Android使用自定义View绘制渐隐渐现动画_Android

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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