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(){@Overridepublic void handleMessage(Message msg){DisappearingDoodleView.this.invalidate();}};public DisappearingDoodleView(Context context){super(context);}public DisappearingDoodleView(Context context, AttributeSet attrs){super(context, attrs);}@Overrideprotected 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();}}@Overridepublic 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 releasemCurrentLine.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 disappearmLines = 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。

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