[Appendix A] A.2 Canvas - e-Reading Library

A.2 Canvas

Table A.5 shows the properties of each item that is supported by the canvas widget and configured by using the widget's itemconfigure method. The widget itself has no particularly interesting properties. However, the methods shown in the latter half of the table are the widget's, even if they mostly apply to the individual canvas item types. Please note that all methods that take an item ID as a parameter can also take a preconfigured tag name as a parameter.

Table A.5: Canvas Class
Item Properties	Description
Common properties
fill, outline	Colors for filling the region and outline.
tags	A list of strings. These are tags applicable to this item. You can add more to this list with `addtag` .
stipple, outlinestipple	Draw the interior or outline with a stipple pattern. The bitmap value specifies the stipple pattern.
width	Width of the outline.
Arc
start, extent	Angles in degrees, moving counterclockwise.
style	`pieslice` , `chord` , `arc` . In the last case, the `fill` option is ignored.
Bitmap
anchor	As in widget properties shown earlier.
bitmap	The bitmap to display.
background, foreground	Colors for each of the bitmap pixels.
Image
anchor	As in widget properties shown earlier.
image	The image to display.
Line
arrow	`First` , `last` , `both` , or `none` . The end at which an arrow should be drawn.
arrowshape	A reference to a list containing three dimensions a, b, and c like this:
fill	Color.
smooth, splinesteps	If 1, draws a bezier curve instead of a polyline. Each spline is approximated with the number of spline steps.
Polygon
smooth, splinesteps	See "Line" above.
Oval
Standard item properties
Rectangle
Standard item properties
Text
text, anchor	Position text with respect to anchor.
justify	Text justification: `left` , `right` , or `center` .
Window
window	Specifies the widget to associate with this item. The widget must have been created as the canvas's child.
Methods
create (type, x, y [x1, y1], [options...])	Type can be one of the above item types (non-capitalized). Returns a unique integer ID.
itemconfigure (ID, options..)	Configures one or more of the above parameters.
addtag, dtag	Add tags to items and delete them. Please see Tk documentation for tag specifications.
bind	Discussed in the section "Event Bindings" .
coords (ID [x0, y0 ...] ), move (id, xamount, yamount)	Move the item to the new location. `coords` is an absolute move, while `move` does it relative to current position.
delete (ID, [ID, ...] )	Delete the item (or items) that correspond to the tag or ID.
find (searchCommand? arg ...? )	Find all items that meet a certain constraint. Constraints are of the form "above $id," "all," "below $id," "closest x y," "enclosed x1 y1 x2 y2," "withtag id," and so on.
postscript (?option value option value ...? )	Generate PostScript representation for part or whole of the canvas. Please check the Tk canvas documentation for the postscript rendering options.
raise, lower	Raise or lower the item.
scale (ID, xOrigin, yOrigin, xScale, yScale )	Rescale all of the items given by ID.

A.2.1 Text

Many of the text widget methods take one or more indices as arguments. An index can be an absolute number ("base") or a relative number ("base" + modifier). Both types of indices are specified as strings. The commonly used base indices are the following:

line.char: Indicates char 'th character on line. Lines are numbered from 1 for consistency with other Unix programs that use this numbering scheme. Within a line, characters are numbered from 0.
end: Indicates the end of the text (the character just after the last newline).
insert: Where the insertion cursor is currently poised.
mark: Indicates the character just after the mark whose name is mark.
tag. first , tag .last: Indicates the first and last character of a tag.

These absolute positions can be modified with one or more qualifiers:

+ count chars , - count chars, + count lines , - count lines: Adjust the base index by count characters or lines.
wordstart , wordend , linestart , lineend: Adjust the index to point to the first character on the word or line referred to by the index, or just after the word or line.

Table A.6 shows some of the more interesting text properties and methods.

Table A.6: Text Properties and Methods
Text	Description
tabs	Specifies a set of tab stops for the window, as a reference to list of strings. Each string is a number followed by "l," "c," or "r" (left, center, right - to specify how the text is aligned respective to the tab).
height, width	Specifies height and width in number of characters.
state	`normal` or `disabled`
Methods
Text Manipulation
insert (index, {string, [tag]}+, )	Insert one or more strings with an optional tag at index. Index can be any of the index forms explained earlier.
delete(index1, [index2])	Delete character at index1, or the range index1 .. index2.
get (index1, [index2])	Get character at index1, or the range index1 .. index2.
see (index)	Scroll widget so that the index position is visible.
search([switches], pattern, index, [stopIndex])	Searches for text and returns the first index that matches the pattern. The search stops at stopIndex, if it is specified; otherwise it wraps around. Switches include `forward` , `backward` , `exact` (exact match - default), `regexp` , `-nocase` (ignore case), `-count` `var` (`var` is the reference to a variable, into which `search` stores the length of the matched string). The list of switches is terminated by " - ".
Indexing
index (index)	Returns an absolute index of the form line.col , given any of the other index forms.
see (index)	Ensures that the text at index is visible.
markSet (markName, index)	Give a logical bookmark name to that index.
markUnset (markName)	Remove bookmark.
Tag Manipulation
tagAdd (tagName, {index1. [index2]}+)	Add tags to positions or ranges of characters. `insert` is another way of tagging text.
tagRemove (tagName, {index1. [index2]}+	Removes the tag from the specified areas but doesn't delete the tag itself.
tagDelete	Removes and delete the tag.
tagConfigure	Configure one or more properties for a tag. The tag properties are given below.
Tag Properties
-foreground, -background, -fgstipple, -bgstipple, -font	The usual stuff. Try not to overuse these tags, or the text will look like a ransom note - bits and pieces cut from different newspapers and magazines.
-justify,	`center` , `left` , `right` .[ 1 ]
-relief, -borderwidth	Should specify both border width and background for relief to show up.
-tabs	Applies only if the first character in that line also belongs to the same tag.
-underline	Boolean option.

[1] Text processing has made it possible to right-justify any idea, even one which cannot be justified on any other grounds.

- J. Finnegan, USC.


A.1 Button		A.3 Entry