cairo_path_t: Cairo: A Vector Graphics Library

## cairo_path_t

The cairo_path_t type is one area in which most language bindings will differ significantly from the C API. The C API for cairo_path_t is designed for efficiency and to avoid auxiliary objects that would be have to be manually memory managed by the application. However, a language binding should not present cairo_path_t as an array, but rather as an opaque that can be iterated over. Different languages have quite different conventions for how iterators work, so it is impossible to give an exact specification for how this API should work, but the type names and methods should be similar to the language's mapping of the following:

    typedef struct cairo_path_iterator cairo_path_iterator_t;
    typedef struct cairo_path_element cairo_path_element_t;

    cairo_path_iterator_t *
    cairo_path_get_iterator (cairo_path_t *path);

    cairo_bool_t
    cairo_path_iterator_has_next (cairo_path_iterator_t *iterator);

    cairo_path_element_t *
    cairo_path_iterator_next (cairo_path_iterator_t *iterator);

    cairo_path_element_type_t
    cairo_path_element_get_type (cairo_path_element_t *element);

    void
    cairo_path_element_get_point (cairo_path_element_t *element,
                                  int                   index,
                                  double                *x,
                                  double                *y);

The above is written using the Java conventions for iterators. To illustrate how the API for PathIterator might depend on the native iteration conventions of the API, examine three versions of the loop, first written in a hypothetical Java binding:

    PathIterator iter = cr.copyPath().iterator();
    while (cr.hasNext()) {
        PathElement element = iter.next();
        if (element.getType() == PathElementType.MOVE_TO) {
            Point p = element.getPoint(0);
            doMoveTo (p.x, p.y);
        }
    }

And then in a hypothetical C++ binding:

    Path path = cr.copyPath();
    for (PathIterator iter = path.begin(); iter != path.end(); iter++) {
        PathElement element = *iter;
        if (element.getType() == PathElementType.MOVE_TO) {
            Point p = element.getPoint(0);
            doMoveTo (p.x, p.y);
        }
    }

And then finally in a Python binding:

    for element in cr.copy_path():
        if element.getType == cairo.PATH_ELEMENT_MOVE_TO:
            (x, y) = element.getPoint(0)
            doMoveTo (x, y);

While many of the API elements stay the same in the three examples, the exact iteration mechanism is quite different, to match how users of the language would expect to iterate over a container.

You should not present an API for mutating or for creating new cairo_path_t objects. In the future, these guidelines may be extended to present an API for creating a cairo_path_t from scratch for use with `cairo_append_path()` but the current expectation is that `cairo_append_path()` will mostly be used with paths from `cairo_copy_path()`.