Android原生的Tablayout下面有一个指示器（指示线、下划线），如图所示：

详情见附录1。

但是Android原生的Tablayout指示器Indicator自定义空间很有限，能设置颜色，如果想把Tablayout指示器Indicator的宽和高做调整适应自己产品开发的UI设计要求，就很难办到了，更是如果要求的Indicator的指示器呈现渐变颜色、圆角，就难上加难了。就必须自己想办法解决这些自定义问题。

比如下面的这个设计要求：

UI设计师对指示器Indicator的宽、高、圆角都提出了规范要求。对于能自定义Indicator的宽、高、圆角及颜色的第三方开源项目很多。但是这次设计要求Indicator必须呈现出一定的颜色渐变，这就比较困难了，因为很多第三方开源的项目仅仅支持对Indicator设计一个单一的颜色，不支持多种颜色的渐变。

找来找去，我在github上找到了一个支持自定义Indicator的开源项目：

https://github.com/hackware1993/MagicIndicator 

但是若直接基于该项目对Indicator进行渐变颜色还是比较麻烦，比如，如果我想对Indicator进行自定义，就需要继承该项目的CommonNavigatorAdapter，然后在CommonNavigatorAdapter里面重写getIndicator，然后在getIndicator里面返回一个自定义的IPagerIndicator，麻烦就麻烦在继承实现IPagerIndicator对于底部下划线Indicator的onDraw方法，该方法将直接对Indicator底部的下划线指示器Indicator进行画线画圆操作等等，但是我的需求只是对一个圆角线性下划线指示器Indicator颜色渐变即可。
我仔细看了原作者的代码时候，终于找到一个简单的做法实现，就是在自定义IPagerIndicator的onDraw里面，对mPaint设置一个线性渐变LinearGradient即可，但是设置LinearGradient需要当前渐变的宽高坐标，而这部分宽高坐标取决于原项目中的mLineRect（mLineRect是一个RectF），该mLineRect代表了真正要绘制下划线指示器Indicator的区域，然而，如果有这个对象，我直接就可以在onDraw里面根据mLineRect获取四个空间位置点，然后构造线性渐变LinearGradient，进而设置给mPaint，就轻松实现颜色渐变了，这样的实现方案最大好处就是无须对原项目做二次复杂定制化开发。

但是遗憾的是，原作者在原项目中把mLineRect设置为私有private性质，且没有提供public的get方法，因此，我把原作者的LinePagerIndicator重新拉下来，增加了一个公开方法，把mLineRect返回，

    /**
     * 返回底部画线的RectF mLineRect。
     *
     * @return
     */
    public RectF getLineRect() {
        return mLineRect;
    }

作为对LinePagerIndicator的扩展，最终的LinePagerIndicatorEx.java：

package zhangphil.test;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.RectF;
import android.view.View;
import android.view.animation.Interpolator;
import android.view.animation.LinearInterpolator;

import net.lucode.hackware.magicindicator.FragmentContainerHelper;
import net.lucode.hackware.magicindicator.buildins.ArgbEvaluatorHolder;
import net.lucode.hackware.magicindicator.buildins.UIUtil;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.abs.IPagerIndicator;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.model.PositionData;

import java.util.Arrays;
import java.util.List;

public class LinePagerIndicatorEx  extends View implements IPagerIndicator {
    public static final int MODE_MATCH_EDGE = 0;   // 直线宽度 == title宽度 - 2 * mXOffset
    public static final int MODE_WRAP_CONTENT = 1;    // 直线宽度 == title内容宽度 - 2 * mXOffset
    public static final int MODE_EXACTLY = 2;  // 直线宽度 == mLineWidth

    private int mMode;  // 默认为MODE_MATCH_EDGE模式

    // 控制动画
    private Interpolator mStartInterpolator = new LinearInterpolator();
    private Interpolator mEndInterpolator = new LinearInterpolator();

    private float mYOffset;   // 相对于底部的偏移量，如果你想让直线位于title上方，设置它即可
    private float mLineHeight;
    private float mXOffset;
    private float mLineWidth;
    private float mRoundRadius;

    private Paint mPaint;
    private List<PositionData> mPositionDataList;
    private List<Integer> mColors;

    private RectF mLineRect = new RectF();

    public LinePagerIndicatorEx(Context context) {
        super(context);
        init(context);
    }

    private void init(Context context) {
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mPaint.setStyle(Paint.Style.FILL);
        mLineHeight = UIUtil.dip2px(context, 3);
        mLineWidth = UIUtil.dip2px(context, 10);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawRoundRect(mLineRect, mRoundRadius, mRoundRadius, mPaint);
    }

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        if (mPositionDataList == null || mPositionDataList.isEmpty()) {
            return;
        }

        // 计算颜色
        if (mColors != null && mColors.size() > 0) {
            int currentColor = mColors.get(Math.abs(position) % mColors.size());
            int nextColor = mColors.get(Math.abs(position + 1) % mColors.size());
            int color = ArgbEvaluatorHolder.eval(positionOffset, currentColor, nextColor);
            mPaint.setColor(color);
        }

