Android触摸事件如何实现笔触画布详解

前言

任何View都有触摸事件,经常在自定义控件时重写setOnTouchListener

本篇通过手绘图片来讲述这个知识点，下面话不多说了，来一起看看详细的介绍吧

本篇分为三个等级：一览图：

直线

曲线 

笔触

LEVEL1:基础实现

在Activity中通过一个全屏的Bitmap创建的Canvas绘制

为ImageView添加触摸事件监听。

1.成员变量

ImageView mIdIvShow;
float downX = 0;
float downY = 0;
float upX = 0;
float upY = 0;
private Canvas mCanvas;
private Paint mPaint;

2.创建画布

//获取屏幕尺寸
Point point = new Point();
getWindowManager().getDefaultDisplay().getSize(point);

//创建一个和屏幕一样大的Bitmap
Bitmap bitmap = Bitmap.createBitmap(point.x, point.y, Bitmap.Config.ARGB_8888);
//创建Canvas对象
mCanvas = new Canvas(bitmap);
mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mPaint.setStrokeWidth(10);
mPaint.setColor(Color.RED);
//将bitmap用ImageView展示
mIdIvShow.setImageBitmap(bitmap);

3.监听事件

 mIdIvShow.setOnTouchListener((v, event) -> {
   switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN:
     downX = event.getX();
     downY = event.getY();
     L.d("按下:(" + downX + "," + downY + ")" + L.l());
     break;
    case MotionEvent.ACTION_CANCEL:
     break;
    case MotionEvent.ACTION_MOVE:
     break;
    case MotionEvent.ACTION_UP:
     upX = event.getX();
     upY = event.getY();
     L.d("抬起:(" + upX + "," + upY + ")" + L.l());
     mCanvas.drawLine(downX, downY, upX, upY, mPaint);
     mIdIvShow.invalidate();//更新视图
     break;
   }
   return true;
  });
 }

升级版：LEVER2

mIdIvShow.setOnTouchListener((v, event) -> {
 switch (event.getAction()) {
  case MotionEvent.ACTION_DOWN:
   downX = event.getX();
   downY = event.getY();
   break;
  case MotionEvent.ACTION_CANCEL:
   break;
  case MotionEvent.ACTION_MOVE:
   upX = event.getX();
   upY = event.getY();
   mCanvas.drawLine(downX, downY, upX, upY, mPaint);
   mIdIvShow.invalidate();
   //更新点位
   downY = upY;
   downX = upX;
   break;
  case MotionEvent.ACTION_UP:
   //抬起点Y>1100,清除笔迹
   if (upY > 1100) {
    Paint paint = new Paint();
    paint.setColor(Color.WHITE);
    mCanvas.drawRect(0, 0, mPoint.x, mPoint.y, paint);
   }
   break;
 }
 return true;
});

再升级版：LEVER3

笔触根据绘制的速度动态改变画笔粗细

float movingX = 0;
float movingY = 0;
private long lastTimestamp = 0L;//最后一次的时间戳

mIdIvShow.setOnTouchListener((view, event) -> {
 switch (event.getAction()) {
  case MotionEvent.ACTION_DOWN:
   lastTimestamp = System.currentTimeMillis();
   downX = event.getX();
   downY = event.getY();
   break;
  case MotionEvent.ACTION_CANCEL:
   break;
  case MotionEvent.ACTION_MOVE:
   movingX = event.getX();
   movingY = event.getY();
   long curTimestamp = System.currentTimeMillis();
   //计算时间差
   long detaT = curTimestamp - lastTimestamp;
   //计算距离差
   float detaS = Logic.disPos2d(movingX, movingY, downX, downY);
   //由于速度是 px/ms
   double v = detaS / detaT;
   //速度转化为画笔宽度的等式
   float width = 14/(float)v;
   L.d(width + L.l());
   //限制极值情况
   if ((width > 0) && width < 30) {
    mPaint.setStrokeWidth(width);
   }
   mCanvas.drawLine(downX, downY, movingX, movingY, mPaint);
   mIdIvShow.invalidate();
   downX = movingX;
   downY = movingY;
   lastTimestamp = curTimestamp;//更新时间
   movePos.add(new PointF(event.getX(), event.getY()));
   break;
 }
 return true;
});

拓展

1.其中可以改变求宽度的等式实现不同的笔触:如

float width = (float) Math.log10(v) * 40;

2.在图片上绘画

//图片原型
Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.iv_500x400);
//图片副本
Bitmap mNewBitmap = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), bitmap.getConfig());
//用副本生成Canvas
mCanvas = new Canvas(mNewBitmap);
mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mPaint.setStrokeCap(Paint.Cap.ROUND);//直线圆头
mCanvas.drawBitmap(bitmap, new Matrix(), mPaint);
mPaint.setStrokeWidth(10);
mPaint.setColor(Color.parseColor("#88164BE6"));
//设置副本图片到ImageView
mIdIvShow.setImageBitmap(mNewBitmap);

总结

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