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<!--

=head2 Adding new methods

For slightly more demanding changes, you may wish to add new methods to the
C<cons> package. Here's an example of a very simple extension,
C<InstallScript>, which installs a tcl script in a requested location, but
edits the script first to reflect a platform-dependent path that needs to be
installed in the script:

  # cons::InstallScript - Create a platform dependent version of a shell
  # script by replacing string ``#!your-path-here'' with platform specific
  # path $BIN_DIR.

  sub cons::InstallScript {
	my ($env, $dst, $src) = @_;
	Command $env $dst, $src, qq(
		sed s+your-path-here+$BIN_DIR+ %< > %>
		chmod oug+x %>
	);
  }

Notice that this method is defined directly in the C<cons> package (by
prefixing the name with C<cons::>). A change made in this manner will be
globally visible to all environments, and could be called as in the
following example:

  InstallScript $env "$BIN/foo", "foo.tcl";

For a small improvement in generality, the C<BINDIR> variable could be
passed in as an argument or taken from the construction environment-,-as
C<%BINDIR>.


=head2 Overriding methods

Instead of adding the method to the C<cons> name space, you could define a
new package which inherits existing methods from the C<cons> package and
overrides or adds others. This can be done using Perl's inheritance
mechanisms.

The following example defines a new package C<cons::switch> which
overrides the standard C<Library> method. The overridden method builds
linked library modules, rather than library archives. A new
constructor is provided. Environments created with this constructor
will have the new library method; others won't.

  package cons::switch;
  BEGIN {@ISA = 'cons'}

  sub new {
	shift;
	bless new cons(@_);
  }

  sub Library {
	my($env) = shift;
	my($lib) = shift;
	my(@objs) = Objects $env @_;
	Command $env $lib, @objs, q(
		%LD -r %LDFLAGS %< -o %>
	);
  }

This functionality could be invoked as in the following example:

  $env = new cons::switch(@overrides);
  ...
  Library $env 'lib.o', 'foo.c', 'bar.c';

-->

  <para>

  Although &SCons; provides many useful methods
  for building common software products:
  programs, libraries, documents.
  you frequently want to be
  able to build some other type of file
  not supported directly by &SCons;
  Fortunately, &SCons; makes it very easy
  to define your own &Builder; objects
  for any custom file types you want to build.
  (In fact, the &SCons; interfaces for creating
  &Builder; objects are flexible enough and easy enough to use
  that all of the the &SCons; built-in &Builder; objects
  are created the mechanisms described in this section.)

  </para>

  <section>
  <title>Writing Builders That Execute External Commands</title>

    <para>

    The simplest &Builder; to create is
    one that executes an external command.
    For example, if we want to build
    an output file by running the contents
    of the input file through a command named
    <literal>foobuild</literal>,
    creating that &Builder; might look like:

    </para>

    <programlisting>
       bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
    </programlisting>

    <para>

    All the above line does is create a free-standing
    &Builder; object.
    The next section will show us how to actually use it.

    </para>

  </section>

  <section>
  <title>Attaching a Builder to a &ConsEnv;</title>

    <para>

    A &Builder; object isn't useful
    until it's attached to a &consenv;
    so that we can call it to arrange
    for files to be built.
    This is done through the &BUILDERS;
    &consvar; in an environment.
    The &BUILDERS; variable is a Python dictionary
    that maps the names by which you want to call
    various &Builder; objects to the objects themselves.
    For example, if we want to call the
    &Builder; we just defined by the name
    <function>Foo</function>,
    our &SConstruct; file might look like:

    </para>

    

    <programlisting>
       bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
       env = Environment(BUILDERS = {'Foo' : bld})
    </programlisting>

    <para>

    With the &Builder; so attached to our &consenv;
    we can now actually call it like so:

    </para>

    <programlisting>
       env.Foo('file.foo', 'file.input')
    </programlisting>

    <para>

    Then when we run &SCons; it looks like:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      foobuild < file.input > file.foo
    </screen>

    <para>

    Note, however, that the default &BUILDERS;
    variable in a &consenv;
    comes with a default set of &Builder; objects
    already defined:
    &Program;, &Library;, etc.
    And when we explicitly set the &BUILDERS; variable
    when we create the &consenv;,
    the default &Builder;s are no longer part of
    the environment:

    </para>

    <programlisting>
       bld = Builder(action = 'foobuild &lt; $SOURCE &gt; $TARGET')
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file.foo', 'file.input')
       env.Program('hello.c')
    </programlisting>

