Android自定义View绘图实现渐隐动画

实现了一个有趣的小东西：使用自定义View绘图，一边画线，画出的线条渐渐变淡，直到消失。效果如下图所示：

用属性动画或者渐变填充（Shader）可以做到一笔一笔的变化，但要想一笔渐变（手指不抬起边画边渐隐），没在Android中找到现成的API可用。所以，自己做了一个。

基本的想法是这样的：

•在View的onTouchEvent中记录触摸点，生成一条一条的线LineElement，放在一个List中。给每个LineElement配置一个Paint实例。
•在onDraw中绘制线段。
•变换LineElement的Paint实例的Alpha值。
•根据Alpha值重组线段列表

别的不说了，上代码：

package com.example.disappearinglines;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.RectF;
import android.os.Handler;
import android.os.Message;
import android.os.SystemClock;
import android.support.annotation.NonNull;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;

public class DisappearingDoodleView extends View {
  final static String TAG = "DoodleView";
  class LineElement {
    static final public int ALPHA_STEP = 5;
    static final public int SUBPATH_DIMENSION = 8;
    public LineElement(){
      mPaint = new Paint();
      mPaint.setARGB(255, 255, 0, 0);
      mPaint.setAntiAlias(true);
      mPaint.setStrokeWidth(16);
      mPaint.setStrokeCap(Paint.Cap.BUTT);
      mPaint.setStyle(Paint.Style.STROKE);
    }
    public LineElement(Paint paint){
      mPaint = paint;
    }

    public void setPaint(Paint paint){
      mPaint = paint;
    }

    public void setAlpha(int alpha){
      mPaint.setAlpha(alpha);
    }

    public float mStartX = -1;
    public float mStartY = -1;
    public float mEndX = -1;
    public float mEndY = -1;
    public Paint mPaint;
  }

  private LineElement mCurrentLine = null;
  private List<LineElement> mLines = null;

  private long mElapsed = 0;
  private Handler mHandler = new Handler(){
    @Override
    public void handleMessage(Message msg){
      DisappearingDoodleView.this.invalidate();
    }
  };

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

  public DisappearingDoodleView(Context context, AttributeSet attrs){
    super(context, attrs);
  }

  @Override
  protected void onDraw(Canvas canvas){
    mElapsed = SystemClock.elapsedRealtime();
    if(mLines != null) {
      for (LineElement e : mLines) {
        if(e.mStartX < 0 || e.mEndY < 0) continue;
        canvas.drawLine(e.mStartX, e.mStartY, e.mEndX, e.mEndY, e.mPaint);
      }
      compactPaths();
    }
  }

  @Override
  public boolean onTouchEvent(MotionEvent event){
    float x = event.getX();
    float y = event.getY();

    int action = event.getAction();
    if(action == MotionEvent.ACTION_UP){// end one line after finger release
      mCurrentLine.mEndX = x;
      mCurrentLine.mEndY = y;
      mCurrentLine = null;
      invalidate();
      return true;
    }

    if(action == MotionEvent.ACTION_DOWN){
      mCurrentLine = new LineElement();
      addToPaths(mCurrentLine);

      mCurrentLine.mStartX = x;
      mCurrentLine.mStartY = y;
      return true;
    }

    if(action == MotionEvent.ACTION_MOVE) {
      mCurrentLine.mEndX = x;
      mCurrentLine.mEndY = y;
      mCurrentLine = new LineElement();
      addToPaths(mCurrentLine);

      mCurrentLine.mStartX = x;
      mCurrentLine.mStartY = y;
    }

    if(mHandler.hasMessages(1)){
      mHandler.removeMessages(1);
    }
    Message msg = new Message();
    msg.what = 1;
    mHandler.sendMessageDelayed(msg, 0);

    return true;
  }

  private void addToPaths(LineElement element){
    if(mLines == null) {
      mLines = new ArrayList<LineElement>() ;
    }

    mLines.add(element);
  }

  public void compactPaths(){

    int size = mLines.size();
    int index = size - 1;
    if(size == 0) return;
    int baseAlpha = 255 - LineElement.ALPHA_STEP;
    int itselfAlpha;
    LineElement line;
    for(; index >=0 ; index--, baseAlpha -= LineElement.ALPHA_STEP){
      line = mLines.get(index);
      itselfAlpha = line.mPaint.getAlpha();
      if(itselfAlpha == 255){
        if(baseAlpha <= 0){
          ++index;
          break;
        }
        line.setAlpha(baseAlpha);
      }else{
        itselfAlpha -= LineElement.ALPHA_STEP;
        if(itselfAlpha <= 0){
          ++index;
          break;
        }
        line.setAlpha(itselfAlpha);
      }
    }

    if(index >= size){
      // all sub-path should disappear
      mLines = null;
    }
    else if(index >= 0){
      //Log.i(TAG, "compactPaths from " + index + " to " + (size - 1));
      mLines = mLines.subList(index, size);
    }else{
      // no sub-path should disappear
    }

    long interval = 40 - SystemClock.elapsedRealtime() + mElapsed;
    if(interval < 0) interval = 0;
    Message msg = new Message();
    msg.what = 1;
    mHandler.sendMessageDelayed(msg, interval);
  }
}

这个示例还可以添加一些效果，比如让线条一边变淡一边变细。

目前还有一些问题，线条粗的话，可以明显看到线段与线段之间有缝隙或裂口，哪位想到怎么优化？

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