package com.viewpagerindicator.sample;

import android.os.Bundle;
import android.support.v4.view.ViewPager;
import com.viewpagerindicator.UnderlinePageIndicator;

public class SampleUnderlinesDefault extends BaseSampleActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.simple_underlines);

        mAdapter = new TestFragmentAdapter(getSupportFragmentManager());

        mPager = (ViewPager)findViewById(R.id.pager);
        mPager.setAdapter(mAdapter);

        mIndicator = (UnderlinePageIndicator)findViewById(R.id.indicator);
        mIndicator.setViewPager(mPager);
    }
}

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="fill_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        />
    <com.viewpagerindicator.UnderlinePageIndicator
        android:id="@+id/indicator"
        android:layout_height="2dp"
        android:layout_width="fill_parent"
        />
</LinearLayout>

com.viewpagerindicator.UnderlinePageIndicator

indicator.setFades(false);

app:selectedColor="#FFCC0000"
        app:fadeDelay="1000"
        app:fadeLength="1000"

indicator.setViewPager(mPager);
        indicator.setSelectedColor(0xFFCC0000);
        indicator.setBackgroundColor(0xFFCCCCCC);
        indicator.setFadeDelay(1000);
        indicator.setFadeLength(1000);

SelectedColor：下划线的颜色
FadeDelay：下划线多长时间消失
FadeLength：不懂。。。