流程
1.一个匀速圆周运动的点
2.多个匀速圆周运动的点
3.多个圆周运动的点，速度由快到慢
4.点与点之间的间距线性减少，动画的最后合为一个点
5.为了让动画看起来更加流畅，需要在动画即将结束的时候手动绘制点
核心控件
PathMeasure：截取Path中的一部分并显示
ValueAnimator：完成动画从初始值平滑的过度到结束值的效果，同时还负责管理动画的播放次数、播放模式、以及对动画设置监听器等
流程一
思路
先用path画一个圆
ValueAnimator设置为0f-1f的平滑
用PathMeasure根据ValueAnimator返回的值截取path上的一个点
private Paint mPaint; private Path mPath; private PathMeasure mPathMeasure; private int mWidth,mHeight; private ValueAnimator valueAnimator; //用这个来接受ValueAnimator的返回值，代表整个动画的进度 private float t;
初始化画笔
mPaint = new Paint(); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeWidth(15); mPaint.setColor(Color.WHITE); //设置画笔为园笔 mPaint.setStrokeCap(Paint.Cap.ROUND); //抗锯齿 mPaint.setAntiAlias(true);
初始化Path和mPathMeasure
这里角度不能选360，否则会测量失误，具体原因和android的内部优化有关
mPath = new Path(); RectF rect = new RectF(-150,-150,150,150); mPath.addArc(rect,-90,359.9f); mPathMeasure = new PathMeasure(mPath,false);
初始化ValueAnimator
valueAnimator = ValueAnimator.ofFloat(0f,1f).setDuration(3000);
valueAnimator.setRepeatCount(-1);
valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override
public void onAnimationUpdate(ValueAnimator animation) {
t = (float) animation.getAnimatedValue();
invalidate();
}
});这里的ValueAnimator设置的是一个时长3秒的动画，再这3秒中，ValueAnimator会返回一个由0f-1f平滑的数字 ValueAnimator.ofFloat(0f,1f).setDuration(3000)
在这里我们用t来接受返回值，同时刷新视图
t = (float) animation.getAnimatedValue(); invalidate();
这里可以看出t的值，有0到1，这里就可以把t理解为我们这个动画的进度
开始绘制
@Override
protected void onDraw(Canvas canvas) { super.onDraw(canvas);
canvas.translate(mWidth/2,mHeight/2);
Path dst = new Path();
mPathMeasure.getSegment(mPathMeasure.getLength()*t,mPathMeasure.getLength()*t+1,dst,true);
canvas.drawPath(dst,mPaint);
}流程二
思路
我们设置让t每间隔0.05就画一个点，总共画4个点，注意这里getSegment()的最后一个要设置为true来保证画出来的是多个点而不是一条线
@Override
protected void onDraw(Canvas canvas) { super.onDraw(canvas);
canvas.translate(mWidth/2,mHeight/2);
Path dst = new Path(); int num = (int) (t/0.05); float s,y,x; switch(num){ default: case 3:
x = t-0.15f;
s = mPathMeasure.getLength();
y = s*x;
mPathMeasure.getSegment(y,y+1,dst,true); case 2:
x = t-0.10f;
s = mPathMeasure.getLength();
y = s*x;
mPathMeasure.getSegment(y,y+1,dst,true); case 1:
x = t-0.05f;
s = mPathMeasure.getLength();
y = s*x;
mPathMeasure.getSegment(y,y+1,dst,true); case 0:
x = t;
s = mPathMeasure.getLength();
y = s*x;
mPathMeasure.getSegment(y,y+1,dst,true); break;
}
canvas.drawPath(dst,mPaint);
}流程三
思路
我们先绘制出路程-时间的函数图像
函数为y = -x*x + 2*x，当x=1时，y=mPathMeasure.getLength();
设s = mPathMeasure.getLength();
最终我们套用函数:y = -s*x*x+2*s*x;
这里的Y轴代表的是path的长度，X轴对应时间
所以把流程二中的y = s*x改成y = -s*x*x+2*s*x即可
switch(num){
default:
case 3: x = t-0.15f; s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true);
case 2: x = t-0.10f; s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true);
case 1: x = t-0.05f; s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true);
case 0: x = t; s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true); break;
}
canvas.drawPath(dst,mPaint);流程四
思路
虽然流程3中点与点的间距已经开始减少，不过这只是因为速度不同间距才改变的，我们的目的是让这些点到最后合并为1个点，也就是说开始的时候每个点的X间距0.05，结束的时候要让他们的X相同
目前点之间X的间距函数如下：
我们最后要让当X=1时，他们的Y值相等，而且他们X的间距由0.05线性平滑到0
看函数图像已经很清楚了，修改后代码如下：
switch(num){
default:
case 3: x = t-0.15f*(1-t); s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true);
case 2: x = t-0.10f*(1-t); s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true);
case 1: x = t-0.05f*(1-t); s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true);
case 0: x = t; s = mPathMeasure.getLength(); y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true); break;
}流程五
思路
这里已经完成了99.9%了，但细心的同学会发现，进度条每次转动一圈聚成一个点后都会闪一下，这是因为重新开始动画刷新视图的原因，这里的补救方法就是我们在动画快结束的时候手动画一个点
if(t>=0.95){ canvas.drawPoint(0,-150,mPaint);
}这样我们就完成了这个进度条
最后这里附上源码：
package com.example.zhangml.view;import android.animation.ValueAnimator;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Path;import android.graphics.PathMeasure;import android.graphics.RectF;import android.util.AttributeSet;import android.view.View;/**
* Created by zhangml on 2016/9/12 0012.
*/public class Win8Search extends View{
private Paint mPaint; private Path mPath; private PathMeasure mPathMeasure; private int mWidth,mHeight; private ValueAnimator valueAnimator; //用这个来接受ValueAnimator的返回值，代表整个动画的进度
private float t; public Win8Search(Context context) { super(context);
} public Win8Search(Context context, AttributeSet attrs) { super(context, attrs);
init();
valueAnimator.start();
} private void init() {
mPaint = new Paint();
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setStrokeWidth(15);
mPaint.setColor(Color.WHITE); //设置画笔为园笔
mPaint.setStrokeCap(Paint.Cap.ROUND); //抗锯齿
mPaint.setAntiAlias(true);
mPath = new Path();
RectF rect = new RectF(-150,-150,150,150);
mPath.addArc(rect,-90,359.9f);
mPathMeasure = new PathMeasure(mPath,false);
valueAnimator = ValueAnimator.ofFloat(0f,1f).setDuration(3000);
valueAnimator.setRepeatCount(-1);
valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override
public void onAnimationUpdate(ValueAnimator animation) {
t = (float) animation.getAnimatedValue();
invalidate();
}
});
} @Override
protected void onDraw(Canvas canvas) { super.onDraw(canvas);
canvas.translate(mWidth/2,mHeight/2);
Path dst = new Path(); if(t>=0.95){
canvas.drawPoint(0,-150,mPaint);
} int num = (int) (t/0.05); float s,y,x; switch(num){ default: case 3:
x = t-0.15f*(1-t);
s = mPathMeasure.getLength();
y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true); case 2:
x = t-0.10f*(1-t);
s = mPathMeasure.getLength();
y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true); case 1:
x = t-0.05f*(1-t);
s = mPathMeasure.getLength();
y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true); case 0:
x = t;
s = mPathMeasure.getLength();
y = -s*x*x+2*s*x;
mPathMeasure.getSegment(y,y+1,dst,true); break;
}
canvas.drawPath(dst,mPaint);
} @Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) { super.onSizeChanged(w, h, oldw, oldh);
mWidth = w;
mHeight = h;
}
}