Android使用自定义View绘制渐隐渐现动画

概述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：

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);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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