hv3(3) Mega-widget building on Tkhtml.

SYNOPSIS


package require snit
package require hv3
::hv3::hv3 pathName ?options?

DESCRIPTION

The [hv3] command creates a new window (given by the pathName argument) and makes it into hv3 widget. The hv3 command returns its pathName argument. At the time this command is invoked, there must not exist a window named pathName, but pathName"s parent must exist. Hv3 is a pure Tcl widget implemented using Tkhtml3 and the excellent mega-widget framework Snit.

An [hv3] widget is not a web-browser. If it were to be used as a component in a web-browser application it would represent a single browser frame (or iframe). The API described in this document is not the whole API offered by the snit object ::hv3::hv3. Instead, it is the subset of that API that is expected not to change. No guarantees of course.

There are two "objects" involved in using the [hv3] widget. One is the widget itself ([::hv3::hv3]). The other is the request-handle ([::hv3::request]). A request-handle is the interface between the hv3 widget and wherever it is getting its data from (i.e. your implementation of http://, https:// etc.). Many users will also wish to understand the Tkhtml3 "node-handle" interface, documented as part of the Tkhtml3 manpage.

An [hv3] widget provides the following features on top of the [html] widget:

*
Built-in scrollbars.
*
Support for selecting text with the pointer.
*
Support for loading linked images and stylesheets from URIs.
*
Support for HTML forms and submission thereof.
*
Support for CSS configured hover (mouseover) effects.
*
Support for loading a new document by clicking on a hyper-link.

The two most important interfaces are the [goto] method and the
 -requestcmd option. The [goto] method tells the widget to load the document identified by the specified absolute or relative URI.

The -requestcmd option must be configured with a callback script that the widget invokes to request the requested document. It is the users responsibility to retrieve the document and pass it back to the widget. If the document contains links to external resources (images or CSS stylesheets), then the widget invokes the
 -requestcmd script to request these. The -requestcmd callback may choose to implement handling for one or more of http:// URIs, file:// URIs or any other existing or invented URI scheme. See the "Example Usage" section below for an example.

STANDARD OPTIONS

-height
-width

HTML OPTIONS

The following Tkhtml options are exposed as public options of this mega-widget. 

-fontscale
-fonttable
-forcefontmetrics
-zoom

HTML COMMANDS

The following Tkhtml commands are exposed as public options of this mega-widget. 

node ? ?-index? x y?

WIDGET-SPECIFIC OPTIONS

Command-Line Name:-enableimages
 .br Database Name: enableimages
 .br Database Class: Enableimages
 .br
 .RS
 .PP Boolean option (default true). True for image support, false otherwise. If this option is set to false, then the -requestcmd script will never be invoked to request an image resource.
 .RE Command-Line Name:-isvisitedcmd
 .br Database Name: isvisitedcmd
 .br Database Class: Isvisitedcmd
 .br
 .RS
 .PP If not an empty string, this option specifies a script for the widget to invoke to determine if a hyperlink (<A>) node should be styled with the :link or :visited pseudo-class. The script is invoked with the node handle appended to it. If true is returned, :visited is used, otherwise :link.
 .RE Command-Line Name:-requestcmd
 .br Database Name: requestcmd
 .br Database Class: Requestcmd
 .br
 .RS
 .PP If this option is not set by the user code, then the Hv3 widget will be unable to display anything.

It should be set to a script that may be invoked by the hv3 widget to request a resource required to display a URI requested via the [goto] method. Each time a resource is required, the -requestcmd script is invoked with a single argument appended to it, the name of a request handle object. See section "Request Handles" for details.
 .RE Command-Line Name:-targetcmd
 .br Database Name: targetcmd
 .br Database Class: Targetcmd
 .br
 .RS
 .PP If this option is not set to an empty string (the default), it should be set to a script that will be invoked each time a hyper-link is clicked or a form submitted in the hv3 widget by the end-user. A single argument is appended to the script before it is evaluated, the Tkhtml3 node-handle for the relevant <FORM> (in the case of form submittal) or <A> (if the end user clicked a hyperlink) node. The script should return the path of an hv3 widget into which the new resource should be loaded. This is useful for implementing browsers that support HTML frames and iframes.

If the script returns an empty string the request is abandoned and the new resource never loaded and the form data (if any) not submitted.

If the option is set to an empty string the new resource is always loaded into the hv3 widget itself.
 .RE

WIDGET COMMAND

The [hv3] command creates a new Tcl command whose name is pathName. This command may be used to invoke various operations on the widget as follows:

pathName cget option

Returns the current value of the configuration option given by option. Option may have any of the values accepted by the [hv3] command.

pathName configure ?option? ?value?

