Add meta reader helper

Since broadcaster has a ring buffer, we can take advantage of this
property to read the meta data for video/audio without losing any data.

meta.go

@@ -0,0 +1,35 @@
+package mediadevices
+
+import (
+	"github.com/pion/mediadevices/pkg/io/audio"
+	"github.com/pion/mediadevices/pkg/io/video"
+	"github.com/pion/mediadevices/pkg/prop"
+)
+
+// detectCurrentVideoProp is a small helper to get current video property
+func detectCurrentVideoProp(broadcaster *video.Broadcaster) (prop.Media, error) {
+	var currentProp prop.Media
+
+	// Since broadcaster has a ring buffer internally, a new reader will either read the last
+	// buffered frame or a new frame from the source. This also implies that no frame will be lost
+	// in any case.
+	metaReader := broadcaster.NewReader(false)
+	metaReader = video.DetectChanges(0, func(p prop.Media) { currentProp = p })(metaReader)
+	_, err := metaReader.Read()
+
+	return currentProp, err
+}
+
+// detectCurrentAudioProp is a small helper to get current audio property
+func detectCurrentAudioProp(broadcaster *audio.Broadcaster) (prop.Media, error) {
+	var currentProp prop.Media
+
+	// Since broadcaster has a ring buffer internally, a new reader will either read the last
+	// buffered frame or a new frame from the source. This also implies that no frame will be lost
+	// in any case.
+	metaReader := broadcaster.NewReader(false)
+	metaReader = audio.DetectChanges(0, func(p prop.Media) { currentProp = p })(metaReader)
+	_, err := metaReader.Read()
+
+	return currentProp, err
+}

meta_test.go

@@ -0,0 +1,98 @@
+package mediadevices
+
+import (
+	"image"
+	"testing"
+
+	"github.com/pion/mediadevices/pkg/io/audio"
+	"github.com/pion/mediadevices/pkg/io/video"
+	"github.com/pion/mediadevices/pkg/wave"
+)
+
+func TestDetectCurrentVideoProp(t *testing.T) {
+	resolution := image.Rect(0, 0, 4, 4)
+	first := image.NewRGBA(resolution)
+	first.Pix[0] = 1
+	second := image.NewRGBA(resolution)
+	second.Pix[0] = 2
+
+	isFirst := true
+	source := video.ReaderFunc(func() (image.Image, error) {
+		if isFirst {
+			isFirst = true
+			return first, nil
+		} else {
+			return second, nil
+		}
+	})
+
+	broadcaster := video.NewBroadcaster(source, nil)
+
+	currentProp, err := detectCurrentVideoProp(broadcaster)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if currentProp.Width != resolution.Dx() {
+		t.Fatalf("Expect the actual width to be %d, but got %d", currentProp.Width, resolution.Dx())
+	}
+
+	if currentProp.Height != resolution.Dy() {
+		t.Fatalf("Expect the actual height to be %d, but got %d", currentProp.Height, resolution.Dy())
+	}
+
+	reader := broadcaster.NewReader(false)
+	img, err := reader.Read()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	rgba := img.(*image.RGBA)
+	if rgba.Pix[0] != 1 {
+		t.Fatal("Expect the frame after reading the current prop is not the first frame")
+	}
+}
+
+func TestDetectCurrentAudioProp(t *testing.T) {
+	info := wave.ChunkInfo{
+		Len:          4,
+		Channels:     2,
+		SamplingRate: 48000,
+	}
+	first := wave.NewInt16Interleaved(info)
+	first.Data[0] = 1
+	second := wave.NewInt16Interleaved(info)
+	second.Data[0] = 2
+
+	isFirst := true
+	source := audio.ReaderFunc(func() (wave.Audio, error) {
+		if isFirst {
+			isFirst = true
+			return first, nil
+		} else {
+			return second, nil
+		}
+	})
+
+	broadcaster := audio.NewBroadcaster(source, nil)
+
+	currentProp, err := detectCurrentAudioProp(broadcaster)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if currentProp.ChannelCount != info.Channels {
+		t.Fatalf("Expect the actual channel count to be %d, but got %d", currentProp.ChannelCount, info.Channels)
+	}
+
+	reader := broadcaster.NewReader(false)
+	chunk, err := reader.Read()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	realChunk := chunk.(*wave.Int16Interleaved)
+	if realChunk.Data[0] != 1 {
+		t.Fatal("Expect the chunk after reading the current prop is not the first chunk")
+	}
+}