利用反射的原理来自定义Tab 下划线indicator的长度

public class SetIndicator {
    public static void setIndicator(Context context, TabLayout tabs, int leftDip, int rightDip) {
        Class<?> tabLayout = tabs.getClass();
        Field tabStrip = null;
        try {
            tabStrip = tabLayout.getDeclaredField("mTabStrip");
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        tabStrip.setAccessible(true);
        LinearLayout ll_tab = null;
        try {
            ll_tab = (LinearLayout) tabStrip.get(tabs);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

        int left = (int) (getDisplayMetrics(context).density * leftDip);
        int right = (int) (getDisplayMetrics(context).density * rightDip);

        for (int i = 0; i < ll_tab.getChildCount(); i++) {
            View child = ll_tab.getChildAt(i);
            child.setPadding(0, 0, 0, 0);
    LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(0, LinearLayout.LayoutParams.MATCH_PARENT, 1);
            params.leftMargin = left;
            params.rightMargin = right;
            child.setLayoutParams(params);
            child.invalidate();
        }
    }

public static DisplayMetrics getDisplayMetrics(Context context) {
        DisplayMetrics metric = new DisplayMetrics();
        ((Activity) context).getWindowManager().getDefaultDisplay().getMetrics(metric);
        return metric;
    }
public static float getPXfromDP(float value, Context context) {
        return TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, value,
                context.getResources().getDisplayMetrics());
    }
public static void setIndicator(TabLayout tabs, int leftDip, int rightDip) {
        Class<?> tabLayout = tabs.getClass();
        Field tabStrip = null;
        try {
            tabStrip = tabLayout.getDeclaredField("mTabStrip");
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }

        tabStrip.setAccessible(true);
        LinearLayout llTab = null;
        try {
            llTab = (LinearLayout) tabStrip.get(tabs);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }

int left = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, leftDip, Resources.getSystem().getDisplayMetrics());
int right = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, rightDip, Resources.getSystem().getDisplayMetrics());
        for (int i = 0; i < llTab.getChildCount(); i++) {
            View child = llTab.getChildAt(i);
            child.setPadding(0, 0, 0, 0);
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(0, LinearLayout.LayoutParams.MATCH_PARENT, 1);
            params.leftMargin = left;
            params.rightMargin = right;
            child.setLayoutParams(params);
            child.invalidate();
        }
    }
}

• 在fragment中的使用:        其中 33,29分别是距离左边和右边的距离

• topicTabLayout.post(new Runnable() {
              @Override
              public void run() {
                  SetIndicator.setIndicator(topicTabLayout,33,29);
              }
          });