Bitmap如何工作以及画布到底是什么？

Question

我正在尝试创建一个绘图应用程序，并从Fingerpaint中找到了一个示例。我对这个例子做了一些修改，并试图了解这是如何完成的。我的例子如下。

public class TestView extends View {
    private Paint mPaint;
    private Paint mBitmapPaint;

    private Bitmap mBitmap;
    private Canvas mCanvas;
    private Path mPath;

    public TestView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public TestView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    public TestView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        init();
    }

    private void init() {
        mPath = new Path();
        mBitmapPaint = new Paint();

        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeWidth(20);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
        canvas.drawPath(mPath, mPaint);
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }
}

我理解mPaint是如何构建和使用的。我不明白的是onSizeChanged()和此行canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint); 中的代码。为什么需要新的Canvas？创建mBitmap的目的是什么？我们为什么要绘制位图？为什么将颜色设置为mBitmapPaint什么都不做？基本上我不明白mBitmap，mCanvas和mBitmapPaint是做什么的。但如果我对这些进行评论，那么这个例子将不起作用。