NOTE-Amaya-970220
Ir�ne Vatton, W3C, <[email protected]>
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This document briefly presents Amaya, the W3C testbed client. It explains the motivations for the development of the software as well as its goals. It also presents the main design choices that have been made to reach these goals.
As the web is based on a client-server architecture, both a client and a server are needed to experiment and demonstrate new specifications for the web. Within W3C, Jigsaw plays this role on the server side. Amaya is developed to play the same role on the client side.
This note presents the main choices that were made in the design of Amaya, as well as its key features. The user interface and the software architecture are also briefly described.
Amaya is both a browser and an authoring tool dedicated to the World Wide Web. It has been specifically conceived to serve as a testbed client to experiment and demonstrate new web protocols and formats as well as new extensions to existing ones. This makes its features a bit specific.
A testbed client must be versatile enough to allow a wide range of experiments and demonstrations. That is the reason why Amaya has been designed as an active client, i.e. a client that not only retrieves documents from the web and presents them to the user, but also an authoring tool that allows an author to create new documents, to edit existing ones and to publish these documents on remote web servers.
To help investigate the impact of new specifications on the many aspects of a web client, Amaya has been designed with both
The web is made of a number of documents linked together. While working on the web, a user needs to access several documents, specially when following, creating, or modifying links. For that reason, Amaya is able to work on several documents at a time and it can act as a browser and as an editor simultaneously on all these documents: the whole client functionality is available at any time for any document.
In its basic version, Amaya implements many web protocols and formats:
HTML is the document format for the Web. As a web tool, Amaya must take all aspects of this format into account. Existing tools have shown that there are different approaches to consider HTML in a web client. Amaya has its own approach to HTML, that we describe below.
Being an editor, Amaya processes HTML files in a specific way. An editor based on the WYSIWYG paradigm must allow the user to act on the formatted document for altering the HTML source. To allow that feature, the HTML source is treated as a data structure definition. When parsing a source file, Amaya builds an internal representation of the document structure, which is basically a tree. This logical structure is then utilized both for formatting and for editing.
In Amaya, HTML is considered as an SGML application. The editor always follows the HTML DTD when manipulating the document structure and it performs only valid operations. The advantage of this approach is that it leads to well structured documents and allows other tools to further process the documents safely. The drawbacks are also well known: (1) manipulations are often complex from the user viewpoint and (2) documents that are not strictly correct are often rejected. The later is a real nuisance, as many documents are syntactically or structurally incorrect on the web.
To address the first drawback, a particular attention has been paid to the user interface. Most commands issued by the user are the same as those proposed by usual word processors, but they are interpreted by the editor in terms of the internal document structure. Specific structure manipulation commands are thus avoided, as well as context sensitive menus.
To address the second drawback, the HTML parser has been designed in such a way Amaya never rejects any document. When it parses a document that is not structurally correct, it tries to transform the structure of the document. Sometimes this structure is so bizarre that it's not possible to automatically generate a fully HTML conformant structure. In that case, the parser nevertheless loads the document, but if the user attempts to modify it, the editor performs only valid operations. Thus, the document structure is improved and, even if it is not guaranteed to be always correct, it's not worse than before when it is saved, and the user is not prevented from working on any document.
Users are not supposed to know the HTML or CSS languages to author web pages. Amaya does not ask them to write nor to read the documents they create under their HTML syntax. The HTML file is automatically produced by the tool, as well as the CSS syntax. But Amaya is not simply a word processor with an HTML filter. As it is intended to implement as many aspects of the web specifications as possible, Amaya should be able to implement every HTML feature. It must then be modelled after HTML while letting users interact with it in a very simple and natural way.
For these reasons, the WYSIWYG paradigm has been chosen for the user interface, with a structured model for the internal representation. Document are presented to the user with the aspect they would have if displayed by the most popular browsers and the user interacts with the editor on the basis of this external representation. Simultaneously, the editor maintains an internal representation that follows the HTML rules and constraints, as stated in the DTD.
A user can manipulate text in the same way as if s/he were using a simple word processor. At any time, the user can select any part of that text and assign to it an HTML type (H1, LI, EM, etc.), by means of the Types menu or of the shortcut buttons. Such a command transforms the selected part into an element of the chosen type. If some other elements need to be created or transformed to keep the document HTML conformant, these transformations are also performed. This allows the author to type in the contents first and to provide structure afterwards, but the opposite is also possible, as the type of an element can also be chosen before entering its contents.
In Amaya, this is the only way to manipulate the document structure. The user does not enter or edit tags. Only the editor directly acts on the document structure, and when it makes a change, it checks the change against the HTML DTD, to make sure that the structure remains correct.
