Android使用自定义View绘制渐隐渐现动画
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：
packagecom.example.disappearinglines;
importandroid.content.Context;
importandroid.graphics.Canvas;
importandroid.graphics.Paint;
importandroid.graphics.Path;
importandroid.graphics.RectF;
importandroid.os.Handler;
importandroid.os.Message;
importandroid.os.SystemClock;
importandroid.support.annotation.NonNull;
importandroid.util.AttributeSet;
importandroid.util.Log;
importandroid.view.MotionEvent;
importandroid.view.View;
importjava.util.ArrayList;
importjava.util.Collection;
importjava.util.Iterator;
importjava.util.List;
importjava.util.ListIterator;
publicclassDisappearingDoodleViewextendsView{
finalstaticStringTAG="DoodleView";
classLineElement{
staticfinalpublicintALPHA_STEP=5;
staticfinalpublicintSUBPATH_DIMENSION=8;
publicLineElement(){
mPaint=newPaint();
mPaint.setARGB(255,255,0,0);
mPaint.setAntiAlias(true);
mPaint.setStrokeWidth(16);
mPaint.setStrokeCap(Paint.Cap.BUTT);
mPaint.setStyle(Paint.Style.STROKE);
}
publicLineElement(Paintpaint){
mPaint=paint;
}
publicvoidsetPaint(Paintpaint){
mPaint=paint;
}
publicvoidsetAlpha(intalpha){
mPaint.setAlpha(alpha);
}
publicfloatmStartX=-1;
publicfloatmStartY=-1;
publicfloatmEndX=-1;
publicfloatmEndY=-1;
publicPaintmPaint;
}
privateLineElementmCurrentLine=null;
privateList<LineElement>mLines=null;
privatelongmElapsed=0;
privateHandlermHandler=newHandler(){
@Override
publicvoidhandleMessage(Messagemsg){
DisappearingDoodleView.this.invalidate();
}
};
publicDisappearingDoodleView(Contextcontext){
super(context);
}
publicDisappearingDoodleView(Contextcontext,AttributeSetattrs){
super(context,attrs);
}
@Override
protectedvoidonDraw(Canvascanvas){
mElapsed=SystemClock.elapsedRealtime();
if(mLines!=null){
for(LineElemente:mLines){
if(e.mStartX<0||e.mEndY<0)continue;
canvas.drawLine(e.mStartX,e.mStartY,e.mEndX,e.mEndY,e.mPaint);
}
compactPaths();
}
}
@Override
publicbooleanonTouchEvent(MotionEventevent){
floatx=event.getX();
floaty=event.getY();
intaction=event.getAction();
if(action==MotionEvent.ACTION_UP){//endonelineafterfingerrelease
mCurrentLine.mEndX=x;
mCurrentLine.mEndY=y;
mCurrentLine=null;
invalidate();
returntrue;
}
if(action==MotionEvent.ACTION_DOWN){
mCurrentLine=newLineElement();
addToPaths(mCurrentLine);
mCurrentLine.mStartX=x;
mCurrentLine.mStartY=y;
returntrue;
}
if(action==MotionEvent.ACTION_MOVE){
mCurrentLine.mEndX=x;
mCurrentLine.mEndY=y;
mCurrentLine=newLineElement();
addToPaths(mCurrentLine);
mCurrentLine.mStartX=x;
mCurrentLine.mStartY=y;
}
if(mHandler.hasMessages(1)){
mHandler.removeMessages(1);
}
Messagemsg=newMessage();
msg.what=1;
mHandler.sendMessageDelayed(msg,0);
returntrue;
}
privatevoidaddToPaths(LineElementelement){
if(mLines==null){
mLines=newArrayList<LineElement>();
}
mLines.add(element);
}
publicvoidcompactPaths(){
intsize=mLines.size();
intindex=size-1;
if(size==0)return;
intbaseAlpha=255-LineElement.ALPHA_STEP;
intitselfAlpha;
LineElementline;
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){
//allsub-pathshoulddisappear
mLines=null;
}
elseif(index>=0){
//Log.i(TAG,"compactPathsfrom"+index+"to"+(size-1));
mLines=mLines.subList(index,size);
}else{
//nosub-pathshoulddisappear
}
longinterval=40-SystemClock.elapsedRealtime()+mElapsed;
if(interval<0)interval=0;
Messagemsg=newMessage();
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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