ANativeWindow是什么

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

• 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.