This kind of user interface may require complex structure transformations to be done by the editor. The user may select a passage that has a given structure (say a bulleted list) and ask for a completely different structure (say a table). For such complex transformations, the editor cannot rely only on the DTD, as some semantics is involved. The problem is solved by a transformation language. This language is used to define complex structure transformations. It allows users to define structure patterns and the corresponding transformed structures. With these definitions, the editor may perform any transformation that the user may need and the user may define new transformations as needed.
This approach has a number of advantages:
In HTML, various attributes can be associated with a number of elements, and the allowed attributes depend on the element type. This is stated by the DTD. To help the user choose the right attributes, a specific menu offers at any time all attributes that can be associated with the selected element(s), and only those attributes.
As there is no other means than this menu to associate attributes with elements, only valid attributes can be entered. Moreover, when attributes are allowed to take only some predefined values, these values are also proposed in a specific menu, in such a way that only valid values are entered.
This enforces the structured approach that guarantees that documents are always structurally correct.
In some cases, the structure of a HTML document may be complex, with a number of nested elements of several types. That complexity may lead to ambiguity, as different structures may look very similar when displayed in their formatted representation. As a help to the user in such situations, Amaya can display the actual structure of the document being edited. This is called the structure view and it is displayed in a separate window that the user can see at the same time as the formatted view.
The structure view presents the document's hierarchical structure as an indented list and displays the type and attributes of every element. This view is not a static representation. The user can act on it. S/he can select any element there; s/he can modify the document exactly in the same way as in the formatted view. Moreover, all editing commands are executed in both views simultaneously, whatever the view chosen for issuing the command.
The principle of showing the same document in different windows simultaneously is not restricted to the structure view. An alternate view is also available. It shows the document as if it would be displayed on a dumb terminal. Like the structure view, the alternate view may be used for editing, as well as the other views. It's not simply a preview that allows to check the document from a different viewpoint; it's a dynamic representation, where the user can make any change.
The concept of a view may be used for showing other aspects of a document. It is fairly simple to define new views and one can imagine a window showing links, another showing headings (a table of contents), etc.
Links are first class citizens in web pages. Therefore a particular attention has been paid to their manipulation. Most often, the user does not have to type URLs to edit links. Instead, the browsing function of Amaya is used to load different documents. Links are then created or modified by simply clicking on the target document or anchor that is displayed on the screen. Thus, the user can see the actual target of a link and avoid any mistake when setting links.
A web page is not only ruled by the HTML specification. It can also include stylistic stuff that conforms the CSS (Cascading Style Sheets) specification. For CSS, the same approach has been taken as for HTML: the user does not write CSS syntax directly. S/he interacts on a formatted document and sees immediately the result of his/her commands on the formatted document and in other views. Amaya is in charge of producing the actual CSS syntax.
As the same result may be achieved by associating some style declarations at several locations in an HTML document, the formatted view is not enough. Other means should be provided for avoiding any ambiguity. Like for HTML, the structure view fulfills this need. It clearly shows where style rules are located: as Style attributes associated with HTML elements, as Style elements in the heading of a document, or as a link to a style sheet.
Amaya is not simply a structured editor. It is a complete web authoring environment. When working with Amaya you are on the web and you can access any web resource. You can browse the web and find the information you need while writing a document. You can copy and paste from any document, directly on the formatted representation. You can create links pointing at any place and you can check these links immediately. All these actions can be performed in a single consistent environment, as the editing and browsing functions are integrated seamlessly in a single tool.
Access to the web is not restricted to browsing: the document you edit can also be published on remote servers where you are allowed to write. Publishing on the web is as simple as saving a document to a local file. Just type an URL instead of a file name when saving a document and Amaya does the rest. Included objects such as pictures are also saved and all URLs are updated accordingly if the document has moved from its original location.
Transferring documents and other resources to and from remote servers is done by the libwww. Access to these servers is done exclusively by means of HTTP methods: GET to load remote documents, POST/GET to send forms, and PUT to publish documents. Amaya takes advantage of the most advanced features of HTTP, such as content negotiation to retrieve the most appropriate picture format, for instance, or keep alive connections to save bandwidth.
Amaya's internal design reflects its testbed role. The software architecture allows easy extensions. Several APIs and mechanisms are available to change and extend its functionality with as less modifications as possible within the source code.
To allow its handling of documents in a structured way, Amaya has been designed as an application on top of the Thot tool kit. Thot is a set of libraries that implement document manipulation functions. It is based on a structured document model that clearly separates content, structure, and presentation.
All editing and formatting functions in Thot rely on the document logical structure. In any application that uses Thot, a document is represented internally as a tree that assembles typed elements, such as headings, paragraphs, lists, highlighted phrases, etc. These elements may have attributes which provide more information about the element's role in the document structure, exactly like in HTML and SGML.
