自定义属性

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <declare-styleable name="ColorTraceTextView">
        <attr name="origin_color" format="color" />
        <attr name="change_color" format="color" />
    </declare-styleable>

</resources>

自定义变色字体类

public class ColorTraceTextView extends AppCompatTextView {

    // 字体默认颜色
    private int originColor = 0xFF000000;
    // 字体选中颜色
    private int changeColor = 0xAAFF0000;
    // 划动的位移
    private float progress = 0f;
    // 划动的方向
    private Direction direction = Direction.LEFT_TO_RIGHT;

    private Paint originPaint;
    private Paint changePaint;
    private Rect bounds;

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

    public ColorTraceTextView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public ColorTraceTextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        obtainStyleAttrs(context, attrs);
    }

    private void obtainStyleAttrs(Context context, AttributeSet attrs) {
        TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.ColorTraceTextView);
        originColor = ta.getColor(R.styleable.ColorTraceTextView_origin_color, originColor);
        changeColor = ta.getColor(R.styleable.ColorTraceTextView_change_color, changeColor);

        originPaint = getPaint(originColor);
        changePaint = getPaint(changeColor);
        bounds = new Rect();

        ta.recycle();
    }

    @Override
    protected synchronized void onDraw(Canvas canvas) {
        int middle = (int) (progress * getWidth());
        if (direction == Direction.LEFT_TO_RIGHT) {
            drawColorText(canvas, changePaint, 0, middle);
            drawColorText(canvas, originPaint, middle, getWidth());
        } else {
            drawColorText(canvas, changePaint, getWidth() - middle, getWidth());
            drawColorText(canvas, originPaint, 0, getWidth() - middle);
        }
    }

    private void drawColorText(Canvas canvas, Paint paint, int start, int end) {
        canvas.save();
        // 截取变色的文本区域
        canvas.clipRect(start, 0, end, getHeight());
        // 获取文本内容
        String text = getText().toString();
        paint.getTextBounds(text, 0, text.length(), bounds);
        int x = getWidth() / 2 - bounds.width() / 2;
        // 基线
        Paint.FontMetricsInt fontMetrics = paint.getFontMetricsInt();
        int dy = (fontMetrics.bottom - fontMetrics.top) / 2 - fontMetrics.bottom;
        int baseLine = getHeight() / 2 + dy;
        // 绘制字体
        canvas.drawText(text, x, baseLine, paint);
        canvas.restore();
    }

    private Paint getPaint(@ColorInt int color) {
        Paint paint = new Paint();
        // 设置画笔颜色
        paint.setColor(color);
        // 防止抖动
        paint.setDither(true);
        // 抗锯齿
        paint.setAntiAlias(true);
        // 设置字体大小
        paint.setTextSize(getTextSize());
        return paint;
    }

    public void setOriginColor(@ColorInt int color) {
        this.originPaint.setColor(color);
    }

    public void setChangeColor(@ColorInt int color) {
        this.changePaint.setColor(color);
    }

    public void setDirection(Direction direction) {
        this.direction = direction;
    }

    public void setProgress(float progress) {
        this.progress = progress;
        invalidate();
    }

    public enum Direction {
        LEFT_TO_RIGHT, RIGHT_TO_LEFT
    }

    public int dp2px(int value) {
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, value,
                getResources().getDisplayMetrics());
    }

}

在Activity中使用

public class MainActivity extends AppCompatActivity {

    private String[] titles = {"推荐", "体育", "经济", "农业", "政治"};
    private List<ColorTraceTextView> list;
    private LinearLayout layout;
    private ViewPager viewPager;

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

        list = new ArrayList<>();
        layout = findViewById(R.id.ll_title);
        viewPager = findViewById(R.id.view_page);

        initLayout();
        initViewPage();
    }

    private void initLayout() {
        for (int i=0; i<titles.length; i++) {
            LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
                    ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
            params.weight = 1;
            ColorTraceTextView traceTextView = new ColorTraceTextView(this);
            traceTextView.setText(titles[i]);
            traceTextView.setTextSize(20);
            traceTextView.setPadding(0, traceTextView.dp2px(10), 0, traceTextView.dp2px(10));
            traceTextView.setLayoutParams(params);
            layout.addView(traceTextView);
            list.add(traceTextView);
        }
    }

    private void initViewPage() {
        viewPager.setAdapter(new FragmentPagerAdapter(getSupportFragmentManager()) {
            @Override
            public Fragment getItem(int position) {
                return PageFragment.instance(titles[position]);
            }

            @Override
            public int getCount() {
                return titles.length;
            }
        });

        viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
                ColorTraceTextView left = list.get(position);
                left.setDirection(ColorTraceTextView.Direction.RIGHT_TO_LEFT);
                left.setProgress(1 - positionOffset);

                if ((position + 1) < titles.length) {
                    ColorTraceTextView right = list.get(position + 1);
                    right.setDirection(ColorTraceTextView.Direction.LEFT_TO_RIGHT);
                    right.setProgress(positionOffset);
                }
            }

            @Override
            public void onPageSelected(int position) {

            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });
    }

}

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">

    <LinearLayout
        android:id="@+id/ll_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" />

    <View
        android:layout_width="match_parent"
        android:layout_height="1dp"
        android:background="#FFD2D2D2" />

    <androidx.viewpager.widget.ViewPager
        android:id="@+id/view_page"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" />

</LinearLayout>