Previously, in the chapter , we have introduced the concept of GObject properties of controlling an element's behaviour. This is a very powerful, but has two big disadvantage: firstly, it is too generic, and secondly, it isn't dynamic. The first disadvantage has to do with customizability of the end-user interface that will be built to control the element. Some properties are more important than others. Some integer properties are better shown in a spin-button widget, whereas others would be better represented by a slider widget. Such things are not possible because the UI has no actual meaning in the application. A UI widget that stands for a bitrate property is the same as an UI widget that stands for the size of a video, as long as both are of the same GParamSpec type. Another problem, related to the one about parameter important, is that things like parameter grouping, function grouping or anything to make parameters coherent, is not really possible. The second argument against parameters are that they are not dynamic. In many cases, the allowed values for a property are not fixed, but depend on things that can only be detected at run-time. The names of inputs for a TV card in a video4linux source element, for example, can only be retrieved from the kernel driver when we've opened the device; this only happens when the element goes into the READY state. This means that we cannot create an enum property type to show this to the user. The solution to those problems is to create very specialized types of controls for certain often-used controls. We use the concept of interfaces to achieve this. The basis of this all is the glib GTypeInterface type. For each case where we think it's useful, we've created interfaces which can be implemented by elements at their own will. We've also created a small extension to GTypeInterface (which is static itself, too) which allows us to query for interface availability based on runtime properties. This extension is called GstImplementsInterface. WRITEME WRITEME WRITEME WRITEME WRITEME WRITEME WRITEME WRITEME WRITEME