The fglsvgcanvas web component

The fglsvgcanvas built-in web component implements a drawing canvas for Scalable Vector Graphics content.


This documentation does not explain SVG drawing principles. Before starting with fglsvgcanvas, consider learning SVG, with tutorials found on the internet. It is especially mandatory to properly understand the root SVG viewport / viewBox / preserveAspectRatio concepts.

The fglsvgcanvas built-in web component is a gICAPI web component.

Since SVG allows you to specify coordinates and sizes with units such as "10em", and including the % percentage unit like in "50%", fglsvgcanvas functions use the STRING type for parameters such as x, y, width, height. In SVG, the decimal separator for numeric values must always be a dot. When computing coordinates and sizes with numeric types such as DECIMAL or when passing decimal values directly to fglsvgcanvas functions, pay attention to numeric to string conversion. By default, the DBMONEY/DBFORMAT settings apply and can produce a decimal separator different from the dot. The fglsvgcanvas library converts automatically decimal coordinates to the ISO format. However, if possible, define a large SVG viewBox (like "0 0 1000 1000"), in order to use only integer numbers for coordinates and sizes, or % percentage units.

The fglsvgcanvas web component HTML page is basically a simple HTML container. It is delivered with the utility library $FGLDIR/src/webcomponents/fglsvgcanvas/fglsvgcanvas.4gl, that can be used to produce SVG content.

The programming pattern is based on the built-in om.* API. Create DOM nodes with the utility functions, and construct the root <svg/> element by adding child nodes created from the fglsvgcanvas functions.

Figure: fglsvgcanvas sample from Fourjs Github

Screenshot of a program using the fglsvgcanvas web component


Defining the fglsvgcanvas web component in the form file

In the .per form definition file, define the SVG container as a WEBCOMPONENT form item with the COMPONENTTYPE attribute set to the "fglsvgcanvas" value.


The form field name will be used in front calls to identify the SVG canvas.

Since the SVG canvas web component provides built-in scrollbars, the SCROLLBARS attribute can be set to NONE.

Use SIZEPOLICY=FIXED and STRETCH=BOTH, to get an SVG canvas that resizes with the parent window.

Additional fglsvgcanvas web component configuration options can be defined with the PROPERTIES attribute (details will be discussed later in this topic).

For example:
[cv                     ]
[                       ]
[                       ]
   COMPONENTTYPE = "fglsvgcanvas",
   PROPERTIES = ( selection="item_selection" ),

The fglsvgcanvas.4gl library

SVG can be used to draw complex content such as an agenda or a graph, with advanced SVG concepts such as CSS styles, patterns, nested <svg/> elements, etc.

To simplify SVG programming, Genero BDL provides the $FGLDIR/src/webcomponents/fglsvgcanvas/fglsvgcanvas.4gl utility library. This library implements a set of functions that produce common SVG elements.

The fglsvgcanvas.4gl library supports following SVG features:
  • Attribute sets
  • CSS styles
  • Patterns
  • Masks
  • Filters
  • Gradients
  • Shapes (rect, circle, polygon, etc)
  • Simple text, text on path, text tspan
  • Animation
  • Clickable elements
  • Clipping paths
  • RGB color utilities (shade, tint)

fglsvgcanvas library initialization and finalization

Library initialization and finalization functions are provided to prepare the library before usage, and free resources when the library is no longer needed:

CALL fglsvgcanvas.initialize()
CALL fglsvgcanvas.finalize()

For more details see initialize() and finalize().

Creating an SVG canvas handler

Before creating new SVG element, you need to create the SVG canvas handler, to get an id that will be used in subsequent fglsvgcanvas calls. Define a SMALLINT variable to hold the SVG canvas handler id that is returned by the create() function. This function takes the WEBCOMPONENT field name as attribute, to bind the form field to the SVG canvas handle:

LET cid = fglsvgcanvas.create("formonly.canvas")

Selecting an SVG canvas handler

The SVG canvas is identified by the id returned by the create() function. After creating an SVG canvas, it is automatically defined as the current canvas, and any subsequent calls to an fglsvgcanvas function will apply to that current canvas handler. If you want to manipulate several SVG canvases, select the current canvas with the setCurrent() function:

CALL fglsvgcanvas.setCurrent(cid)

The root SVG node

The root SVG DOM node is created when calling the create() function. However, before drawing your SVG, you need to define essential root SVG attributes.

To define the root SVG attributes, use the setRootSVGAttributes() function.

