Using ConstraintLayout with Circular Positioning on Android

I was working on a feature for a client that involved custom views with Android. While researching ConstraintLayout, I saw that it had a circular positioning feature. It seemed pretty neat, so I thought I would circle around when finished and build a demo.

The basics of Circular Positioning are fairly straight forward:

  1. You define a view as the center
  2. You create a circular constraint for each view you want to surround it

I learned a lot from the following article:


You can build a circular restraint in a layout file. It’s a great first exercise. Yet, I wanted to be able to change the number of views that circled the center. To do that I needed to create a custom view.

It seemed like it would be a good learning experience. I have never had to create constraints in code before. Judging by what I saw on the internet, few others do either.

My demo lays out my items in a circular layout and gives the option to the user to change the number of items. When changed, the view re-constrains everything in a balanced circle.

For example:
2 items = 50%, 3 items = 33.3%, 4 items = 25%, and so on

circular layout with faces
My custom view – with 8 images surrounding the center

The Custom View

My custom view is called CircularIconLayout

  • It extends ConstraintLayout – simply because I am building a ConstraintLayout!
  • I base the views in the circle on a section count
  • I pass in a layout file that is used for the views in the circle. This allows me to use any type of view I want

When putting everything together, I follow these general steps:

  1. I create a TextView and constrain it to the center (centerView)
  2. Create a separate view for each section of the circle
  3. I calculate where to place the views around the circle:
    1. divide the number of sections by 360
      • example: 360 / 4 = 25 degrees for each section
    2. determine the center of each section
      • example: 25 / 2 = 12.5 degrees is the middle of each section
  4. Create a constraint for each view using constrainCircle()

I keep a list of the views for reference (sectionList). When the user changes the number of sections, I update the list and re-constrain everything to the centerView again.

Here is some code showing how to setup the constraints:

// constrain sections to the centerView
val constraintSet = ConstraintSet()

// constrain centerView to the middle of our (parent) view
constraintSet.connect(, ConstraintSet.TOP,, ConstraintSet.TOP)
constraintSet.connect(, ConstraintSet.BOTTOM,, ConstraintSet.BOTTOM)
constraintSet.connect(, ConstraintSet.START,, ConstraintSet.START)
constraintSet.connect(, ConstraintSet.END,, ConstraintSet.END)

// calculate where to place each view surrounding the center
val distanceFromCenter = circleRadius.toDensityPixels()
val segmentDegrees: Float = 360.0f / sectionCount.toFloat()
var angle: Float = segmentDegrees / 2.0f

// constrain each section to the centerView
sectionList.forEach { section ->
    constraintSet.constrainCircle(,, distanceFromCenter, angle)
    angle += segmentDegrees


Formatting the views in the circle is left up to the Activity or Fragment that calls it. For example, the activity could use the sectionList to load images into the ImageViews that are in the circle.

Extra features

I tried to make the demo simple, but I got a little carried away in the Activity:

  • The user can change the number of items displayed in the circle
  • The user ALSO can change the type of views used in the circle. You can switch between images and text

I also added a couple other features:

icu4j or RuleBasedNumberFormat – a library for spelling out numbers. i.e. 1 = “one”, 12 = “twelve”, and so forth.

I found an example for creating random colors from a pool of acceptable colors on Stack Overflow. I made some improvements and used it to generate random colors.

The Demo

For me, code speaks louder than words. Checkout the demo at my github repo:

demo video gif

Building a Washington State Ferry decoration

Night shot of ferry outside

I built a working WA State Ferry decoration for my yard.

I like to work on projects that are a little outside of things I usually work on. I have no idea how to achieve it, and spend most of the year working on it in my spare time.

Some of my past projects:

For this post I want to expand on how I made some challenges, and how I overcame them.

You can see an overview of the entire process here:

Movement and Control

My original idea for the ferry was to use an aluminum rail with a wheeled sled. I found one while I was working on a different project. I had aspirations of building something similar to a 3d printer to control it. As I was working through how to achieve this, it became clear it would be too complicated and hard to maintain.

