實現了一個有趣的小東西：使用自定義View繪圖，一邊畫線，畫出的線條漸漸變淡，直到消失。效果如下圖所示：
用屬性動畫或者漸變填充（Shader）可以做到一筆一筆的變化，但要想一筆漸變（手指不抬起邊畫邊漸隱），沒在Android中找到現成的API可用。所以，自己做了一個。
基本的想法是這樣的：
•在View的onTouchEvent中記錄觸摸點，生成一條一條的線LineElement，放在一個List中。給每個LineElement配置一個Paint實例。
•在onDraw中繪制線段。
•變換LineElement的Paint實例的Alpha值。
•根據Alpha值重組線段列表
別的不說了，上代碼：
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); }} 這個示例還可以添加一些效果，比如讓線條一邊變淡一邊變細。
目前還有一些問題，線條粗的話，可以明顯看到線段與線段之間有縫隙或裂口，哪位想到怎么優化？
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持武林網。
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