Query or modify the configuration options of the widget. If no option is specified, returns a list describing all of the available options for pathName (see Tk_ConfigureInfo for information on the format of this list). If option is specified with no value, then the command returns a list describing the one named option (this list will be identical to the corresponding sublist of the value returned if no option is specified). If one or more option-value pairs are specified, then the command modifies the given widget option(s) to have the given value(s); in this case the command returns an empty string. Option may have any of the values accepted by the [hv3] command.

pathName goto uri

Load the resource at uri into the widget. If uri is not an absolute URI, it is resolved with respect to the widget"s current document URI (or <BASE> element contents, if present).

pathName stop

Abandon all pending requests. All request handle objects that are still outstanding are destroyed (it is an error to use such a request handle after calling [stop]).

REQUEST HANDLES

To be useful, the user must provide the hv3 with some way to request resources ((X)HTML documents, CSS stylesheets and binary image files) identified by URI for display. To this end, the user configures a -requestcmd script with the hv3 widget. Each time a resource is required, the -requestcmd script is evaluated with a single argument, a request handle object identifier, appended to it.

A request handle object is a snit object. The -requestcmd script can query the object to determine the parameters of the request and then invoke object methods to return data and meta data. The key APIs are the -uri option and the [finish] method.

Data may be returned asynchronously. That is, it is not necessary to return data from within the -requestcmd evaluation, the request handle may be stored and data returned at some later time.

REQUEST HANDLE OPTIONS

Command-Line Name:-enctype
 .br Database Name: enctype
 .br Database Class: Enctype
 .br
 .RS
 .PP This option is used by "POST" requests, which may be made by an hv3 widget if the loaded document contains a form and the end-user submits it. For a "GET" request (all other requests, the usual case) it is set to an empty string.

The cannonical test to check if a given request is a POST or GET request is: 

if {[$handle cget -postdata] ne ""} {
  # This is a POST request.
} else {
  # This is a GET request.
}

For POST requests, this option may be set by the Hv3 widget to contain the Content-Type of the data stored in the -postdata option. For example "application/x-www-form-urlencoded".
 .RE

Command-Line Name:-header
 .br Database Name: header
 .br Database Class: Header
 .br
 .RS
 .PP The Hv3 widget sets this option to an empty string before passing the request handle to the user code.

The user code may set this option to a list containing data to be handled by the hv3 widget as if it had been returned as the HTTP header for an HTTP request. The list consists of alternating HTTP header-names and values. This is the same format as the "meta" element of the "state array" interface used by Tcl"s built-in http package.

The Hv3 widget interprets the following HTTP headers:

*
TODO.

 .RE Command-Line Name:-mimetype
 .br Database Name: mimetype
 .br Database Class: Mimetype
 .br
 .RS
 .PP The Hv3 widget sets this option to the expected mime type of the resource requested.

If the user code knows the mime type of the resource being returned, it should set this option before the first invocation of the [append] method. Useful values recognized by the hv3 widget include "text/xhtml" and "image/gif".
 .RE Command-Line Name:-postdata
 .br Database Name: postdata
 .br Database Class: Postdata
 .br
 .RS
 .PP This option is used by "POST" requests, which may be made by an hv3 widget if the loaded document contains a form and the end-user submits it.

It contains the data to be posted.
 .RE Command-Line Name:-requestheader
 .br Database Name: requestheader
 .br Database Class: Requestheader
 .br
 .RS
 .PP The Hv3 widget sets this option to a list of HTTP header-names and values to be handled as request parameters for an HTTP request (i.e. the "referrer" header).

The user code should not change the value of this option.
 .RE Command-Line Name:-uri
 .br Database Name: uri
 .br Database Class: Uri
 .br
 .RS
 .PP This option is always set by the Hv3 widget before passing the request handle to the user code. It contains the absolute URI of the resource required by the widget.

The user code should not change the value of this option.
 .RE

REQUEST HANDLE METHODS

requestHandle append data

This method should be invoked one or more times to return data to the hv3 widget.

The data passed to this method should always be binary data. If the data is actually text data for a document or stylesheet, it"s encoding is determined based on either a HTTP header returned via the [header] option, or a <meta> element in the header section of an HTML or XHTML document. If neither of these are present, the assumed encoding is either the document encoding in the case of linked CSS stylesheet, or the value returned by [encoding system] for an HTML or XHTML document.

requestHandle finish

This method should be called after all data has been obtained. The request handle object is deleted by the system from within this call, so the object may not be used after this method has been invoked.

