add example which registers and uses go custom elements

this example has an assert that checks wether or nor the finalizers ran correctly. If they don't then a mem leak will happen

examples/plugins/registered_elements/internal/customsrc/element.go

@@ -0,0 +1,139 @@
+package customsrc
+
+import (
+	"fmt"
+	"math"
+	"time"
+
+	"github.com/go-gst/go-glib/glib"
+	"github.com/go-gst/go-gst/examples/plugins/registered_elements/internal/common"
+	"github.com/go-gst/go-gst/gst"
+)
+
+// default: 1024, this value makes it easier to calculate num buffers with the sample rate
+const samplesperbuffer = 4800
+
+const samplerate = 48000
+
+var properties = []*glib.ParamSpec{
+	glib.NewInt64Param(
+		"duration",
+		"duration",
+		"duration the source",
+		0,
+		math.MaxInt64,
+		0,
+		glib.ParameterReadWrite,
+	),
+}
+
+type customSrc struct {
+	// self   *gst.Bin
+	source *gst.Element
+	volume *gst.Element
+
+	duration time.Duration
+}
+
+// ClassInit is the place where you define pads and properties
+func (*customSrc) ClassInit(klass *glib.ObjectClass) {
+	class := gst.ToElementClass(klass)
+	class.SetMetadata(
+		"custom test source",
+		"Src/Test",
+		"Demo source bin with volume",
+		"Wilhelm Bartel <bartel.wilhelm@gmail.com>",
+	)
+	class.AddPadTemplate(gst.NewPadTemplate(
+		"src",
+		gst.PadDirectionSource,
+		gst.PadPresenceAlways,
+		gst.NewCapsFromString(fmt.Sprintf("audio/x-raw,channels=2,rate=%d", samplerate)),
+	))
+	class.InstallProperties(properties)
+}
+
+// SetProperty gets called for every property. The id is the index in the slice defined above.
+func (s *customSrc) SetProperty(self *glib.Object, id uint, value *glib.Value) {
+	param := properties[id]
+
+	bin := gst.ToGstBin(self)
+
+	switch param.Name() {
+	case "duration":
+		state := bin.GetCurrentState()
+		if !(state == gst.StateNull || state != gst.StateReady) {
+			return
+		}
+
+		gv, _ := value.GoValue()
+
+		durI, _ := gv.(int64)
+
+		s.duration = time.Duration(durI)
+
+		s.updateSource()
+	}
+}
+
+// GetProperty is called to retrieve the value of the property at index `id` in the properties
+// slice provided at ClassInit.
+func (o *customSrc) GetProperty(self *glib.Object, id uint) *glib.Value {
+	param := properties[id]
+
+	switch param.Name() {
+	case "duration":
+		v, _ := glib.GValue(int64(o.duration))
+		return v
+	}
+
+	return nil
+}
+
+func (*customSrc) New() glib.GoObjectSubclass {
+	return &customSrc{}
+}
+
+// InstanceInit should initialize the element. Keep in mind that the properties are not yet present. When this is called.
+func (s *customSrc) InstanceInit(instance *glib.Object) {
+	self := gst.ToGstBin(instance)
+
+	s.source = common.Must(gst.NewElement("audiotestsrc"))
+	s.volume = common.Must(gst.NewElement("volume"))
+
+	klass := instance.Class()
+	class := gst.ToElementClass(klass)
+
+	self.AddMany(
+		s.source,
+		s.volume,
+	)
+
+	srcpad := s.volume.GetStaticPad("src")
+
+	ghostpad := gst.NewGhostPadFromTemplate("src", srcpad, class.GetPadTemplate("src"))
+
+	gst.ElementLinkMany(
+		s.source,
+		s.volume,
+	)
+
+	self.AddPad(ghostpad.Pad)
+
+	s.updateSource()
+}
+
+func (s *customSrc) Constructed(o *glib.Object) {}
+
+func (s *customSrc) Finalize(o *glib.Object) {
+	common.FinalizersCalled++
+}
+
+// updateSource will get called to update the audiotestsrc when a property changes
+func (s *customSrc) updateSource() {
+	if s.source != nil {
+		numBuffers := (float64(s.duration / time.Second)) / (float64(samplesperbuffer) / float64(samplerate))
+
+		s.source.SetProperty("num-buffers", int(math.Ceil(numBuffers)))
+	}
+}