Thot provides four languages that are used to write fout types of schemas. A schema is a set of rules which define the behaviour of the editor regarding the logical structure of documents, their presentation, their external syntax, or some specific treatments.
The logical structure of a document is constrained by certain rules. Structure rules specify the available element types and attributes, they indicate how these elements may be assembled to make a valid structure, and they state what attributes can be associated with each element type. In Thot, structure rules are gathered in structure schemas. A structure schema defines the logical structure of a document type; it is written in a simple declarative language, called S.
Amaya uses a structure schema to specify the logical structure of HTML documents. This schema is equivalent to the HTML DTD.
The document image displayed in different views is defined by a presentation schema, written in language P. A presentation schema specifies the views to be proposed to the user and it indicates in presentation rules how the elements and attributes defined in the corresponding structure schema should be displayed in these views.
Amaya uses several presentation schemas, which specify different layouts and styles for HTML documents: one for color screens, one for printers, etc. These schemas define the default presentation to be applied to HTML documents, when no particular style information is associated. If CSS is used, these presentation schemas are dynamically extended with presentation rules that correspond to the CSS rules included in or referred from the document.
A structure schema only specifies the internal logical structure of documents, but not their external syntax. The same structure can be conveyed by different syntaxes, and when the document is saved several syntaxes can be used. The output syntax is defined by a translation schema, written in language T. This language is also declarative. It specifies how each element and attribute defined by a structure schema should be represented in the external representation.
A translation schema is used in Amaya to define the HTML syntax. Other schemas can be used to save the document in ASCII or in LATEX, for instance.
The fourth language of Thot is language A. It is used to define the user interface of an application as well as its specific commands (basic editing commands are provided by Thot). With an application schema, written in language A, a developer specifies the menu bar that appears at the top of each view, the contents of the corresponding pull-down menus and the functions that are called by these menus. An application schema also allows to extend or modify the basic editing functions provided by Thot.
Amaya is a Thot application described in language A. It uses the basic
editing functions provided by Thot, to which it adds some particular commands
that are specific to the web and the HTML structure.
Each language has its own compiler. The S, P, and T compilers generate files that are loaded dynamically by the Thot library at running time, while the A compiler generates C code that must be compiled and linked with the modules implementing the application.
The main Thot library provides developers with an API that allows them to handle all entities and objects that make a document and its environment. There are functions for creating, modifying, deleting, opening, accessing, finding, moving, etc. such things as documents, logical structures, elements, attributes, links, views, etc.
All document manipulation functions are based on the schemas presented
above. As an example, an application program needs only to call two functions
(NewTree and InsertTree) to create a new subtree in
the document structure. Function NewTree receives as a parameter
the type of the element that must become the root of the new tree, and
according to the structure schema, it creates all the required elements in the
subtree. Function InsertTree uses the structure schema to check
whether the tree can be inserted at the required place in the document
structure and it uses the presentation schema to redisplay only what is needed
in all open views. The developer cares only about the document logical
structure and all the rest is performed by Thot. This makes the development of
applications very simple.
Thot also provides a library of graphical user interface functions that, along with language A, makes the development of a user interface also very simple.
A number of standard editing functions are provided by Thot, but these functions may need to be modified, extended or replaced by the application in some cases. For that reason, application schemas allow developers to specify some specific functions (that they have to write) that must be called before, after or instead of the standard functions. They can specify different functions that are performed according to the element type or attribute concerned. This allows to change the behavior of the standard editing functions without making any change within the Thot code itself.
As an example, activating an element by double-clicking it with the mouse is a standard Thot function, but in Amaya, when an anchor is clicked, a specific function must be performed to get and display the referred document. Language A allows a developer to declare that, when an element of type anchor is clicked, a specific function must be called instead of the standard Thot function. That function, written by the developer, uses the Thot library to access the HREF attribute associated with the clicked anchor, then it uses libwww to get the corresponding document, and again the Thot library to display that document.
Amaya inherits a lot of functions from the Thot library, such as text editing, formatting, basic structure manipulations, user dialogue, picture display, etc. More specific functions are implemented by the following software components:
HEAD of a HTML file or in a STYLE attribute,
and generates the corresponding presentation rules for Thot.The architecture described above allows Amaya to be extended in many ways and most extensions can be made very simply, often without programming. Due to the Thot library and its API, most basic editing functions are available and can be reused very easily. Due to the available declarative languages, many changes to the software can be made simply by changing some declarations.
These features have already been used to experiment some proposed extensions to HTML, such as an extended table model or the OBJECT element associated with a plug-in mechanism. An experiment with web collections is also in progress for printing large documents published as a set of smaller HTML pages. Additional experiments are planned in the W3C team, but other interested parties are encouraged to use Amaya for experimenting other extensions and develop their own applications.
The following documents are available on the web: