A simple Page indicator for Android ViewPager

PagerIndicator.java

package com.example;

import android.annotation.TargetApi;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.os.Build;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.view.View;

import com.example.R;


/**
 * Simple page indicator
 * <p>
 * Created by gazer on 9/13/16.
 */
public class PagerIndicator extends View implements ViewPager.OnPageChangeListener {
    private Paint paint;
    private Paint currentPaint;
    private ViewPager pager;
    private int desiredRadius;

    public PagerIndicator(Context context) {
        super(context);
        setup();
    }

    public PagerIndicator(Context context, AttributeSet attrs) {
        super(context, attrs);
        setup();
    }

    public PagerIndicator(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        setup();
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public PagerIndicator(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        setup();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        desiredRadius = getContext().getResources().getDimensionPixelSize(R.dimen.pager_indicator_radius);

        int desiredWidth = 2 * desiredRadius + getPaddingLeft() + getPaddingRight();
        int desiredHeight = 2 * desiredRadius + getPaddingTop() + getPaddingBottom();

        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        int width;
        int height;

        //Measure Width
        if (widthMode == MeasureSpec.EXACTLY) {
            //Must be this size
            width = widthSize;
        } else if (widthMode == MeasureSpec.AT_MOST) {
            //Can't be bigger than...
            width = Math.min(desiredWidth, widthSize);
        } else {
            //Be whatever you want
            width = desiredWidth;
        }

        //Measure Height
        if (heightMode == MeasureSpec.EXACTLY) {
            //Must be this size
            height = heightSize;
        } else if (heightMode == MeasureSpec.AT_MOST) {
            //Can't be bigger than...
            height = Math.min(desiredHeight, heightSize);
        } else {
            //Be whatever you want
            height = desiredHeight;
        }

        //MUST CALL THIS
        setMeasuredDimension(width, height);
    }

    private void setup() {
        paint = new Paint();
        paint.setColor(Color.LTGRAY);
        paint.setStyle(Paint.Style.FILL_AND_STROKE);
        paint.setAntiAlias(true);

        currentPaint = new Paint();
        currentPaint.setColor(Color.WHITE);
        currentPaint.setStyle(Paint.Style.FILL_AND_STROKE);
        currentPaint.setAntiAlias(true);
    }

    Rect bounds = new Rect();

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        if (isInEditMode()) {
            drawDots(canvas, 5, 2);
            return;
        }

        if (pager != null) {
            int total = pager.getAdapter().getCount();
            int currentPage = pager.getCurrentItem();

            drawDots(canvas, total, currentPage);
        }
    }

    private void drawDots(Canvas canvas, int total, int currentPage) {
        canvas.getClipBounds(bounds);
        float centerX = bounds.centerX();
        float centerY = bounds.centerY();

        float totalWidth = total * desiredRadius + (total - 1) * desiredRadius;

        float x = centerX - totalWidth + desiredRadius;
        for (int i = 0; i < total; i++) {
            canvas.drawCircle(x, centerY, desiredRadius, i == currentPage ? currentPaint : paint);
            x += desiredRadius * 4;
        }
    }

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        // Not used
    }

    @Override
    public void onPageSelected(int position) {
        invalidate();
    }

    @Override
    public void onPageScrollStateChanged(int state) {
        // Not used
    }

    public void setPager(ViewPager pager) {
        this.pager = pager;
        pager.addOnPageChangeListener(this);
    }
}