This function returns the root om.DomNode of the SVG tree:

DEFINE root_svg, n om.DomNode
LET root_svg = fglsvgcanvas.setRootSVGAttributes(
                      "10em", "5em", -- viewport
                      "0 0 500 200",  -- viewbox
                      "xMidYMid meet" -- preserveAspectRatio
Specify the following properties to define your root SVG element:
  • the viewport defines the viewing area for the SVG image (use NULL,NULL for auto-resize; default unit is px, consider using em unit),
  • the viewBox defines the internal coordinate system (with a (0,0,500,200) viewBox, point (250,100) is the middle),
  • the parameters to preserve the aspect ratio.

For more details about the <svg/> element attributes, see the W3 SVG specification.

Destroying an SVG canvas handler

When the SVG canvas handler is no longer needed (for example, before closing the form/window displaying the corresponding WEBCOMPONENT), you can release resources allocated for the SVG handle by calling the destroy() function:
LET cid = fglsvgcanvas.create("formonly.canvas")
CALL fglsvgcanvas.destroy( cid )
After calling the destroy() function, the SVG canvas handler that was created before the destroyed handler, will be set as the new current SVG canvas:
LET cid1 = fglsvgcanvas.create("formonly.canvas1") -- current canvas is cid1
LET cid2 = fglsvgcanvas.create("formonly.canvas2") -- current canvas is cid2
CALL fglsvgcanvas.destroy( cid2 ) -- current canvas is cid1
CALL fglsvgcanvas.destroy( cid1 ) -- no current canvas

Building the SVG DOM tree

After creating the root SVG DOM node with the setRootSVGAttributes() function, create other DOM element with fglsvgcanvas functions, and append the child nodes to the root element or sub-elements:
DEFINE root_svg, n, g om.DomNode
LET n = fglsvgcanvas.svg( ... )  -- creates an <svg/> sub-node.
CALL root_svg.appendChild( n )
LET g = fglsvgcanvas.g( ... )  -- creates a <g/> sub-node.
CALL n.appendChild( g )

Cleaning the SVG canvas

To clean up the SVG canvas content, use the clean() function:
CALL fglsvgcanvas.clean( cid )

Defining CSS styles

SVG supports CSS styling. Styles must be defined in the <defs/> element, and can then be referenced in SVG drawing elements by using the class attribute.

To create styles, start by defining a set of SVG attributes with an om.SaxAttributes object. For attribute names, use the predefined SVGATT_* constants available in the fglsvgcanvas library.

Define a dynamic array of om.SaxAttributes, to define several reusable attribute sets.


DEFINE attr DYNAMIC ARRAY OF om.SaxAttributes
LET attr[COLORS_OCEAN] = om.SaxAttributes.create()
CALL attr[COLORS_OCEAN].addAttribute(SVGATT_FILL,           "cyan" )
CALL attr[COLORS_OCEAN].addAttribute(SVGATT_FILL_OPACITY,   "0.3" )
CALL attr[COLORS_OCEAN].addAttribute(SVGATT_STROKE,         "blue" )

LET attr[COLORS_SAHARA] = om.SaxAttributes.create()
CALL attr[COLORS_SAHARA].addAttribute(SVGATT_FILL,          "yellow" )
An SVG attribute set defined in an om.SaxAttributes object can be used in different manners:
  1. To explicitly set attributes in an SVG DOM node, with the setAttributes() function.
  2. To define a CSS style with a selector and a list of name:value; pairs, with the styleDefinition() function.
  3. To set an inline-style in an DOM node, defining a list of name:value; pairs, with the styleAttributeList() function.

To create a set of CSS styles, create a <defs/> SVG element, containing a <style/> element including your attributes sets.

The <style/> element is created with the styleList() function, and each CSS style string is created with the styleDefinition() function, from the om.SaxAttributes objects:

DEFINE defs om.DomNode
LET defs = fglsvgcanvas.defs( NULL )
CALL defs.appendChild( fglsvgcanvas.styleList(
                       || fglsvgcanvas.styleDefinition(".style_sahara",attr[COLORS_SAHARA])
CALL root_svg.appendChild( defs )

Then, you can, for example, define a <rect/> element that references the style defined with in the <defs/> node:

DEFINE r om.DomNode
LET r = fglsvgcanvas.rect(10,10,30,40,NULL,NULL)
CALL r.setAttribute(SVGATT_CLASS, "style_ocean")

Displaying the SVG content

After creating the SVG content, it must be sent to the front-end for rendering.

