Android中的ANativeWindow

ANativeWindow是什么

ANativeWindow是C/C++中定义的一个结构体,等同于Java中的Surface.

Android NDK中可以访问到ANativeWindow.

ANativeWindow中存放像素信息的结构是:ANativeWindow_Buffer:

typedef struct ANativeWindow_Buffer {
    /// The number of pixels that are shown horizontally.
    int32_t width;

    /// The number of pixels that are shown vertically.
    int32_t height;

    /// The number of *pixels* that a line in the buffer takes in
    /// memory. This may be >= width.
    int32_t stride;

    /// The format of the buffer. One of AHardwareBuffer_Format.
    int32_t format;

    /// The actual bits.
    void* bits;

    /// Do not touch.
    uint32_t reserved[6];
} ANativeWindow_Buffer;

主要API

• 获取与surface对应的ANativeWindow

  ANativeWindow* ANativeWindow_fromSurface(JNIEnv* env, jobject surface);
  

• 保持/释放ANativeWindow对象的引用

  void ANativeWindow_acquire(ANativeWindow* window);
  
  void ANativeWindow_release(ANativeWindow* window);
  

• 向buffer中写入数据并提交

  int32_t ANativeWindow_lock(ANativeWindow* window, ANativeWindow_Buffer* outBuffer,
          ARect* inOutDirtyBounds);
          
  // 这之间的代码可以执行一些向buffer中写入数据的操作
  
  int32_t ANativeWindow_unlockAndPost(ANativeWindow* window);
  

• 获取Window Surface的信息:宽/高/像素格式

  int32_t ANativeWindow_getWidth(ANativeWindow* window);
  
  int32_t ANativeWindow_getHeight(ANativeWindow* window);
  
  int32_t ANativeWindow_getFormat(ANativeWindow* window);
  

  像素格式定义在AHARDWAREBUFFER_FORMAT_*

• 改变Window Buffer的格式和大小

  int32_t ANativeWindow_setBuffersGeometry(ANativeWindow* window,
          int32_t width, int32_t height, int32_t format);
  

一般的流程

1. 通过ANativeWindow_fromSurface获取与Surface对应的ANativeWindow对象
2. ANativeWindow_setBuffersGeometry设置buffer的尺寸和格式
3. ANativeWindow_acquire获取引用对象
4. 利用ANativeWindow_lock/ANativeWindow_unlockAndPost与之间的绘制代码绘制图像
5. ANativeWindow_release释放引用.

一个简单的例子

写个简单的例子:利用ANativeWindow绘制一个灰色的背景.

• C++代码

  #include <jni.h>
  #include <string>
  #include <android/native_window.h>
  #include <android/native_window_jni.h>
  
  void drawColor(JNIEnv *env,jobject obj,jobject surface, jint colorARGB) {
      int alpha = colorARGB >> 24 & 0xFF;
      int red = colorARGB >> 16 & 0xFF;
      int green = colorARGB >> 8 & 0xFF;
      int blue = colorARGB & 0xFF;
  
      int colorABGR = alpha << 24 | (blue << 16) | (green << 8) | red;
      
      ANativeWindow *window = ANativeWindow_fromSurface(env,surface);
  
      int32_t result = ANativeWindow_setBuffersGeometry(window,640,640,AHARDWAREBUFFER_FORMAT_R8G8B8A8_UNORM);
      if (result < 0){
          ANativeWindow_release(window);
          window = nullptr;
          return;
      }
      ANativeWindow_acquire(window);
      ANativeWindow_Buffer buffer;
      if (ANativeWindow_lock(window,&buffer, nullptr) < 0){
          ANativeWindow_release(window);
          window = nullptr;
          return;
      }
  
      auto *line = (uint32_t *) buffer.bits;
      for (int y = 0; y < buffer.height; ++y) {
          for (int x = 0; x < buffer.width; ++x) {
              line[x] = colorABGR;
          }
          line += buffer.stride;
      }
  
      if (ANativeWindow_unlockAndPost(window) < 0){
          return;
      }
  
      ANativeWindow_release(window);
  }
  
  extern "C"
  JNIEXPORT void JNICALL
  Java_com_jiangkang_androiddemos_MainActivity_drawColor(JNIEnv *env, jobject thiz, jobject surface,
                                                         jint color) {
      drawColor(env,thiz,surface,color);
  }
  

• Activity代码

  import android.graphics.Color
  import android.os.Bundle
  import android.view.Surface
  import android.view.SurfaceHolder
  import androidx.appcompat.app.AppCompatActivity
  import kotlinx.android.synthetic.main.activity_main.*
  
  class MainActivity : AppCompatActivity() {
  
      override fun onCreate(savedInstanceState: Bundle?) {
          super.onCreate(savedInstanceState)
          setContentView(R.layout.activity_main)
          surface_view.holder.addCallback(object : SurfaceHolder.Callback2 {
              override fun surfaceRedrawNeeded(holder: SurfaceHolder?) {
              }
  
              override fun surfaceChanged(
                  holder: SurfaceHolder?,
                  format: Int,
                  width: Int,
                  height: Int
              ) {
              }
  
              override fun surfaceDestroyed(holder: SurfaceHolder?) {
              }
  
              override fun surfaceCreated(holder: SurfaceHolder?) {
                  holder?.let {
                      drawColor(it.surface, Color.GRAY)
                  }
              }
          })
      }
  
      external fun drawColor(surface: Surface, color: Int)
  
      companion object {
          init {
              System.loadLibrary("native-lib")
          }
      }
  }
  

总结

1. Native层中的ANativeWindow对应Java层中的Surface,因此可以利用SurfaceView + ANativeWindow,用C++代码绘制屏幕;
2. 在图像/视频处理中会利用到ANativeWindow,也可以与EGL,FFMPEG等结合使用
3. Flutter Engine中正是通过 ANativeWindow才将Dart中的UI与Android中UI联系到一起的.