Black-box composition of mismatched software components

Chapter 5. Evaluating the Cake language 141
values (embed: EphyBaseEmbed,
history : EphyHistory) ←→
{ /∗ ... ∗/ }
(web view: WebKitWebView,
scrolled window: GtkScrolledWindow,
load state: WebKitEmbedLoadState,
loading uri: char [])
Figure 5.15: Associating corresponding objects in Cake
struct WebKitEmbed { // from webkit-embed.h line 56
EphyBaseEmbed parent instance;
/∗< private >∗/
WebKitEmbedPrivate ∗priv;
};
struct WebKitEmbedPrivate // from webkit-embed.c line 65
{
WebKitWebView ∗web view;
GtkScrolledWindow ∗scrolled window;
WebKitEmbedLoadState load state;
char ∗loading uri;
EphyHistory ∗history;
};
Figure 5.16: Subclassing-like containment and pointer-based association in C
a pointer to the subordinate object, but associatively, by maintaining references in the
table implementing the co-object relation (§4.4.1).
Instantiation These rules present a complication: how do we ensure that the Epiphany
client binary, which was compiled to instantiate and manipulate instances of some subclass,
will instead instantiate and manipulate this association state using the logic in our
Cake rules? This requires a combination of interposing on instantiation of state and interposing
onaccess to that state. As ofteninCake programming, more thanonemix of these
two tactics can lead to a viable solution. We can choose not to interpose directly, instead
relying on the behaviour that an EphyBaseEmbed object will be a subobject instantiated
within some enclosing object. The form of this enclosing object will be determined by
whatever alternative HTML rendering library the client binary was compiled against (the
Mozilla renderer in our case). Such an object is nevertheless usable—its EphyBaseEmbed
subobject will be used as is, and its extra fields from the containing object will go unused.
5.6.2 Method dispatch
We must next make sure that calling code calls through our dispatch table (created in the
Cake code with an instantiate helper invocation, not shown). In the GObject library, the
dispatch table is located by a complex sequence of steps—by library calls which access