To display the SVG content to the WEBCOMPONENT field associated with the SVG canvas handle, use the display() function:

CALL fglsvgcanvas.display( cid )

Detecting SVG element selection

To enable clickable SVG elements, define the WEBCOMPONENT form field with a "selection" property in the PROPERTIES attribute. This property defines the action that will be fired when the user clicks on an SVG element. Use lowercase action names:

   PROPERTIES = (selection="item_selection"),

Define clickable SVG elements by using the <g/> SVG grouping element contain child elements.

The child elements of the clickable group must not use the fill:none style, otherwise they are not clickable.

The group element must have the SVGATT_ONCLICK/"onclick" attribute set to SVGVAL_ELEM_CLICKED: The JavaScript function defined by SVGVAL_ELEM_CLICKED is predefined in fglsvgcanvas.html, and will trigger the action defined by the selection property defined by the WEBCOMPONENT form field.

The <g/> group element must be defined with an "id" attribute, that will be used to identify the clicked elements:

DEFINE root_svg, g, c om.DomNode
CALL root_svg.appendChild( g := fglsvgcanvas.g("shape_1") )
CALL g.appendChild( c :=,20,3) )
CALL c.setAttribute(SVGATT_STYLE, 'stroke:gray;fill:blue;fill-opacity:0.3' )
CALL g.appendChild( c :=,12,2) )
CALL c.setAttribute(SVGATT_STYLE, 'stroke:gray;fill:yellow;fill-opacity:0.8' )
Note that child elements can be created, assigned to a variable with the := operator and the resulting expression can be directly passed as appendChild() parameter.
CALL root_svg.appendChild( g := fglsvgcanvas.g("shape_1") )

In the program code, detect SVG element selection with an ON ACTION handler using the action name that matches the "selection" property of the WEBCOMPONENT field. When the action is fired, the id of the selected SVG element is provided in the WEBCOMPONENT field value:

          canvas STRING, -- Variable bound to the WEBCOMPONENT form field
      DISPLAY "Clicked element:", rec.canvas

Information about the clicked SVG element is provided in JSON notation, for example:


It is also possible to use the fglsvgcanvas.getItemId() function to get the id of the clicked SVG element. However, this produces a front call that can be avoided, since the element id is available in the WEBCOMPONENT field value. The getItemId() function should only be used when the WEBCOMPONENT field does not have the focus, for example to handle mouse hovering events with mouse_event_focus = false.

For a complete example, see Example 2: Basic clickable SVG shapes with fglsvgcanvas.

Detecting mouse double-clicks on SVG elements

To detect a double-click on an SVG element, create SVG elements with the SVGATT_ONCLICK attribute as described in Detecting SVG element selection, and define the selection2 property in the WEBCOMPONENT field:


And define the corresponding action handler in the code:

   ON ACTION item_selection2
Simple click and double-click actions can be used individually (when only double-click is required, there is no need to define the selection property for single-clicks)

When both single-click (selection property) and double-click (selection2 property) actions are used, the single-click (selection) action will be fired after a delay, because this is the only why to distinguish from double-click mouse events in SVG).

For a complete example, see Example 2: Basic clickable SVG shapes with fglsvgcanvas.

Detecting mouse right-clicks on SVG elements

To detect a right-click (for context-menu) on an SVG element, define the selection3 property in the WEBCOMPONENT field:


And define the corresponding action handler in the code:

   ON ACTION item_selection3

For right-click/context-menu events, no SVGATT_ONCLICK attribute needs to be defined for the SVG elements: The action defined by the selection3 property will automatically be fired for any SVG elements having an id attribute.

For a complete example, see Example 2: Basic clickable SVG shapes with fglsvgcanvas.

Detecting mouse hovering on SVG elements

SVG elements can be defined with the mouse hovering events "onmouseover" and "onmouseout", respectively defined as the SVGATT_ONMOUSEOVER and SVGATT_ONMOUSEOUT constants in fglsvgcanvas.4gl for convenience and code readability.

If you want to implement display effects only, consider using well-known SVG + JavaScript techniques to modify element attributes directly in the web browser context, and thus avoid unnecessary network round-tips. For example, to produce some responsive rendering effect when the mouse goes over an SVG element, you can do following:
DEFINE n om.DomNode
LET n = fglsvgcanvas.rect(50,50,20,30,2,2)
CALL n.setAttribute(SVGATT_ONMOUSEOVER, "'opacity', '0.5');")
CALL n.setAttribute(SVGATT_ONMOUSEOUT,  "'opacity', '1.0');")

The fglsvgcanvas web component can be configured to bind the onmouseover/onmouseout SVG mouse hovering events to ON ACTION handlers and trigger code, when the mouse goes over the SVG elements.