I had researched garage door openers in the past for my ski lift. I ruled it out since it wasn’t designed to run all the time – I was afraid it would overheat. I revisited the solution again for the ferry. I researched various models and features, and kept an on craigslist until one showed up.

At first I thought I would have the ferry run on a schedule. Every 15 minutes it would automatically switch sides. It didn’t take long to come to the conclusion that it was better to have the user press a button.

Testing a LED button with alligator clips
You can never have too many alligator clips

Garage door openers already have an option for a button. One downside is that the button can be used at any time to stop progress of the opener. I did not want the button press to be so responsive. Once the process started I wanted it to finish. This is where I decided to use an Arduino to control the button. I have a separate blog post showing how I achieved this:

The button behavior was fairly straight forward, but it would take some experimentation. I learned long ago that a good test environment gives you the highest chance of success. I had to be diligent about making my project easy to pickup after leaving it alone for a month . This meant well organized code, version control, and pictures of my breadboard setup.

I organized actions into functions:

  • instead of writing digitalWrite(LED_BUILTIN, HIGH) to turn off an LED
  • I had a functions called turnBuiltinLEDOn() and turnBuiltinLEDOff()

Staying organized made it a lot easier to crawl back into my code when needed.

What things should look like

I knew I had to make:

  • a ferry that looked like a ferry
  • a cover for the motor to keep it dry, and it should look like part of the scenery
  • something else on the other side, that also fit the scenery
A model made to look like a ferry boat.
A boxy Washington State Ferry

The Ferry
There are lots of examples of ferry online. I looked at a few pictures of real WA State Ferries, and a few drawings as well. I’ve found that people like decorations that look more like a cartoon than the real thing. I also wanted the ferry to look homemade. I settled on something with more rounded port holes, and the same color scheme as real thing. I also wanted some room for some cars.

A diecast model on the ferry
Our first customer

I thought it would be neat to have all our cars represented on the ferry. Finding a model for a classic VW Beetle was not that hard. Finding a Eurovan and a Chrysler minivan were a bit harder. I found some cheap diecast models on ebay. They looked close enough, I painted them to look like our vehicles.

waiting for a painted diecast model to dry
A makeshift drying station

There seems to be a large diecast model community. There was plenty of advice on how to paint the models. I follow most of the advice, but I cut a few corners here and there. I didn’t strip the old paint, and I had to paint with a brush. For some reason every model was advertised as 1:32 scale. But when you compared them next to each other the scale is not accurate. I did my best with the time allotted.

The Terminal
For the side of the Garage Door opener with a motor, I made something that looked like a terminal. The only problem was that the real terminals do not look like much. You would be hard pressed to see one if you were not paying attention. I settled on a bland white building, with a large version of one of the logos on the side. I borrowed this idea from some of the airlines that paint a huge logo on their hangars at airports. I wanted to have windows that could light up, but I decided that could be a feature for another time.

A picture of a box painted to look like a ferry terminal in a cluttered garage.
A ferry terminal hanging out in a cluttered garage

The Dock
While researching terminals, I saw a lot of pictures of docks. The main thing about the docks is that they had a large structure that went high in the air. I decided to build a similar looking structure as a dock for the other side. I got a little fancy and came up with a pattern that made it look pretty good.

Assembling the ferry dock
Assembling the ferry dock


Storage is a big deal. We knew from past experience that you must account for storage. Any time I have made an exception it has a certain cost in the future. From the outset I had a plan for how I was going to store everything:

  • The ferry is held together by bolts, and comes completely apart
  • The ferry dock is also held together by bolts
  • The terminal serves as a box that holds all the other parts
  • I’m going to store the garage door opener, in the garage! Since the track is attached to the motor, I can hang it up high as one piece

I built the platforms as I needed them. I originally thought I’d use a ladder for one side, but it didn’t work well when I had everything together. Both these platforms come completely apart. I learned a lot from watching a youtube video for building modular stages:


One of the things I like about this project is that I got it to the point where I have a lot of flexibility. At this point the project is using the same techniques used by model train builders for their scenery. I have a few ideas:

  • I want to light up the terminal at night. I plan to cut out the windows (the black rectangles), and install some translucent material with lights behind them
  • Some friends got me models of their cars for the ferry. One of them had LED headlights. I rewired them so that they work on external power instead of batteries. I’d like to put this car on the dock and power it so that its lights shine at night
  • I want to install some lights on the dock
  • Someone suggested I add a small horn when the ferry starts, but I’m thinking that will start to annoy the neighbors after a while 🙂
Night shot of ferry outside

In Summary

This was a fun project! I felt like I got to apply some lessons learned from past projects and it went fairly smoothly. This ferry incorporates more features than the other projects. The interactive part really makes it a crowd-pleaser.

