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17.1.3 How to Represent Dia Data in Scheme

For all but the most trivial applications, you will probably want to allow some representation of your domain objects to exist on the Scheme level. This is where the idea of SMOBs comes in, and with it issues of lifetime management and garbage collection.

To get more concrete about this, let's look again at the example we gave earlier of how application users can use Guile to build higher-level functions from the primitives that Dia itself provides.

(define (change-squares'-fill-pattern new-pattern)
  (for-each-shape current-page
    (lambda (shape)
      (if (square? shape)
          (change-fill-pattern shape new-pattern)))))

Consider what is stored here in the variable shape. For each shape on the current page, the for-each-shape primitive calls (lambda (shape) ...) with an argument representing that shape. Question is: how is that argument represented on the Scheme level? The issues are as follows.

One resolution of these issues is for the Scheme-level representation of a shape to be a new, Scheme-specific C structure wrapped up as a SMOB. The SMOB is what is passed into and out of Scheme code, and the Scheme-specific C structure inside the SMOB points to Dia's underlying C structure so that the code for primitives like square? can get at it.

To cope with an underlying shape being deleted while Scheme code is still holding onto a Scheme shape value, the underlying C structure should have a new field that points to the Scheme-specific SMOB. When a shape is deleted, the relevant code chains through to the Scheme-specific structure and sets its pointer back to the underlying structure to NULL. Thus the SMOB value for the shape continues to exist, but any primitive code that tries to use it will detect that the underlying shape has been deleted because the underlying structure pointer is NULL.

So, to summarize the steps involved in this resolution of the problem (and assuming that the underlying C structure for a shape is struct dia_shape):

As far as memory management is concerned, the SMOB values and their Scheme-specific structures are under the control of the garbage collector, whereas the underlying C structures are explicitly managed in exactly the same way that Dia managed them before we thought of adding Guile.

When the garbage collector decides to free a shape SMOB value, it calls the SMOB free function that was specified when defining the shape SMOB type. To maintain the correctness of the guile_shape field in the underlying C structure, this function should chain through to the underlying C structure (if it still exists) and set its guile_shape field to NULL.

For full documentation on defining and using SMOB types, see Defining New Types (Smobs).