    <screen>
      % <userinput>scons -Q</userinput>
      AttributeError: SConsEnvironment instance has no attribute 'Program':
        File "SConstruct", line 4:
          env.Program('hello.c')
    </screen>

    <para>

    To be able use both our own defined &Builder; objects
    and the default &Builder; objects in the same &consenv;,
    you can either add to the &BUILDERS; variable
    using the &Append; function:

    </para>

    

    <programlisting>
       env = Environment()
       bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
       env.Append(BUILDERS = {'Foo' : bld})
       env.Foo('file.foo', 'file.input')
       env.Program('hello.c')
    </programlisting>

    <para>

    Or you can explicitly set the appropriately-named
    key in the &BUILDERS; dictionary:

    </para>

    <programlisting>
       env = Environment()
       bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
       env['BUILDERS']['Foo'] = bld
       env.Foo('file.foo', 'file.input')
       env.Program('hello.c')
    </programlisting>

    <para>

    Either way, the same &consenv;
    can then use both the newly-defined
    <function>Foo</function> &Builder;
    and the default &Program; &Builder;:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      foobuild < file.input > file.foo
      cc -c -o hello.o hello.c
      cc -o hello hello.o
    </screen>

  </section>

  <section>
  <title>Letting &SCons; Handle The File Suffixes</title>

    <para>

    By supplying additional information
    when you create a &Builder;,
    you can let &SCons; add appropriate file
    suffixes to the target and/or the source file.
    For example, rather than having to specify
    explicitly that you want the <literal>Foo</literal>
    &Builder; to build the <literal>file.foo</literal>
    target file from the <literal>file.input</literal> source file,
    you can give the <literal>.foo</literal>
    and <literal>.input</literal> suffixes to the &Builder;,
    making for more compact and readable calls to
    the <literal>Foo</literal> &Builder;:

    </para>

    

    <programlisting>
       bld = Builder(action = 'foobuild < $SOURCE > $TARGET',
                     suffix = '.foo',
                     src_suffix = '.input')
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file1')
       env.Foo('file2')
    </programlisting>

    <screen>
      % <userinput>scons -Q</userinput>
      foobuild < file1.input > file1.foo
      foobuild < file2.input > file2.foo
    </screen>

    <para>

    You can also supply a <literal>prefix</literal> keyword argument
    if it's appropriate to have &SCons; append a prefix
    to the beginning of target file names.

    </para>

  </section>

  <section>
  <title>Builders That Execute Python Functions</title>

    <para>

    In &SCons;, you don't have to call an external command
    to build a file.
    You can, instead, define a Python function
    that a &Builder; object can invoke
    to build your target file (or files).
    Such a &buildfunc; definition looks like:

    </para>

    <programlisting>
       def build_function(target, source, env):
           # Code to build "target" from "source"
           return None
    </programlisting>

    <para>

    The arguments of a &buildfunc; are:

    </para>

    <variablelist>

      <varlistentry>
      <term>target</term>

      <listitem>
      <para>

      A list of Node objects representing
      the target or targets to be
      built by this builder function.
      The file names of these target(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>
      </varlistentry>

      <varlistentry>
      <term>source</term>

      <listitem>
      <para>

      A list of Node objects representing
      the sources to be
      used by this builder function to build the targets.
      The file names of these source(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>
      </varlistentry>

      <varlistentry>
      <term>env</term>

      <listitem>
      <para>

      The &consenv; used for building the target(s).
      The builder function may use any of the
      environment's construction variables
      in any way to affect how it builds the targets.

      </para>
      </listitem>
      </varlistentry>

    </variablelist>

    <para>

    The builder function must
    return a <literal>0</literal> or <literal>None</literal> value
    if the target(s) are built successfully.
    The builder function
    may raise an exception
    or return any non-zero value
    to indicate that the build is unsuccessful,

    </para>

    <para>

    Once you've defined the Python function
    that will build your target file,
    defining a &Builder; object for it is as
    simple as specifying the name of the function,
    instead of an external command,
    as the &Builder;'s
    <literal>action</literal>
    argument:

    </para>

    <programlisting>
       def build_function(target, source, env):
           # Code to build "target" from "source"
           return None
       bld = Builder(action = build_function,
                     suffix = '.foo',
                     src_suffix = '.input')
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file')
    </programlisting>

