finish AppSink impl and add example

examples/appsink/main.go

@@ -0,0 +1,113 @@
+package main
+
+import (
+	"fmt"
+	"math"
+	"time"
+
+	"github.com/tinyzimmer/go-gst/examples"
+	"github.com/tinyzimmer/go-gst/gst"
+	"github.com/tinyzimmer/go-gst/gst/app"
+)
+
+func createPipeline() *gst.Pipeline {
+	gst.Init(nil)
+
+	pipeline, err := gst.NewPipeline("")
+	if err != nil {
+		panic(err)
+	}
+
+	src, err := gst.NewElement("audiotestsrc")
+	if err != nil {
+		panic(err)
+	}
+
+	sink, err := app.NewAppSink()
+	if err != nil {
+		panic(err)
+	}
+
+	pipeline.AddMany(src, sink.Element)
+	src.Link(sink.Element)
+
+	// Tell the appsink what format we want. It will then be the audiotestsrc's job to
+	// provide the format we request.
+	// This can be set after linking the two objects, because format negotiation between
+	// both elements will happen during pre-rolling of the pipeline.
+	sink.SetCaps(gst.NewCapsFromString(
+		"audio/x-raw, format=S16LE, layout=interleaved, channels=1",
+	))
+
+	// Getting data out of the appsink is done by setting callbacks on it.
+	// The appsink will then call those handlers, as soon as data is available.
+	sink.SetCallbacks(&app.SinkCallbacks{
+		// Add a "new-sample" callback
+		NewSampleFunc: func(sink *app.Sink) gst.FlowReturn {
+
+			// Pull the sample that triggered this callback
+			sample := sink.PullSample()
+			if sample == nil {
+				return gst.FlowEOS
+			}
+
+			// Retrieve the buffer from the sample
+			buffer := sample.GetBuffer()
+			if buffer == nil {
+				return gst.FlowError
+			}
+
+			// At this point, buffer is only a reference to an existing memory region somewhere.
+			// When we want to access its content, we have to map it while requesting the required
+			// mode of access (read, read/write).
+			//
+			// We also know what format to expect because we set it with the caps. So we convert
+			// the map directly to signed 16-bit integers.
+			samples := buffer.Map(gst.MapRead).AsInt16Slice()
+
+			// Calculate the root mean square for the buffer
+			// (https://en.wikipedia.org/wiki/Root_mean_square)
+			var square float64
+			for _, i := range samples {
+				square += float64(i * i)
+			}
+			rms := math.Sqrt(square / float64(len(samples)))
+			fmt.Println("rms:", rms)
+
+			return gst.FlowOK
+		},
+	})
+
+	return pipeline
+}
+
+func mainLoop(pipeline *gst.Pipeline) error {
+	defer pipeline.Destroy()
+
+	pipeline.SetState(gst.StatePlaying)
+
+	bus := pipeline.GetPipelineBus()
+
+	for {
+		msg := bus.TimedPop(time.Duration(-1))
+		if msg == nil {
+			break
+		}
+		switch msg.Type() {
+		case gst.MessageEOS:
+			break
+		case gst.MessageError:
+			return msg.ParseError()
+		}
+	}
+
+	return nil
+}
+
+func main() {
+	examples.Run(func() error {
+		pipeline := createPipeline()
+		return mainLoop(pipeline)
+	})
+
+}