Writing a Library with Vala:
Contents
ABI and API Design Choices
ABI
abi-compliance-checker is a cross-platform tool for checking the stability of an ABI. Supported platforms are GNU/Linux, FreeBSD, Mac OS X, MS Windows. See ABI Compliance Checker for more details.
The tool uses debug symbols to generate an ABI profile of a shared library:
abi-dumper my_library.so -o ABI-0.dump -lver 0
This is then repeated for the new version of the library:
abi-dumper my_library.so -o ABI-1.dump -lver 1
A report is then generated with:
abi-compliance-checker -l my_library -old ABI-0.dump -new ABI-1.dump
The report is an HTML file showing changes in the ABI. An example report for Vala is available online.
API Design
Avoid Custom Constructors
The following example contains a custom constructor:
public class MyClass : Object {
private string value;
public MyClass (string contents) {
value = contents;
}
}
This will be translated to C as the function name my_class_new and can be called with a string argument to create the object.
The problem is GObject has an alternative way of creating an object. In Vala this is of the form:
void main () {
Object.new (typeof(MyClass));
}
The Vala Object.new method is bound to GObject g_object_new function in C and is often used to instantiate GObjects. This can be from C, but also from languages using GObject introspection bindings. The problem is the construction defined Vala is not called.
Vala does have a way to run a function at instantiation time that is the use of the construct {} in the class. See GObject Style Construction in the Vala tutorial.
Avoid Using varargs
A function with a variable number of arguments is not introspectable. Although the GObject Introspection Repository will contain a method or function that can be called with a variable number of arguments, the method or function will be marked as introspectable="0". This causes binding generators to ignore the method or function. In Vala this can be overridden using skip = false in the metadata, but such techniques are not available in all bindings.
Avoid Using Generics
Since GObject Introspection does not handle generics, using them in APIs is harmful, since GI will generate 3 new parameters in the constructors of each generic class: one for the GType function, one for the duplication function and another for the destruction. These parameters are quite complicated to handle in languages like Python or Javascript.
In addition to this, the properties that expose the generic type parameter will be exposed as gpointers, which makes it even more complicated. Even generic methods like Gee's add () will expect a gpointer in GI, so doing something like this in Python will result in an error, contrary to what you expect.
list = get_a_list_of_strings ()
list.add ('Hi')
Further Reading
Some useful links for further reading:
APIs, like diamonds, are forever - some criteria for good API design
Things to avoid - when writing bindable APIs for GObject Introspection
Minimalistic example of the GLib’s GBoxedType usage - explanation of basic types when used with GObject Introspection and how to bind structs