    <para>

    And notice that the output changes slightly,
    reflecting the fact that a Python function,
    not an external command,
    is now called to build the target file:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      build_function(["file.foo"], ["file.input"])
    </screen>

  </section>

  <section>
  <title>Builders That Create Actions Using a &Generator;</title>

    <para>

    &SCons; Builder objects can create an action "on the fly"
    by using a function called a &generator;.
    This provides a great deal of flexibility to
    construct just the right list of commands
    to build your target.
    A &generator; looks like:

    </para>

    <programlisting>
       def generate_actions(source, target, env, for_signature):
           return 'foobuild < %s > %s' % (target[0], source[0])
    </programlisting>

    <para>

    The arguments of a &generator; are:

    </para>

    <variablelist>

      <varlistentry>
      <term>source</term>

      <listitem>
      <para>

      A list of Node objects representing
      the sources to be built
      by the command or other action
      generated by this function.
      The file names of these source(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>

      </varlistentry>

      <varlistentry>
      <term>target</term>

      <listitem>
      <para>

      A list of Node objects representing
      the target or targets to be built
      by the command or other action
      generated by this function.
      The file names of these target(s)
      may be extracted using the Python &str; function.

      </para>
      </listitem>

      </varlistentry>

      <varlistentry>
      <term>env</term>

      <listitem>
      <para>

      The &consenv; used for building the target(s).
      The generator may use any of the
      environment's construction variables
      in any way to determine what command
      or other action to return.

      </para>
      </listitem>

      </varlistentry>

      <varlistentry>
      <term>for_signature</term>

      <listitem>
      <para>

      A flag that specifies whether the
      generator is being called to contribute to a build signature,
      as opposed to actually executing the command.

      <!-- XXX NEED MORE HERE -->

      </para>
      </listitem>

      </varlistentry>

    </variablelist>

    <para>

    The &generator; must return a
    command string or other action that will be used to
    build the specified target(s) from the specified source(s).

    </para>

    <para>

    Once you've defined a &generator;,
    you create a &Builder; to use it
    by specifying the generator keyword argument
    instead of <literal>action</literal>.

    </para>

    

    <programlisting>
       def generate_actions(source, target, env, for_signature):
           return 'foobuild < %s > %s' % (source[0], target[0])
       bld = Builder(generator = generate_actions,
                     suffix = '.foo',
                     src_suffix = '.input')
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file')
    </programlisting>

    <screen>
      % <userinput>scons -Q</userinput>
      foobuild < file.input > file.foo
    </screen>

    <para>

    Note that it's illegal to specify both an
    <literal>action</literal>
    and a
    <literal>generator</literal>
    for a &Builder;.

    </para>

  </section>

  <section>
  <title>Builders That Modify the Target or Source Lists Using an &Emitter;</title>

    <para>

    &SCons; supports the ability for a Builder to modify the
    lists of target(s) from the specified source(s).

    </para>

    

    <programlisting>
       def modify_targets(target, source, env):
           target.append('new_target')
           source.append('new_source')
           return target, source
       bld = Builder(action = 'foobuild $TARGETS - $SOURCES',
                     suffix = '.foo',
                     src_suffix = '.input',
                     emitter = modify_targets)
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file')
    </programlisting>

    <screen>
      % <userinput>scons -Q</userinput>
      foobuild file.foo new_target - file.input new_source
    </screen>

    <programlisting>
       bld = Builder(action = 'XXX',
                     suffix = '.foo',
                     src_suffix = '.input',
                     emitter = 'MY_EMITTER')
       def modify1(target, source, env):
           return target, source
       def modify2(target, source, env):
           return target, source
       env1 = Environment(BUILDERS = {'Foo' : bld},
                          MY_EMITTER = modify1)
       env2 = Environment(BUILDERS = {'Foo' : bld},
                          MY_EMITTER = modify2)
       env1.Foo('file1')
       env2.Foo('file2')
    </programlisting>

  </section>

  <!--

  <section>
  <title>Builders That Use Other Builders</title>

    <para>

    XXX

    </para>

    <scons_example name="ex8">
       <file name="SConstruct" printme="1">
       env = Environment()
       #env.SourceCode('.', env.BitKeeper('XXX'))
       env.Program('hello.c')
       </file>
       <file name="hello.c">
       hello.c
       </file>
    </scons_example>

    <scons_output example="ex8">
      <command>scons -Q</command>
    </scons_output>

  </section>

  -->