自定义 View：多点触摸画笔的实现

2021 年 11 月 17 日
本文字数：1196 字
阅读完需：约 4 分钟

1.知识点

View.onTouchEvent 方法中的 MotionEvent 参数的事件类型对比
Paint.setStrokeCap 方法是什么意思
Paint.setStrokeJoin 方法是什么意思

2.原理

未完待续，此处会有一个表格，对比各个 ActionMasked 的差异

3.代码

package com.besmart.components;
import android.content.Context;import android.graphics.Canvas;import android.graphics.Paint;import android.graphics.Path;import android.util.AttributeSet;import android.util.SparseArray;import android.view.MotionEvent;import android.view.View;
import androidx.annotation.Nullable;
public class TestView extends View {    Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    SparseArray<Path> paths = new SparseArray<>();
    public TestView(Context context, @Nullable AttributeSet attrs) {        super(context, attrs);    }
    {        paint.setStyle(Paint.Style.STROKE);        paint.setStrokeWidth(Util.dp2px(4));        paint.setStrokeCap(Paint.Cap.ROUND);        paint.setStrokeJoin(Paint.Join.ROUND);    }
    @Override    protected void onDraw(Canvas canvas) {        for (int i = 0; i < paths.size(); i++) {            Path path = paths.valueAt(i);            canvas.drawPath(path, paint);        }    }
    @Override    public boolean onTouchEvent(MotionEvent event) {        switch (event.getActionMasked()){            case MotionEvent.ACTION_DOWN:            case MotionEvent.ACTION_POINTER_DOWN:                int actionIndex = event.getActionIndex();                int pointerId = event.getPointerId(actionIndex);                Path path = new Path();                path.reset();                path.moveTo(event.getX(actionIndex), event.getY(actionIndex));                paths.append(pointerId, path);                break;            case MotionEvent.ACTION_MOVE:                for (int i = 0; i < event.getPointerCount(); i++) {                    pointerId = event.getPointerId(i);                    path = paths.get(pointerId);                    path.lineTo(event.getX(i), event.getY(i));                }                invalidate();                break;            case MotionEvent.ACTION_UP:            case MotionEvent.ACTION_POINTER_UP:                actionIndex = event.getActionIndex();                pointerId = event.getPointerId(actionIndex);                paths.remove(pointerId);                invalidate();                break;        }        return true;    }}