android - 如何从 SurfaceView 绘制到 Canvas？

Question

我正在尝试做简单的画家。问题貌似Android有3个独立的Canvas给我顺序画。

我用 SurfaceView 制作了 UI，从中取出了 Holder。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    sv = (SurfaceView) findViewById(R.id.sv);
    holder = sv.getHolder();

    holder.addCallback(callback);
}

然后拿了Surface。

@Override
    public void surfaceCreated(SurfaceHolder holder) {
    surface = holder.getSurface();
}

并通过来自 OnTouchListener() 的事件绘制点和线。

private void paintStartDot(float x, float y) {
    Canvas canvas = surface.lockCanvas(null);

    canvas.drawPoint(x, y, drawPaint);
    surface.unlockCanvasAndPost(canvas);

    lastX = x;
    lastY = y;
}

private void paintEndDot(float x, float y) {
    Canvas canvas = surface.lockCanvas(null);

    canvas.drawLine(lastX, lastY, x, y, drawPaint);

    surface.unlockCanvasAndPost(canvas);

    lastX = x;
    lastY = y;
}

截屏视频:

https://youtu.be/NNDnzrtMLZI

怎么了？

完整源代码可在此处获得: https://github.com/tseglevskiy/canvasdemo1/blob/error/app/src/main/java/ru/jollydroid/canvasdemo1/MainActivity.java

Answer 1

Surface.lockCanvas 为您提供的 Canvas 不持久。在您调用 unlockCanvasAndPost 的那一刻，表面缓冲区的内容被推送到屏幕上。每次调用 lockCanvas 时，您都需要从头开始重新绘制图片。

如果你想增量更新 Canvas ，你应该保留一个“离屏”canvas backed by a Bitmap您响应用户操作而更新。然后将位图绘制到 Surface Canvas 上。

private void paintStartDot(float x, float y) {
    if (mBitmap == null) return;  // not ready yet
    Canvas canvas = new Canvas(mBitmap);
    canvas.drawPoint(x, y, drawPaint);

    // draw the bitmap to surface 
    canvas = surface.lockCanvas(null);
    canvas.drawBitmap(mBitmap, 0, 0, null);
    surface.unlockCanvasAndPost(canvas);
}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
    // good place to create the Bitmap
    mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config. ARGB_8888);
    // also here we need to transform the old bitmap to new one
}

我不确定您是否真的需要 SurfaceView。您可以只扩展 View 并覆盖其 View.onDraw 方法。在这种情况下，调用 view.invalidate() 以指示它需要重绘。

另请参阅:Android View.onDraw() always has a clean Canvas