问题Canvas.rotate（）Android中。如何它究竟工作？ [英] Issue with Canvas.rotate() in Android .How does it exactly work?

问题描述

请找我下面的OnDraw方法的code（.I'm试图通过25度拉弧后，旋转画布（//旋转调用-b）。但我发现，电弧仍然绘制从0到50 degrees.I期待它由另一个25度移动至

Please find the code of my onDraw method below(.I'm trying to rotate the canvas(//Rotate call -b) by 25 degrees after drawing the arc .But i find that the arc is still drawn from 0 to 50 degrees.I was expecting it to move by another 25 degrees.

public class CustomView extends View {

    public CustomView(Context context) {
        super(context);

    }

    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs);

    }
    protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);

            Paint paint = new Paint();
            paint.setColor(Color.RED);
            int px = getMeasuredWidth() / 2;
            int py = getMeasuredHeight() / 2;

            // radius - min 
            int radius = 130;

            // Defining bounds for the oval for the arc to be drawn
            int left = px - radius;
            int top = py - radius;
            int right = left + (radius * 2);
            int bottom = top + (radius * 2);

            RectF rectF = new RectF(left, top, right, bottom);
            paint.setColor(Color.RED);
            paint.setStyle(Style.FILL);

                //canvas.rotate(25,px,py);//Rotate call -a

        canvas.drawArc(rectF, 0, 50, true, paint);

            canvas.rotate(25,px,py);//Rotate call  -b

        }
}

但是，如果我画弧我看到拉弧是25度多。什么究竟是怎么回事移入前放置旋转调用（//旋转调用-a）？有人可以给我解释一下？

But if i place the rotate call(//Rotate call -a) before drawing the arc i see that the arc drawn is shifted by 25 degrees more .What exactly is happening here? Can someone explain to me?

感谢

推荐答案

在画布维护矩阵这是负责就可以了所有的转换。即使对于旋转。正如你可以在<一见href="http://developer.android.com/reference/android/graphics/Canvas.html#rotate%28float,%20float,%20float%29"相对=nofollow>文档，在旋转法说：

The Canvas maintains a Matrix that is responsible for all transformations on it. Even for rotation. As you can see in the documentation, the rotate method says:

preconcat当前矩阵与指定的旋转。

所有的转换是在画布 矩阵，因此，在画布<完成/ code>。你画圆弧不旋转。你先旋转画布再画就可以了。

All transformations are done on the Canvas Matrix,hence, on the Canvas. The arc you draw isn't rotated. You first rotate the Canvas and then draw on it.

因此​​，在code，调用-a 的作品，而不是调用-b 。

Hence, in your code, call -a works, not call -b.

编辑： 对于像postrotate和prerotate查看矩阵类（问题 postRotate 和 preRotate 方法）。

For issues like postrotate and prerotate check the Matrix class(postRotate and preRotate methods).

几个例子：这和这。

和几件事情，你可能需要阅读：这个并<一href="http://stackoverflow.com/questions/3855578/android-matrix-what-is-the-different-between-$p$pconcat-and-postconcat">this.

And few things you might want to read : this and this.

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