Update README.md

README.md

@@ -19,3 +19,86 @@ dependencies: [
     .package(url: "https://github.com/sunlubo/SwiftFFmpeg.git", from: "1.0.0")
 ]
 ```
+
+## Usage
+
+```swift
+import Foundation
+import SwiftFFmpeg
+
+func main() throws {
+    if CommandLine.argc < 2 {
+        print("Usage: \(CommandLine.arguments[0]) <input file>")
+        return
+    }
+    let input = CommandLine.arguments[1]
+
+    let fmtCtx = AVFormatContext()
+    try fmtCtx.openInput(input)
+    try fmtCtx.findStreamInfo()
+
+    let stream = fmtCtx.videoStream!
+    let codecpar = stream.codecpar
+    guard let codec = AVCodec.findDecoderById(codecpar.codecId) else {
+        print("Codec not found")
+        return
+    }
+    guard let codecCtx = AVCodecContext(codec: codec) else {
+        print("Could not allocate video codec context.")
+        return
+    }
+    try codecCtx.setParameters(stream.codecpar)
+    try codecCtx.openCodec()
+
+    let pkt = AVPacket()!
+    let frame = AVFrame()!
+
+    var num = 50
+    while let _ = try? fmtCtx.readFrame(into: pkt) {
+        if pkt.streamIndex != stream.index {
+            pkt.unref()
+            continue
+        }
+
+        do {
+            try codecCtx.sendPacket(pkt)
+        } catch {
+            print("Error while sending a packet to the decoder: \(error)")
+            return
+        }
+
+        while true {
+            do {
+                try codecCtx.receiveFrame(frame)
+            } catch let err as AVError where err == .EAGAIN || err == .EOF {
+                break
+            } catch {
+                print("Error while receiving a frame from the decoder: \(error)")
+                return
+            }
+
+            let str = String(format: "Frame %2d (type=%@, size=%5d bytes) pts %6lld key_frame %d [DTS %2d]",
+                             codecCtx.frameNumber,
+                             frame.pictType.description,
+                             frame.pktSize,
+                             frame.pts,
+                             frame.isKeyFrame,
+                             frame.codedPictureNumber)
+            print(str)
+
+            frame.unref()
+        }
+
+        num -= 1
+        if num <= 0 { break }
+
+        pkt.unref()
+    }
+}
+
+do {
+    try main()
+} catch {
+    print(error)
+}
+```