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Guidelines for writing new Components
Mathieu Baudet <>
(C) MLstate 2010
asciidoc format: use 'asciidoc GUIDELINES' to compile into HTML (or not)
== Inherited guidelines ==
The following sections of the guidelines of widgets (see +../widgets/GUIDELINES+) apply as well to components:
- Definitions
- Package and naming conventions
(for modules, replace +W+ by +C+, as in +CDataSheet+)
- Configuration of widgets
- Management of DOM identifiers
- HTML styles and CSS classes
- Tests and examples
== General design of components ==
Components are active, in the sense that they use one or several OPA sessions to operate an internal state.
Components should never be connected directly to the database. They must rely on an API provided by the developer to do so.
.Graphical components (client-side only)
Purely graphical components are created and installed in one step.
More precisely, a client-side call to creation function will:
- initialize the internal session(s) using the given configuration and parameters
- compute and then install the HTML of the components on the given id
- (optionnaly) process the already given data to fill the view in a synchroneous way
- return an abstract value containing the channel(s) of the created session(s) as well as the constant parameters (e.g. the id).
.General components (client-side and server-side)
More complex components (e.g. Chat, ApplicationFrame) that are not purely graphical may require a prior initialization on the server to create a server-side object (generally a session containing a network of clients).
The graphical part of such a general component is designed exactly as a purely graphical component.
Components that are often used to render the content of the initial home page of a web site may include a 'bootstrapping widget' (see section below) to make content indexation possible.
== Client/server slicing ==
Graphical components should be entirely run on the client-side, unless they are explicitly provided with server functions, or connected to server sessions. (This applies in particular to the graphical part of a general component).
Default configurations must be available on both sides and the functions that they contain must be serializable.
=> In current version of OPA S3, this means that the functions must be defined at top-level and themselves available on both sides.
CAUTION: Please ensure these properties by testing your components, e.g. by deconnecting the server after the page is loaded. OPA experts may also use the compiler option +--dump-slicer+.
== Public interface of graphical components ==
.Functions and types
The minimal API of a component includes
- one creation function
- a function to send messages to the component's session
The main expected functions are
create(config : CXxx.config, id : string, initial_display : CXxx.display, initial_content : CXxx.content, data_writer : CXxx.data_writer, data_requester : CXxx.data_requester) : CXxx.instance
send(obj : CXxx.instance, message : CXxx.message) : void
- +CXxx.config+ is the type of configurations,
- +CXxx.content+ is the type for the content of the component
- +CXxx.instance+ is the type for an instance of the component itself,
- +CXxx.display+ is the type of the display parameters (display parameters may evolve during time as opposed to a configuration),
- +CXxx.data_writer+ is an automaton that will process the updates coming from the user interface and forward them (by side-effect) to the ``outside world".
- +CXxx.data_requester+ is a set of callbacks to request some content for the component.
A typical definition for the data_writer is:
type CXxx.data_writer = CXxx.value -> void
but more complex definitions may assume that a state is passed along as in:
type CXxx.data_writer('state) = {
initial_state : 'state
on_change : 'state, CXxx.content -> CXxx.instruction('state)
type CXxx.instruction('a) = Session.instruction('a)
NOTE: The implementation of +CXxx.instance+ should be private to the package.
A typical definition for the data_requester is:
type CXxx.data_requester = () -> option(CXxx.content)
A +data_requester+ is expected to return +{ none }+ if the data is not immediately available. Otherwise, the data may be sent later to the component using a message (see below).
IMPORTANT: By default, data should be requested whenever the display is being updated and a value to be displayed is missing.
NOTE: For bigger components with multiple contents, +CXxx.content+ may be replaced by a map from +CXxx.address+ to +CXxx.value+s.
create(...initial_content : list(CXxx.address, CXxx.value) ...)
type CXxx.data_requester = CXxx.address -> option(CXxx.value)
type CXxx.data_writer('state) = {
initial_state : 'state
on_change : 'state, CXxx.address, Cxxx.value -> CXxx.instruction('state)
.Typical messages handled by the components session
type CXxx.message = // public interface of the session
/ { setDisplay : Cxxx.display }
/ { setContent : CXxx.content)) }
/ { clearContent : void } // mark the content as undefined (i.e. missing)
/ { getContent : option(CXxx.content) -> void) } // iterate on the content
/ { stop } // Terminate the session of the component
type CXxx.internal_message = // internal messages (if needed)
/ { notifyContent : option(CXxx.content))) } // may be needed internally when the value has just changed, and the data_writer needs to be called
Compared to the +data_writer+ and +data_requester+, the messages +setContent+, +getContent+, +clearContent+ above provide an alternative asynchroneous interface to execute read/write operations on the data.
== Interface of general components ==
In the case of general components, a +server+ must be initiated first:
init(config : CXxx.config, ...) : CXxx.server
create(config : CXxx.config, server : CXxx.server, id : string, initial_display : CXxx.display, initial_content : CXxx.content, data_writer : CXxx.data_writer, data_requester : CXxx.data_requester) : CXxx.instance
send(obj : CXxx.instance, message : CXxx.message) : void
Server-side operations acting on the objects +CXxx.server+ may be defined as well.
== Typical usage of the interface ==
The typical way to install a new component somewhere in the DOM is
- select an +id+ to install the component and that can be used as a prefix
- create a config :
config = { default_config with foo = myvalue }
- (for general component only) initiate a server-side object:
server = init(config, ...)
- create and install the component,
for graphical components:
obj = create(config, id, ...)
for general components:
obj = create(config, server, id, ...)
== Bootstrapping widget (optional) ==
The 'bootstrapping widget' of a component is a pure function
html(config : CXxx.config, id : string, initial_display : CXxx.display, initial_value : CXxx.content, connect_me : CXxx.type -> void) : xhtml
or for general components
html(config : CXxx.config, server : CXxx.server, id : string, initial_display : CXxx.display, initial_value : CXxx.content, connect_me : CXxx.type -> void) : xhtml
that is used by the server to create a temporary, robot-friendly version of the page (typically for indexation).
It is expected that the server uses this function like this
<div id=#{id}>
{ html(config, id, ...) }
The computed HTML will include a handler +onready+ that will call the function +create+ of the component so that the boostrapping widget disappears as soon as the JS runtime is operational on the client side.
This handler will also install the object returned by +create+ by calling the appropriate callback (named +connect_me+ in the example above).
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