在flutter中，想要实现一个视频播放器，或者调用摄像头来拍照或者录制视频，这样就需要将预览画面显示到Flutter UI中，除了使用PlatformView方案外，还可以使用Texture（外接纹理）的渲染方式来实现。

Texture可以理解为GPU内保存将要绘制的图像数据的一个对象，Flutter engine会将Texture的数据在内存中直接进行映射（而无需在原生和Flutter之间再进行数据传递），Flutter会给每一个Texture分配一个id，使用的时候只要有textureId即可显示。通过外接纹理的方式，实际上Flutter和Native传输的数据载体就是PixelBuffer，Native端的数据源（摄像头、播放器等）将数据写入PixelBuffer，Flutter拿到PixelBuffer以后转成OpenGLES Texture，交由Skia绘制。

const Texture({
  super.key,
  required this.textureId,
  this.freeze = false,
  this.filterQuality = FilterQuality.low,
});

flutter 外接纹理实现流程

1. 注册纹理
   Native创建一个对象，实现FlutterTexture协议，该对象用来管理具体的纹理数据
   通过FlutterTextureRegistry来注册第一步的FlutterTexture对象，获取一个Flutter纹理id
   将该id通过channel机制传递给dart侧，dart侧就能够通过Texture这个widget来使用纹理了，参数就是id

2. 纹理渲染
   dart侧声明一个Texture widget，表明该widget实际渲染的是native提供的纹理
   engine侧拿到layerTree，layerTree的TextureLayer节点负责外接纹理的渲染
   首先通过dart侧传递的id，找到先注册的FlutterTexture，该flutterTexture是我们自己用native代码实现的，其核心是实现了copyPixelBuffer方法
   flutter engine调用copyPixelBuffer拿到具体的纹理数据，然后交由底层进行GPU渲染

• iOS端实现关键代码（ CameraManager需要实现FlutterTexture协议 ）

HeartRateHelperPlugin

public static func register(with registrar: FlutterPluginRegistrar) {
  let _channel = FlutterMethodChannel(name: "heart_rate_plugin", binaryMessenger: registrar.messenger())
  channel = _channel
  let instance = HeartRateHelperPlugin()
  registrar.addMethodCallDelegate(instance, channel: _channel)
  //管理texture
  texture = registrar.textures()
  let camera = CameraManager(cameraType: .front)
     //获取textureId，通过MethodChannel传给flutter
  let textureId = texture!.register(camera)
  camera.updateHandle = { [weak self]  in
    //主动调用textureFrameAvailable，告诉TextureRegistry更新画面
    texture?.textureFrameAvailable(textureId)
   }

}

typealias onFrameAvailable = () -> Void

class CameraManager: NSObject,FlutterTexture {
    //...
    var updateHandle: onFrameAvailable?
    private var pixelBufferSynchronizationQueue:dispatch_queue_t?
    private var latestPixelBuffer:CVPixelBuffer?
    
    func copyPixelBuffer() -> Unmanaged<CVPixelBuffer>? {
    var pixelBuffer:Unmanaged<CVPixelBuffer>? = nil
    //实现获取外界纹理的逻辑
    // Use `dispatch_sync` because `copyPixelBuffer` API requires synchronous return.
    pixelBufferSynchronizationQueue?.sync {
        if let newBuffer = latestPixelBuffer {
            pixelBuffer = Unmanaged.passRetained(newBuffer)
        }
        latestPixelBuffer = nil
    }
    return pixelBuffer
   }
   
}
//获取每帧的数据，
extension CameraManager: AVCaptureVideoDataOutputSampleBufferDelegate {
    // MARK: - Export buffer from video frame
    func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        if(!CMSampleBufferDataIsReady(sampleBuffer)) {
            print("sample buffer is not ready. Skipping sample")
            return
        }
        // Use `dispatch_sync` to avoid unnecessary context switch under common non-contest scenarios;
        // Under rare contest scenarios, it will not block for too long since the critical section is
        // quite lightweight.
        pixelBufferSynchronizationQueue?.sync {
            if let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) {
                self.latestPixelBuffer = pixelBuffer
                if let updateHandle = updateHandle {
                    updateHandle()
                }
            }
        }
    }
   

• flutter端实现

//通过MethodChannel接受的外接纹理ID
Texture(
  textureId: state.textureId.value,
),

3. 源码分析
   以下是IOS端的TextureLayer节点的最终绘制代码（android类似，但是纹理获取方式略有不同），整体过程可以分为三步
   1)：调用external_texture copyPixelBuffer，获取CVPixelBuffer
   2)：CVOpenGLESTextureCacheCreateTextureFromImage创建OpenGL的Texture(这个是真的Texture)
   3)：将OpenGL Texture封装成SKImage，调用Skia的DrawImage完成绘制。

void IOSExternalTextureGL::Paint(SkCanvas& canvas, const SkRect& bounds) {
  if (!cache_ref_) {
    CVOpenGLESTextureCacheRef cache;
    CVReturn err = CVOpenGLESTextureCacheCreate(kCFAllocatorDefault, NULL,
                                                [EAGLContext currentContext], NULL, &cache);
    if (err == noErr) {
      cache_ref_.Reset(cache);
    } else {
      FXL_LOG(WARNING) << "Failed to create GLES texture cache: " << err;
      return;
    }
  }
  fml::CFRef<CVPixelBufferRef> bufferRef;
  bufferRef.Reset([external_texture_ copyPixelBuffer]);
  if (bufferRef != nullptr) {
    CVOpenGLESTextureRef texture;
    CVReturn err = CVOpenGLESTextureCacheCreateTextureFromImage(
        kCFAllocatorDefault, cache_ref_, bufferRef, nullptr, GL_TEXTURE_2D, GL_RGBA,
        static_cast<int>(CVPixelBufferGetWidth(bufferRef)),
        static_cast<int>(CVPixelBufferGetHeight(bufferRef)), GL_BGRA, GL_UNSIGNED_BYTE, 0,
        &texture);
    texture_ref_.Reset(texture);
    if (err != noErr) {
      FXL_LOG(WARNING) << "Could not create texture from pixel buffer: " << err;
      return;
    }
  }
  if (!texture_ref_) {
    return;
  }
  GrGLTextureInfo textureInfo = {CVOpenGLESTextureGetTarget(texture_ref_),
                                 CVOpenGLESTextureGetName(texture_ref_), GL_RGBA8_OES};
  GrBackendTexture backendTexture(bounds.width(), bounds.height(), GrMipMapped::kNo, textureInfo);
  sk_sp<SkImage> image =
      SkImage::MakeFromTexture(canvas.getGrContext(), backendTexture, kTopLeft_GrSurfaceOrigin,
                               kRGBA_8888_SkColorType, kPremul_SkAlphaType, nullptr);
  if (image) {
    canvas.drawImage(image, bounds.x(), bounds.y());
  }
}

截屏2024-07-15 23.40.30.png

PlatformView和Texture的优缺点

最开始调用摄像头录制视频采用的是PlatformView的方式(UiKitView组件)，但是进入录制页面出现了内存暴涨，并且退出录制页面后没有明显下降，后面参考camera插件，改成了Texture的方案，性能上有明显提升

截屏2024-07-15 23.42.35.png

参考链接：
https://blog.csdn.net/gloryFlow/article/details/139090371
https://juejin.cn/post/6844903662548942855