實現(xiàn)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(xiàn)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(jù)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 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() ;

}

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(yōu)化？

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