        // 计算锚点位置
        PositionData current = FragmentContainerHelper.getImitativePositionData(mPositionDataList, position);
        PositionData next = FragmentContainerHelper.getImitativePositionData(mPositionDataList, position + 1);

        float leftX;
        float nextLeftX;
        float rightX;
        float nextRightX;
        if (mMode == MODE_MATCH_EDGE) {
            leftX = current.mLeft + mXOffset;
            nextLeftX = next.mLeft + mXOffset;
            rightX = current.mRight - mXOffset;
            nextRightX = next.mRight - mXOffset;
        } else if (mMode == MODE_WRAP_CONTENT) {
            leftX = current.mContentLeft + mXOffset;
            nextLeftX = next.mContentLeft + mXOffset;
            rightX = current.mContentRight - mXOffset;
            nextRightX = next.mContentRight - mXOffset;
        } else {    // MODE_EXACTLY
            leftX = current.mLeft + (current.width() - mLineWidth) / 2;
            nextLeftX = next.mLeft + (next.width() - mLineWidth) / 2;
            rightX = current.mLeft + (current.width() + mLineWidth) / 2;
            nextRightX = next.mLeft + (next.width() + mLineWidth) / 2;
        }

        mLineRect.left = leftX + (nextLeftX - leftX) * mStartInterpolator.getInterpolation(positionOffset);
        mLineRect.right = rightX + (nextRightX - rightX) * mEndInterpolator.getInterpolation(positionOffset);
        mLineRect.top = getHeight() - mLineHeight - mYOffset;
        mLineRect.bottom = getHeight() - mYOffset;

        invalidate();
    }

    @Override
    public void onPageSelected(int position) {
    }

    @Override
    public void onPageScrollStateChanged(int state) {
    }

    @Override
    public void onPositionDataProvide(List<PositionData> dataList) {
        mPositionDataList = dataList;
    }

    public float getYOffset() {
        return mYOffset;
    }

    public void setYOffset(float yOffset) {
        mYOffset = yOffset;
    }

    public float getXOffset() {
        return mXOffset;
    }

    public void setXOffset(float xOffset) {
        mXOffset = xOffset;
    }

    public float getLineHeight() {
        return mLineHeight;
    }

    public void setLineHeight(float lineHeight) {
        mLineHeight = lineHeight;
    }

    public float getLineWidth() {
        return mLineWidth;
    }

    public void setLineWidth(float lineWidth) {
        mLineWidth = lineWidth;
    }

    public float getRoundRadius() {
        return mRoundRadius;
    }

    public void setRoundRadius(float roundRadius) {
        mRoundRadius = roundRadius;
    }

    public int getMode() {
        return mMode;
    }

    public void setMode(int mode) {
        if (mode == MODE_EXACTLY || mode == MODE_MATCH_EDGE || mode == MODE_WRAP_CONTENT) {
            mMode = mode;
        } else {
            throw new IllegalArgumentException("mode " + mode + " not supported.");
        }
    }

    public Paint getPaint() {
        return mPaint;
    }

    /**
     * 返回底部画线的RectF mLineRect。
     *
     * @return
     */
    public RectF getLineRect() {
        return mLineRect;
    }

    public List<Integer> getColors() {
        return mColors;
    }

    public void setColors(Integer... colors) {
        mColors = Arrays.asList(colors);
    }

    public Interpolator getStartInterpolator() {
        return mStartInterpolator;
    }

    public void setStartInterpolator(Interpolator startInterpolator) {
        mStartInterpolator = startInterpolator;
        if (mStartInterpolator == null) {
            mStartInterpolator = new LinearInterpolator();
        }
    }

    public Interpolator getEndInterpolator() {
        return mEndInterpolator;
    }

    public void setEndInterpolator(Interpolator endInterpolator) {
        mEndInterpolator = endInterpolator;
        if (mEndInterpolator == null) {
            mEndInterpolator = new LinearInterpolator();
        }
    }
}

然后再写一个HXLinePagerIndicator继承自LinePagerIndicatorEx：

package zhangphil.test;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.LinearGradient;

public class HXLinePagerIndicator extends LinePagerIndicatorEx {
    public HXLinePagerIndicator(Context context) {
        super(context);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        LinearGradient lg = new LinearGradient(getLineRect().left, getLineRect().top, getLineRect().right, getLineRect().bottom, new int[]{0xFFD3B151, 0xFFF6DD99}, null, LinearGradient.TileMode.CLAMP);
        getPaint().setShader(lg);
        canvas.drawRoundRect(getLineRect(), getRoundRadius(), getRoundRadius(), getPaint());
    }
}

在HXLinePagerIndicator，要做到事情仅仅就是在onDraw时候，针对mLineRect进行一个颜色渐变处理。对于其他属性控制如圆角、宽、高、动画等等，不做特殊处理。