When SVG element selection is enabled, the corresponding action has a higher importance than mouse hovering actions: Depending on the mouse hovering timeout (mouse_event_timeout), a click on an SVG element may cancel the mouse over / mouse out actions, even if corresponding SVG onmouseover and onmouseout events occurred respectively before and after the onclick SVG event.

The next code example defines the web component field properties to handle mouse hovering SVG events:
       mouse_over = "item_mouse_over",
       mouse_out  = "item_mouse_out",
       mouse_event_timeout    = 600,
       mouse_event_focus      = false
Mouse hovering properties description:
  1. The mouse_over = "action-name" property identifies the ON ACTION action-name handler to be triggered, when the onmouseover SVG event occurs. Use lowercase to define mouse hovering action names. This property is mandatory to catch mouse hovering events.
  2. The mouse_out = "action-name" property identifies the ON ACTION action-name handler to be triggered, when the onmouseout SVG event occurs. Use lowercase to define mouse hovering action names. The property is typically used to reset mouse hovering management in the program code (the mouse goes away from that SVG element)
  3. mouse_event_timeout = milliseconds is used to avoid network clogging: This property defines the number of milliseconds to wait before sending the dialog action, after an SVG mouse event has occurred. A JavaScript setTimeout() timer is created with this value. The timer is re-initialized each time the event occurs. If not defined, default is 500 milliseconds.
  4. mouse_event_focus = {true|false} specifies if the web component field must have the focus, in order to fire the mouse hovering actions. If not defined, default is true.
When creating your SVG elements, bind mouse hovering SVG events to predefined JavaScript function corresponding to the SVG event. The functions take the current SVG element (this) as parameter.
For example:
DEFINE n om.DomNode
LET n = fglsvgcanvas.rect(x, y, w, h, NULL, NULL)

In the dialog code, implement the ON ACTION handlers to execute code, when the actions are fired.

If the mouse hovering action requires field focus (mouse_event_focus=true), the involved SVG element id is available in the field value, like for the item selection action. If the mouse hovering action can be fired when the web component field does not have the focus, use the fglsvgcanvas.getItemId() function to get the id of the SVG element.

        ON ACTION item_mouse_over ATTRIBUTES(DEFAULTVIEW = NO)
           MESSAGE SFMT("%1 : mouse over item : %2",
                        CURRENT HOUR TO FRACTION(5),
                        fglsvgcanvas.getItemId(cid) )

           MESSAGE ""

For a complete example, see Example 2: Basic clickable SVG shapes with fglsvgcanvas.

Getting the bounding box of an SVG element

After rendering your SVG, it is possible to get the bounding box of an element.

With elements such as SVG text using a specific font, it is difficult to compute the bounding box of the text, until it has been rendered.

First define a variable with the fglsvgcanvas.t_svg_rect type, then (after displaying the SVG with fglsvgcanvas.display(cid)), call the fglsvgcanvas.getBBox(cid, element-id) function, to get the bounding box of the element identified by element-id:
DEFINE rect fglsvgcanvas.t_svg_rect
ON ACTION get_bbox
   CALL fglsvgcanvas.getBBox(cid, "label_23") RETURNING rect.*
   DISPLAY rect.x, rect.y, rect.width, rect.height

The bounding box coordinates and size are returned in the current user space.

If the bounding box is required to place and size SVG element based on the position and size of other elements, consider using SVG scripting instead of the getBBox() function: this will avoid several network roundtrips to render the final SVG image.

For example:
<svg version="1.1" baseProfile="full"
     width="500px" height="500px" viewBox="0 0 2000 2000"

<script type="text/ecmascript">
// <![CDATA[

function setup_rect(box,bbox){
   box.setAttributeNS(null, "x", bbox.x - 2);
   box.setAttributeNS(null, "y", bbox.y - 2);
   box.setAttributeNS(null, "width", bbox.width + 4);
   box.setAttributeNS(null, "height", bbox.height + 4);

function setup(evt){
   setup_rect( document.getElementById("label1Box"), label1.getBBox() );

// ]]>
<g id="label1">
<text x="150" y="250" font-family="Verdana" font-size="55">Hello everybody!</text>
<rect id="label1Box" stroke="red" stroke-width="3px" fill="none"/>