Jay Inslee, Governor of Washington State, said "Nice work" on Twitter.

Parsing dates using java.time on Android

Having fun using java.time and Retrofit for ISO 8601 dates on Android

I was assigned a task to parse ISO 8601 dates with timezone information from an endpoint. I decided to use Java 8’s java.time library. I had to write a custom converter to use it with Retrofit because the endpoint provided the date in a different format. This is a summary of my journey.

What is java.time, and how do I use it with Android?

java.time aims to improve the date and time handling in Java. It was added to Java in JSR-310. It was included in Android as of API 26. But if you want to use it with earlier versions, you must use a library.

There are a couple of solutions, each with their own drawbacks. I settled on a library by Jake Wharton called ThreeTenABP. It has been optimized for the Android platform. Jake goes into his reasoning on the project’s main page.

To get this to work with Retrofit, I used a library called ThreeTen-Backport-Gson-Adapter. It’s a library that provides json serialization/deserialization for java.time using Gson.

Let’s run some tests!

There are lots of ways to document your code. One of my favorites is with tests. A good test can express how to do something, and why you are doing it. It also allows a future developer to play around with the code in a safe environment.

All my examples are tests. I’ve included the classes under test with the test itself. It makes it easier to show here.

Make sure you click to expand the test, otherwise you can’t see it!

Setup environment

You can set these up in Android Studio, but you can also use IntelliJ Community Edition.

I like having IntelliJ around to run test code. Create a new Project with Gradle, and include Java and Kotlin. Then use the build.gradle file to include your dependencies just like you would in Android Studio.

My dependencies are below. Keep in mind that these will be old versions by the time you read this.

Click to see code
dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
    testCompile group: 'junit', name: 'junit', version: '4.12'

    /* Java 8 java.time (JSR-310) backport for Android */
    implementation 'com.jakewharton.threetenabp:threetenabp:1.2.0'
    implementation("org.aaronhe:threetenbp-gson-adapter:1.0.2") {
        exclude module: 'threetenbp'
    testImplementation 'org.threeten:threetenbp:1.3.8'

java.time tests

I wrote some tests to see how java.time behaves. Since it’s a newer library, I figured I could encourage my team to use them to learn java.time in a playground-like environment.

Click to see java.time tests
import junit.framework.Assert.assertEquals
import org.junit.Test
import org.threeten.bp.LocalDateTime
import org.threeten.bp.ZoneId
import org.threeten.bp.ZoneOffset
import org.threeten.bp.ZonedDateTime
import org.threeten.bp.format.DateTimeFormatter

class JavaTimeTests {

    fun test_basicParsing() {
        // GIVEN a date at 12:25am UTC
        val apiDate = LocalDateTime.parse("2019-01-18T00:25:00.0000000")
        val zonedDate = ZonedDateTime.of(apiDate, ZoneId.of("UTC"))

        // WHEN I convert my date to PST
        val zoneID = ZoneId.of("PST", ZoneId.SHORT_IDS)
        val inMyTimezone = zonedDate.withZoneSameInstant(zoneID)

        val isoDateTime = inMyTimezone.format(DateTimeFormatter.ISO_DATE_TIME)

        // THEN I get the expected date and time in PST (a day earlier)
        val isoDate = inMyTimezone.format(DateTimeFormatter.ISO_DATE)
        assertEquals("2019-01-17-08:00", isoDate.toString())

        // WHEN I format my date in a different pattern (using slashes)
        val patternDate = inMyTimezone.format(DateTimeFormatter.ofPattern("yyyy/MM/dd"))
        // THEN I receive a date use the expected pattern
        assertEquals("2019/01/17", patternDate.toString())

