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    String::BufferStack - Nested buffers for templating systems

      my $stack = String::BufferStack->new;
      $stack->push( filter => sub {return uc shift} );

    "String::BufferStack" provides a framework for storing nested buffers.
    By default, all of the buffers flow directly to the output method, but
    individual levels of the stack can apply filters, or store their output
    in a scalar reference.

    Creates a new buffer stack and returns it. Possible arguments include:

        Preallocate this many bytes in the output buffer. This can reduce
        reallocations, and thus speed up appends.

        The method to call when output trickles down to the bottom-most
        buffer and is flushed via flush_output. The default "out_method"
        prints the content to "STDOUT". This method will always be called
        with non-undef, non-zero length content.

    Pushes a new frame onto the buffer stack. By default, the output from
    this new frame connects to the input of the previous frame. There are a
    number of possible options:

        A string reference, into which the output from this stack frame will
        appear. By default, this is the input buffer of the previous frame.

        If a true value is passed for "private", it creates a private string
        reference, and uses that as the buffer -- this is purely for
        convenience. That is, the following blocks are equivilent:

          my $buffer = "";
          $stack->push( buffer => \$buffer );
          # ...
          print $buffer;

          $stack->push( private => 1 );
          # ...
          print $stack->pop;

        A callback, which will be called with a reference to the
        "String::BufferStack" object, and the arguments to append, whenever
        this stack frame has anything appended to the input buffer, directly
        or indirectly.

        Within the context of the pre-append callback, "append",
        "direct_append", and "set_pre_append" function on the frame the
        pre-append is attached to, not the topmost trame. Using "append"
        within the pre-append callback is not suggested; use "direct_append"
        instead. "set_pre_append" can be used to alter or remove the
        pre-append callback itself -- this is not uncommon, in the case
        where the first append is the only one which needs be watched for,
        for instance.

        A callback, used to process data which is appended to the stack
        frame. By default, filters are lazy, being called only when a frame
        is popped. They can be forced at any time by calling
        "flush_filters", however.

    Returns the current depth of the stack. This starts at 0, when no frames
    have been pushed, and increases by one for each frame pushed.

  append STRING [, STRING, ...]
    Appends the given strings to the input side of the topmost buffer. This
    will call all pre-append hooks attached to it, as well. Note that if the
    frame has a filter, the filter will not immediately run, but will be
    delayed until the frame is "pop"'d, or "flush_filters" is called.

    When called with no frames on the stack, appends the stringins directly
    to the "output_buffer".

  direct_append STRING [, STRING, ...]
    Similar to "append", but appends the strings to the output side of the
    frame, skipping pre-append callbacks and filters.

    When called with no frames on the stack, appends the strings directly to
    the "output_buffer".

    Removes the topmost frame on the stack, flushing the topmost filters in
    the process. Returns the output buffer of the frame -- note that this
    may not contain only strings appended in the current frame, but also
    those from before, as a speed optimization. That is:

       $stack->append(" two");
       $stack->pop;   # returns "one two"

    This operation is a no-op if there are no frames on the stack.

  set_pre_append CALLBACK
    Alters the pre-append callback on the topmost frame. The callback will
    be called before text is appended to the input buffer of the frame, and
    will be passed the "String::BufferStack" and the arguments to "append".

  set_filter FILTER
    Alters the filter on the topmost frame. Doing this flushes the filters
    on the topmost frame.

    Filters the topmost stack frame, if it has outstanding unfiltered data.
    This will propagate content to lower frames, possibly calling their
    pre-append hooks.

    If there are no frames on the stack, calls "flush_output". Otherwise,
    calls "flush_filters".

    Flushes all filters. This does not flush output from the output buffer;
    see "flush_output".

    Returns the contents of the output buffer of the topmost frame; if there
    are no frames, returns the output buffer.

    Returns a reference to the output buffer of the topmost frame; if there
    are no frames, returns a reference to the output buffer. Note that
    adjusting this skips pre-append and filter hooks.

    Returns the number of characters appended to the current frame; if there
    are no frames, returns the length of the output buffer.

    Flushes all filters using "flush_filters", then flushes output from the
    output buffer, using the configured "out_method".

    Returns the pending output buffer, which sits below all existing frames.

    Returns a reference to the pending output buffer, allowing you to modify

    Clears *all* buffers in the stack, including the output buffer.

    Clears the topmost buffer in the stack; if there are no frames on the
    stack, clears the output buffer.

  out_method [CALLBACK]
    Gets or sets the output method callback, which is given content from the
    pending output buffer, which sits below all frames.

    Many concepts were originally taken from HTML::Mason's internal buffer

    Alex Vandiver ""

    Copyright 2008-2009, Best Practical Solutions.

    This package is distributed under the same terms as Perl itself.

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