Android TabLayout custom indicator width

if a tab indicator with a fixed size is what you want, to achieve you can create an indicator shape with a fixed size by drawable:

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:gravity="center">
        <shape android:shape="rectangle">
            <solid android:color="@color/colorAccent" />
            <corners
                android:topLeftRadius="2dp"
                android:topRightRadius="2dp" />
            <size
                android:width="16dp"
                android:height="2dp" />
        </shape>
    </item>
</layer-list>

Then on the TabLayout, you can then just set the tab indicator drawable. at the same time, you also have to set the tabIndicatorColor

 app:tabIndicatorColor="@color/tabIndicatorColor"
 app:tabIndicator="@drawable/tab_indicator"

Because the drawable itself center the shape inside its bounds, the indicator will have a fixed size.

if you want the indicator to match the width of the label, you can remove the tabIndicator android:gravity.

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape android:shape="rectangle">
            <solid android:color="@color/colorAccent" />
            <corners
                radius="2dp" />
            <size
                android:width="16dp"
                android:height="2dp" />
        </shape>
    </item>
</layer-list>

For anyone that ends up on this issue, I found a simple solution with vector drawables for a ~90% of the tab width indicator:

ic_tab_indicator_24dp:

<vector
android:height="24dp"
android:width="24dp"
android:viewportHeight="24.0"
android:viewportWidth="24.0"
xmlns:android="http://schemas.android.com/apk/res/android">
<path
    android:strokeWidth="4"
    android:fillColor="@android:color/white"
    android:pathData="M2,0 L22,0 L22,24 L2,24 z"/>

And then set tabIndicator in the layout:

app:tabIndicator="@drawable/ic_tab_indicator_24dp"

Or in styles.xml:

<item name="tabIndicator">@drawable/ic_tab_indicator_24dp</item>

Edit: As my solution is getting attention, I'd add that you can play with the width just modifying the pathData ("M2,0 L22,0 L22,24 L2,24 z") changing 2 and 22 values. The amount added to 2 should be subtracted from 22. i.e.: "M4,0 L20,0 L20,24 L4,24 z" or "M6,0 L18,0 L18,24 L6,24 z"...

Tried all those solutions,but not satisfied, and finally I found a solution myself, we just need to modify the indicatorLeft and indicatorRight fileds of SlidingTabIndicator, extends the tablayout and call setIndicaotrWidth() when need, for example:

setIndicatorWidth(70);

The whole code:

import android.content.Context;
import android.content.res.Resources;
import android.util.AttributeSet;
import android.util.TypedValue;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewTreeObserver;
import android.widget.LinearLayout;
import android.widget.TextView;

import com.google.android.material.tabs.TabLayout;

import java.lang.reflect.Field;

import androidx.annotation.NonNull;

public class TabLayoutEx extends TabLayout {

    public TabLayoutEx(Context context) {
        this(context, null);
    }

    public TabLayoutEx(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public TabLayoutEx(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(attrs);
    }

    private void init(AttributeSet attrs) {
        setTabIndicatorFullWidth(false);
        setIndicatorWidth(70);
    }



    private class DefPreDrawListener implements ViewTreeObserver.OnPreDrawListener {

        private LinearLayout tabStrip = null;
        private int tabWidth;
        private Field fieldLeft;
        private Field fieldRight;

        public void setTabStrip(LinearLayout tabStrip, int width) {
            try {
                this.tabStrip = tabStrip;
                this.tabWidth = width;
                Class cls = tabStrip.getClass();
                fieldLeft = cls.getDeclaredField("indicatorLeft");
                fieldLeft.setAccessible(true);
                fieldRight = cls.getDeclaredField("indicatorRight");
                fieldRight.setAccessible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        @Override
        public boolean onPreDraw() {
            try {
                if (tabWidth > 0) {
                    int left = fieldLeft.getInt(this.tabStrip);
                    int right = fieldRight.getInt(this.tabStrip);
                    //根据目标宽度及现在的宽度调整为合适的left和right
                    int diff = right - left - tabWidth;
                    left = left + diff / 2;
                    right = right - diff / 2;
                    fieldLeft.setInt(this.tabStrip, left);
                    fieldRight.setInt(this.tabStrip, right);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
            return true;
        }
    }

    private DefPreDrawListener defPreDrawListener = new DefPreDrawListener();

    public void setIndicatorWidth(int widthDp) {
        Class<?> tabLayout = TabLayout.class;
        Field tabStrip = null;
        try {
            tabStrip = tabLayout.getDeclaredField("slidingTabIndicator");
            tabStrip.setAccessible(true);
            LinearLayout tabIndicator = (LinearLayout) tabStrip.get(this);
            int width = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, widthDp, Resources.getSystem().getDisplayMetrics());
            //avoid add preDrawListener multi times
            tabIndicator.getViewTreeObserver().removeOnPreDrawListener(defPreDrawListener);
            tabIndicator.getViewTreeObserver().addOnPreDrawListener(defPreDrawListener);
            defPreDrawListener.setTabStrip(tabIndicator, width);
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

you could copy TabLayout, and modify the logic of calculating the width of Indicator. See updateIndicatorPosition() and animateIndicatorToPosition(). A simple demo https://github.com/xybean/CustomTabLayout