        // WHEN I format my date in a different patter (using dashes)
        val localDate = inMyTimezone.toLocalDate()
        // THEN I receive a date use the expected pattern
        assertEquals("2019-01-17", localDate.toString())

    fun test_convertDatesToUTC() {
        // GIVEN a string date and string timezone
        val usZone = ZoneId.of("America/Los_Angeles")
        val str = "1926-09-23 00:00"

        // WHEN I parse the string date to a ZonedDateTime
        val formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm")
        val localDateAndTime = LocalDateTime.parse(str, formatter)
        val dateAndTimeInUS = ZonedDateTime.of(localDateAndTime, usZone)

        // THEN I get the expected date in PST
        // 0 AM PST
        assertEquals(0, dateAndTimeInUS.hour)

        // THEN I get the expected date in UTC
        // 8 AM UTC
        val utcDate = dateAndTimeInUS.withZoneSameInstant(ZoneOffset.UTC)

        assertEquals(8, utcDate.hour)


Parsing dates with gson

I’ve included the smallest amount of code to see how to use gson to format the custom date.

  • I don’t show how to use retrofit here, I just use a gson builder to incorporate the converter factory
  • The Converter parses a custom shaped json object, noted in the test
Click to see code

// for EndpointFactory
import org.aaronhe.threetengson.ThreeTenGsonAdapter

// for EndpointDate
import org.junit.Assert.assertEquals
import org.junit.Test
import org.threeten.bp.LocalDateTime
import org.threeten.bp.Month
import org.threeten.bp.ZoneId
import org.threeten.bp.ZonedDateTime
import org.threeten.bp.format.DateTimeParseException
import java.lang.reflect.Type

 * The data class for the date
data class EndpointDate(
    val dateTime: ZonedDateTime,
    val timeZone: String

 * Deserialize an EndpointDate for Gson.  This will load the proper ZonedDateTime, in the given timeZone
class EndpointDateDeserializer : JsonDeserializer<EndpointDate> {
    override fun deserialize(json: JsonElement?, typeOfT: Type?, context: JsonDeserializationContext?): EndpointDate {
        json?.let {
            val jsonObject = it.asJsonObject

            try {
                val apiDate = LocalDateTime.parse(jsonObject.get("dateTime").asString)
                val zoneString = jsonObject.get("timeZone").asString
                val zone = ZoneId.of(zoneString)

                val zonedDate = ZonedDateTime.of(apiDate, zone)
                return EndpointDate(zonedDate, zoneString)
            } catch (e: DateTimeParseException) {
                throw JsonParseException(e)
        throw IllegalArgumentException("unknown type: $typeOfT")

 * I just supply the gson builder
object EndpointFactory {

    fun getGson(): Gson {
        val gsonBuilder = GsonBuilder()
        gsonBuilder.registerTypeAdapter(, EndpointDateDeserializer())
        return ThreeTenGsonAdapter.registerZonedDateTime(gsonBuilder).create()

class DateParseTests {

    fun test_parseEndpointDate() {

        // GIVEN a gson object with a deserializer that can parse EndpointDate objects
        val gson = EndpointFactory.getGson()

        // GIVEN json of an EndpointDate
        val json = """
                "dateTime": "1564-04-23T11:25:00.0000000",
                "timeZone": "UTC"

        // WHEN the json is parsed into a EndpointDate data class
        val endpointDate = gson.fromJson(json,

        // THEN I get the expected date with proper time zone
        assertEquals(1564, endpointDate.dateTime.year)
        assertEquals(Month.APRIL, endpointDate.dateTime.month)
        assertEquals(23, endpointDate.dateTime.dayOfMonth)

        assertEquals(11, endpointDate.dateTime.hour)
        assertEquals(25, endpointDate.dateTime.minute)


This was an interesting problem to solve. We had some weird timezone issues that kept popping up. Our app was going to be heavily reliant on time. I knew we needed to stop writing short-term hacks and come up with something more concrete. This process took a little more time to put in place, but it paid off in the end by clearing the road ahead of us.