary. For replace, we can just swap the remove and insert arguments. retain 1 replace [] ["h", "e", "l"] retain 3

Selection
In the user interface, a user describes where in the document they want to change by selecting. Although rendered very differently, a blinking cursor is just a zero-length selection. This selection mechanism is used throughout VisualEditor, and especially within the document through the use of ranges. A range is simply a pair of 2 offsets, from and to. Because selection is sometimes drawn from a high offset to a low offset (opposite of the reading direction) it's common to work with the start and end values of a range, which are always in ascending order respectively.

Document offsets
An offset describes a position within the document that is either at the beginning (0), at the end (n+1), or between two elements or characters. All elements in a document are wrapped in a pair of elements with the exception of text and the document itself. Due to this wrapping, in a document that contains a single paragraph, the offset before the first character is offset 1.

To select the contents of an element, a range must start and end inside of the element.

To select the element itself, the range must completely contain the element.

Because wrappers around text nodes are implied, if the cursor is inside of an element that can contain content, it's effectively inside a text node, even if the element is empty.

Working with ranges
Many functions take ranges as arguments, while others provide ranges as return values. In the case that you need to create a new range, the ve.Range class can be instantiated with from and to arguments, each an offset within a document.

A common way that a range object is obtained is when getting the current selection.

You can also modify the selection of a surface by providing a range object as an argument. When the selection is changed on the model, the user interface responds by selecting the corresponding content in the DOM.

Fragments
When working with a surface, a surface fragment object is used to abstract away the complexity of getting information about a surface and making changes to it while keeping everything in sync. Interactions with this API are similar to working with a jQuery selection in many ways, which is done intentionally to make it easier to learn.

Chaining
Once you have a fragment, you can get information about the surface using a getter function, get another fragment based on the one you have, or make changes to the document. The latter two of these classes of operations are chainable.

Null fragments
Sometimes getting a new fragment based on the one you have results in an invalid fragment, such as asking to expand the range to the nearest paragraph while in a pre-formatted node. In this case a null fragment is returned, from which all getters return empty values, only null fragments can be created and changes to the document are ignored.

Ranges
Each fragment's range will automatically be updated to remain relevant any time a document is modified. This is done internally by using the ve.dm.Transaction.translateRange feature.

ve.dm.SurfaceFragment( surface, range )
Selected portion of a surface.
 * surface {ve.dm.Surface}
 * Target surface


 * range {ve.Range} [optional]
 * Range within target document, current selection used by default


 * isNull
 * Responds to transactions being processed on the document
 * Returns  {Boolean}  Fragment is a null fragment


 * adjustRange( start, end )
 * Gets a new fragment with an adjusted position
 * Returns  {ve.dm.SurfaceFragment}  Adjusted fragment


 * start {Number} [optional]
 * Adjustment for start position


 * end {Number} [optional]
 * Adjustment for end position