EXAMPLES

CUSTOM URI SCHEMES

The hv3 widget may seem a little unusual at first in that there is no interface to feed data directly from the users script to the widget. Instead, the widget requests the required data by invoking the -requestcmd script. Data is identified by the -uri option of the request handle passed as an argument.

The reason for this is that the widget often deals with documents that contain linked resources (external CSS stylesheets or images). The resources are not always known when the user script initiates loading the document. For example, if the following document is to be loaded from URI "http://tkhtml.tcl.tk": 

<HTML>
  <BODY>
    <IMG src="image.gif">
  </BODY>
</HTML>

then the -requestcmd must implement the HTTP protocol. The user calls: 

$hv3 goto http://tkhtml.tcl.tk

which causes the -requestcmd script to be invoked with a request handle argument specifying the URI "http://tkhtml.tcl.tk".

When the -requestcmd returns the data for the URI "http://tkhtml.tcl.tk", the widget invokes the -requestcmd a second time, with a request handle argument specifying the URI "http://tkhtml.tcl.tk/image.gif". If the document contained links to CSS stylesheets or other images, the _-requestcmd- script would be invoked for each of these also.

All this is fine if you are fetching data from http servers, but a little inconvenient if the user script already has the document to display ready in a Tcl variable (or variables). The solution here is to invent a custom URI scheme to use within the application. For example, the following example demonstrates a
 -requestcmd script that implements the "tclvar:", URI scheme for refering to global Tcl variables. 

proc tclvar_requestcmd {R} {
  # Get the URI from the request handle. The URI should look 
  # something like:
  #
  #   tclvar:///<global varname>
  #
  set uri [$R cget -uri]
  # Strip "tclvar:///" from the start of the URI.
  set var [string range $uri 10 end]
  # Return the data in the global variable $var to the widget.
  global $var
  $R finish [set $var]
}

And a simple script for using this -requestcmd: 

set my_document {
  <HTML>
    <LINK rel="stylesheet" href="my_stylesheet">
    <BODY>
      <P>Some red text.</P>
    </BODY>
  </HTML>
}
set my_stylesheet {
  P { color : red }
}
::hv3::hv3 .hv3 
pack .hv3 -fill both -expand true
 .hv3 configure -requestcmd tclvar_requestcmd
 .hv3 goto tclvar:///my_document

Note the complication in the code above - the string "tclvar:///" is found at the start of each URI passed to [tclvar_requestcmd]. This is because Hv3 resolves and escapes all URIs against the base URI of the currently loaded document before passing them to the
 -requestcmd. This means you need to be careful with special characters. If the name of the variable storing the stylesheet document in the above example were ::css::my_stylesheet, then markup like this: 

<LINK rel="stylesheet" href="::css::my_stylesheet">

would not work. The string "::css::my_stylesheet" is not a valid relative or absolute URI, so the results of resolving it against the base URI of the document, "tclvar:///my_document", are not defined. The solution is to escape the variable names using URI escapes. The Tkhtml3 package provides the [::tkhtml::encode] and [::tkhtml::decode] commands for escaping and unescaping strings, respectively. After modifying the -requestcmd proc to support escaped strings, it looks like this: 

proc tclvar_requestcmd {R} {
  # Get the URI from the request handle. The URI should look 
  # something like:
  #
  #   tclvar:///<global varname>
  #
  set uri [$R cget -uri]
  # Strip "tclvar:///" from the start of the URI.
  set var [::thtml::decode [string range $uri 10 end]]
  # Return the data in the global variable $var to the widget.
  global $var
  $R finish [set $var]
}

This could be used with a script like this: 

set my/document {
  <HTML>
    <LINK rel="stylesheet" href="%3A%3Acss%3A%3Amy_stylesheet">
    <BODY>
      <P>Some red text.</P>
    </BODY>
  </HTML>
}
namespace eval ::css {
  set my_stylesheet {
    P { color : red }
  }
}
::hv3::hv3 .hv3
pack .hv3 -fill both -expand true
 .hv3 configure -requestcmd tclvar_requestcmd
 .hv3 goto tclvar:///[::tkhtml::encode my/document]

In this case the two invocation of tclvar_requestcmd are made with request handle arguments with the following -uri option values: 

tclvar:///my%2Fdocument
tclvar:///%3A%3Acss%3A%3Amy_stylesheet

Other custom URI scheme handlers could retrieve data by evaluating Tcl scripts, querying a database or accessing any other part of the application.

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