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Import Filters
Skencil uses import filters to read vector drawings from files.
Configuration Variables
For import filter configuration, Skencil uses the following variables:
typeThe type must be Import. This identifier is defined in the
globals() dictionary.
	
class_nameA string containing the name of a Python class defined in the
module. See below for the interface this class has to provide.
		
rx_magicA string containing a regular expression suitable for the re
module. This regular expression is matched against the first
line of the file. if it matches, the import mechanism assumes
that the filter understands the file's format and then procedes
to use that filter to load the file. So, if possible, this
regular expression should match those files the filter can
handle and only those.
While this method of identifying files is not really sufficient
to handle all file types, it works well enough for now. In the
future we will probably have to extend this mechanism.
tk_file_typeA tuple containing two elements. The first is the name of the
file format presented to the user in the file open dialog. The
second element is eithe a string or a tuple of strings. Each of
the strings defines a file name extension used for that
filename.
Examples:
#tk_file_type = ('Skencil Document', '.sk')
#tk_file_type = ('PostScript file', ('.ps', '.eps'))
		
unload (optional)A boolean. True means that the filter module should be unloaded
after use, false means that it shouldn't be unloaded. If the
module is unloaded it will be imported again the next time the
filter is needed. Infrequently used filters should set this to
true, frequently used filters (like the filter for Skencil's own
format) should set it to true.
If omitted, it defaults to false.
It's a good idea to set this variable to true at least while
developing a filter because that way the filter will be
automatically reloaded when the source file changes.
format_nameThe name of the format as a string. This name is currently used
internally to find out whether a document was loaded from a file
in Skencil's format or from a different format (in the latter
case `Save' is treated like `Save As...', i.e. the user has to
provide a filename).
standard_messages (optional)A boolean indicating whether the messages in the plugin are
standard messages that are defined in Skencil's message catalogs.
For third-party plugins this should be omitted.
The Class Interface
External Interface
The interface a loader class presents to the document loading mechanism
is quite simple. The loading mechanism works roughly as follows:
Read the first line of the file
Match it in turn against all the regular expressions
defined for import filter plugins (defined by the 
#N4rx_magic variable ).
If a match is found, instantiate the plugin's class
(defined by the 
#N3class_name variable ) with
three positional parameters 
file, filename and
match.
file is the open input file object, positioned just after
the first line. 
filename is the name of the file.
match is the match-object returned from regular expression
match.
If possible, filters should read the input file sequentially and
not use seeks. Skencil automatically pipes gzipped files
through gzip and in that case the file object is actually a
pipe, so seeks would be impossible. If a filter needs to access
the first line it can do so by using 
match.string or by
defing suitable groups in the regular expression.
Call the instance's Load() method (without arguments).
This method does the actual work and returns an instance of the
document class.
Call the instance's Messages() method without arguments.
The return value is expected to be a list of strings with
messages that are usually presented to the user by the UI.
If the #N5unload configuration
variable
 is true, unload the plugin's module.
Common Base Classes
To make things easier, Skencil defines some base classes that plugins
can inherit, 
LoaderWithComposites and, derived from
that, 
#N10GenericLoader  and SimplifiedLoader, derived in turn from GenericLoader.
GenericLoader
The class GenericLoader provides a default implementation of the
constructor, a mechanism for warning and error messages and methods to
easily create standard objects like rectangles, ellipses and groups.
GenericLoader defines these methods:
__init__(file, filename, match)Assign the arguments to self.file, self.filename and
self.match respectively.
document()Start the construction of a document. This method must be called
before any other of the object creation methods are called.
layer([name])Start a new layer. This method must be called after calling
#N12document  and before any of the other object
creation methods. 
name is the name of the layer. If omitted, it defaults to
"Layer 1".
begin_group()Start a new group. Groups can be arbitrarily nested. Each call
to 
begin_group must have a corresponding call to #N15end_group .
end_group()End the current group.
rectangle(m11, m21,
m12, m22, v1, v2[, radius1 = 0][,
radius2 =
0])
Create a new rectangle object and append it to the current
compound object.
The first six positional parameters (m11, m21,
m12, m22, v1, v2) define a
transformation matrix that transforms the unit square (opposite
corners (0,0) and (1,1)) onto the rectangle.
radius1 and radius2 define the rounded corners.
For more details see the documentation for the class devguide-18.html#N2Rectangle 
ellipse(m11, m21,
m12, m22, v1, v2[,
start_angle = 0.0][, end_angle = 0.0][,
arc_type = ArcPieSlice])Create a new ellipse object and append it to the current
compound object.
The first six positional parameters (m11, m21,
m12, m22, v1, v2) define a
transformation matrix that transforms the unit circle (center
(0,0) and radius 1) onto the ellipse.
start_angle and end_angle, if given, define the
start end end angles for pie slices, arc and chords.
arc_type defines whether and how the arc of an incomplete
ellipse is closed. Valid values are 
ArcArc,
ArcPieSlice and ArcChord. These constants are defined
in 
Sketch.const
For more details see the documentation for the class Ellipse
bezier(paths)Create a new bézier object in the current compound object.
paths must be a tuple of devguide-8.htmlcurve
objects
.
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