最终抽象完毕后，可以在上层直接使用：

package zhangphil.test;

import android.content.Context;
import android.graphics.Color;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.view.Gravity;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.AccelerateInterpolator;
import android.view.animation.DecelerateInterpolator;
import android.widget.TextView;

import net.lucode.hackware.magicindicator.MagicIndicator;
import net.lucode.hackware.magicindicator.ViewPagerHelper;
import net.lucode.hackware.magicindicator.buildins.UIUtil;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.CommonNavigator;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.abs.CommonNavigatorAdapter;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.abs.IPagerIndicator;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.abs.IPagerTitleView;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.indicators.LinePagerIndicator;
import net.lucode.hackware.magicindicator.buildins.commonnavigator.titles.SimplePagerTitleView;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class TabActivity extends AppCompatActivity {
    private ArrayList<String> mTitles;

    private static final String[] CHANNELS = new String[]{"zhang", "phil", "zhang phil", "csdn", "zhang phil csdn", "zhang phil @ csdn", "blog.csdn.net/zhangphil", "android"};
    private List<String> mDataList = Arrays.asList(CHANNELS);
    private ExamplePagerAdapter mExamplePagerAdapter = new ExamplePagerAdapter(mDataList);

    private ViewPager mViewPager;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.tab_avtivity);

        mTitles = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            mTitles.add(String.valueOf(i));
        }

        mViewPager = findViewById(R.id.view_pager);
        mViewPager.setAdapter(mExamplePagerAdapter);

        MagicIndicator magicIndicator = findViewById(R.id.magic_indicator);
        magicIndicator.setBackgroundColor(Color.WHITE);

        CommonNavigator commonNavigator = new CommonNavigator(this);

        //true 选项卡均分父布局宽度。这种情况仅仅在选项卡较少且文字比较短时候合适
        //     如果文字很长，且选项卡很多，那么久会挤在一起密密麻麻，视觉上比较难看。
        //通常设置false。
        commonNavigator.setAdjustMode(false);

        commonNavigator.setAdapter(new CommonNavigatorAdapter() {
            @Override
            public int getCount() {
                return mDataList == null ? 0 : mDataList.size();
            }

            @Override
            public IPagerTitleView getTitleView(Context context, final int index) {
                SimplePagerTitleView simplePagerTitleView = new SimplePagerTitleView(context);
                simplePagerTitleView.setText(mDataList.get(index));
                simplePagerTitleView.setNormalColor(getResources().getColor(android.R.color.holo_orange_light));
                simplePagerTitleView.setSelectedColor(getResources().getColor(android.R.color.holo_red_light));
                simplePagerTitleView.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        mViewPager.setCurrentItem(index);
                    }
                });

                return simplePagerTitleView;
            }

            @Override
            public IPagerIndicator getIndicator(Context context) {
                HXLinePagerIndicator indicator = new HXLinePagerIndicator(context);
                indicator.setMode(LinePagerIndicator.MODE_EXACTLY);
                indicator.setLineHeight(UIUtil.dip2px(context, 6));
                indicator.setLineWidth(UIUtil.dip2px(context, 50));
                indicator.setRoundRadius(UIUtil.dip2px(context, 3));

                indicator.setStartInterpolator(new AccelerateInterpolator());
                indicator.setEndInterpolator(new DecelerateInterpolator(2.0f));

                return indicator;
            }
        });

        magicIndicator.setNavigator(commonNavigator);

        ViewPagerHelper.bind(magicIndicator, mViewPager);
    }

    public class ExamplePagerAdapter extends PagerAdapter {
        private List<String> mDataList;

        public ExamplePagerAdapter(List<String> dataList) {
            mDataList = dataList;
        }

        @Override
        public int getCount() {
            return mDataList == null ? 0 : mDataList.size();
        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
            return view == object;
        }

        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            TextView textView = new TextView(container.getContext());
            textView.setText(mDataList.get(position));
            textView.setGravity(Gravity.CENTER);
            textView.setTextColor(Color.BLACK);
            textView.setTextSize(24);
            container.addView(textView);
            return textView;
        }

        @Override
        public void destroyItem(ViewGroup container, int position, Object object) {
            container.removeView((View) object);
        }

        @Override
        public int getItemPosition(Object object) {
            TextView textView = (TextView) object;
            String text = textView.getText().toString();
            int index = mDataList.indexOf(text);
            if (index >= 0) {
                return index;
            }

            return POSITION_NONE;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return mDataList.get(position);
        }
    }
}

Xml布局文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <net.lucode.hackware.magicindicator.MagicIndicator
        android:id="@+id/magic_indicator"
        android:layout_width="match_parent"
        android:layout_height="50dp" />

    <android.support.v4.view.ViewPager
        android:id="@+id/view_pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

代码运行的最终结果如图：

实现对MagicIndicator的增强，满足了UI设计师的颜色渐变需求。

附录：
1，《Android Material Design TabLayout属性app:tabMode和app: tabGravity》链接：https://blog.csdn.net/zhangphil/article/details/48931483