add queries example

examples/queries/main.go

@@ -0,0 +1,103 @@
+// This example demonstrates how to use GStreamer's query functionality.
+//
+// These are a way to query information from either elements or pads.
+// Such information could for example be the current position within
+// the stream (i.e. the playing time). Queries can traverse the pipeline
+// (both up and downstream). This functionality is essential, since most
+// queries can only answered by specific elements in a pipeline (such as the
+// stream's duration, which often can only be answered by the demuxer).
+// Since gstreamer has many elements that itself contain other elements that
+// we don't know of, we can simply send a query for the duration into the
+// pipeline and the query is passed along until an element feels capable
+// of answering.
+// For convenience, the API has a set of pre-defined queries, but also
+// allows custom queries (which can be defined and used by your own elements).
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/tinyzimmer/go-gst/examples"
+	"github.com/tinyzimmer/go-gst/gst"
+)
+
+func queries(mainLoop *gst.MainLoop) error {
+
+	if len(os.Args) < 2 {
+		fmt.Println("USAGE: queries <pipeline>")
+		os.Exit(1)
+	}
+
+	gst.Init(nil)
+
+	// Let GStreamer create a pipeline from the parsed launch syntax on the cli.
+	pipelineStr := strings.Join(os.Args[1:], " ")
+	pipeline, err := gst.NewPipelineFromString(pipelineStr)
+	if err != nil {
+		return err
+	}
+
+	// Get a reference to the pipeline bus
+	bus := pipeline.GetPipelineBus()
+
+	// Start the pipeline
+	pipeline.SetState(gst.StatePlaying)
+
+	// Kick of a goroutine that will send a query to the pipeline
+	// every second.
+	go func() {
+		for range time.NewTicker(time.Second).C {
+			// Create a new position query and send it to the pipeline.
+			// This will traverse all elements in the pipeline, until one feels
+			// capable of answering the query.
+			pos := gst.NewPositionQuery(gst.FormatTime)
+			if ok := pipeline.Query(pos); !ok {
+				fmt.Println("Failed to query position from pipeline")
+			}
+			// Create a new duration query and send it to the pipeline.
+			// This will traverse all elements in the pipeline, until one feels
+			// capable of answering the query.
+			dur := gst.NewDurationQuery(gst.FormatTime)
+			if ok := pipeline.Query(dur); !ok {
+				fmt.Println("Failed to query duration from pipeline")
+			}
+
+			// The values from the queries above are both in nanoseconds (this may be abstracted later).
+			// We can convert them to durations.
+			_, posVal := pos.ParsePosition() //  If either of the above queries failed, these values
+			_, durVal := dur.ParseDuration() //  will be 0.
+			posDur := time.Duration(posVal) * time.Nanosecond
+			durDur := time.Duration(durVal) * time.Nanosecond
+
+			fmt.Println(posDur, "/", durDur)
+		}
+	}()
+
+	bus.AddWatch(func(msg *gst.Message) bool {
+		switch msg.Type() {
+		case gst.MessageEOS:
+			mainLoop.Quit()
+		case gst.MessageError:
+			gstErr := msg.ParseError()
+			fmt.Printf("Error from %s: %s\n", msg.Source(), gstErr.Error())
+			if debug := gstErr.DebugString(); debug != "" {
+				fmt.Println("go-gst-debug:", debug)
+			}
+			mainLoop.Quit()
+		}
+		return true
+	})
+
+	mainLoop.Run()
+
+	bus.RemoveWatch()
+
+	return pipeline.Destroy()
+}
+
+func main() {
+	examples.RunLoop(queries)
+}