android学习6——canvas的save,restore作用

先看如下代码

public class SaveRestoreActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(new SaveRestoreView(this));
    }
}

public class SaveRestoreView extends View {
    public SaveRestoreView(Context context) {
        super(context);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Paint bgPaint = new Paint();
        bgPaint.setColor(Color.RED);
        int px = 100;
        int py = 100;
        canvas.drawRect(0, 0, px, py, bgPaint);

//        canvas.save();
        canvas.rotate(45);
        canvas.drawRect(px, py, px + 100, py + 100, bgPaint);
//        canvas.restore();

        canvas.drawRect(px + 100, py + 100, px + 200, py + 200, bgPaint);
    }
}

画一个矩形,然后将坐标系转动45度,再画一个矩形.发现矩形旋转了45度.再画一个矩形.矩形仍转动了45度.效果如下图所示:

将注释点的save,resotre打开,效果如下图所示:

save,restore确保对坐标系的改动只发生在save,restore之间的代码.