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。

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