[移动开发]Android Paint ANTI_ALIAS_FLAG 引起的一个小问题

平台

RK3399 + Android 8.1

问题

代码中, 屏幕分辨率为1024x600, 自定义View的尺寸为300x300, 在窗口居中显示.
自定义View绘制的顺序为:

1. 绘制白色背景-Canvas.drawColor
2. 绘制黑色矩形-Canvas.drawRect
3. 绘制红色交叉线, 分别为左上-右下 和 右上-左下 两条
4. 绘制四个角圆, 半径为10个像素

效果图:

在代码中,增加了左右拖动旋转控件的功能

上图的虚线, 非代码绘制

在没有旋转控件前, 并未出现图中的虚线, 而且虚线还会根据旋转的角度而产生变化, 虚线的颜色会根据自定义View的背景色发生改变.

源码

canvas_test.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/rlRoot"
    android:background="#FF000000"/>

CanvasTest.java

package com.android.test;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RectF;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.widget.RelativeLayout;

import com.android.test.R;

public class CanvasTest extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.canvas_test);
        RelativeLayout rlRoot = (RelativeLayout)findViewById(R.id.rlRoot);
        CanvasView cv = new CanvasView(this);
        RelativeLayout.LayoutParams lp = new RelativeLayout.LayoutParams(300, 300);
        lp.addRule(RelativeLayout.CENTER_IN_PARENT);
        rlRoot.addView(cv, lp);
    }

    class CanvasView extends View {

        Paint p_anti = new Paint(Paint.ANTI_ALIAS_FLAG);
        Paint p_norm = new Paint();

        public CanvasView(Context context) {
            super(context);
        }

        int W, H;
        int CX, CY;
        RectF viewPort = new RectF();
        @Override
        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
            super.onSizeChanged(w, h, oldw, oldh);
            W = w;
            CX = W/2;
            H = h;
            CY = H/2;

            viewPort.set(0, 0, W, H);
        }

        float dx, dy, cx, cy;
        float rotZ = 0;
        @Override
        public boolean onTouchEvent(MotionEvent event) {
            float x = event.getRawX();
            float y = event.getRawY();
            cx = x;
            cy = y;
            switch(event.getAction()){
                case MotionEvent.ACTION_DOWN:
                    dx = x;
                    dy = y;
                    rotZ = getRotation();
                    break;
                case MotionEvent.ACTION_MOVE:
                    float rotate = (dx - cx) / W;
                    setRotation(rotZ + 90f * rotate);
                    break;

            }
            return true;
        }

        @Override
        protected void onDraw(Canvas canvas) {
            Paint p = p_anti;
            canvas.drawColor(Color.WHITE);

            //RECT
            p.setColor(Color.BLACK);
            canvas.drawRect(viewPort, p);

            //LINES
            p.setColor(Color.RED);
            float[] lines = {viewPort.left, viewPort.top, viewPort.right, viewPort.bottom,
                            viewPort.right, viewPort.top, viewPort.left, viewPort.bottom};
            canvas.drawLines(lines, p);

            //CIRCLE
            for(int i = 0; i < lines.length / 2; i ++){
                canvas.drawCircle(lines[i * 2], lines[i * 2 + 1], 10, p);
            }
        }
    }
}

最终发现, 这是由Paint引起的.
在代码中定义了两个Paint

        Paint p_anti = new Paint(Paint.ANTI_ALIAS_FLAG);
        Paint p_norm = new Paint();

在onDraw函数中, 使用p_norm并不会出现虚线的问题, 而使用p_anti才会出现.
放大图像: