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README.md

Views DSL

Create UIs with readable Kotlin code.

There's a whole document about Views DSL vs xml layouts if you are not convinced yet.

TL;DR: Kotlin code is more concise than xml, and a small library like this one is the proof of what is already possible with this great language.

Splitties Views DSL has been designed to be:

  • Simple
  • Concise
  • Expressive
  • Explicit
  • Efficient
  • Reliable
  • Flexible

That's 7 key considerations, which are all necessary to make a great library.

Introduction

As said above, Splitties Views DSL has been designed to be simple.

Consequently, you'll find no class in this split (API-wise, as strictly speaking, all functions and properties, even top-level ones and extensions belong to a class in the bytecode). That means you won't have to learn a whole new API to use Splitties Views DSL. You'll just have to discover the extension functions and properties as you need them to craft your Android user interfaces with Kotlin code.

It turns out that you just need a few extension functions and properties to make UI-related code at least as readable as xml counterparts. Note that while putting all of your UI code directly in an Activity or a Fragment is possible with Splitties Views DSL (and can surely help for throwaway prototyping), we will be recommending a cleaner, yet simple approach (spoiler: a custom class).

Before we dive into the details of the API, let's take a look at a simple example:

val launchDemoBtn = button {
    textResource = R.string.go_to_the_demo
}

This example was meaningless, because no one ever publishes an app with only one button. Also, the snippet above just creates a button. If you want it into a ViewGroup, or as the content of an Activity or a Fragment, you need to do so explicitly.

There are real examples in the sample. You can start by taking a look at MainUi. You can also see a simple example that uses ConstraintLayout in AboutUi.

Opening the project in your IDE and navigating the sample UI code while reading this documentation may certainly help you have a hands-on experience and be comfortable more quickly writing UIs with Kotlin, a programming language that is probably already familiar to you.

Table of contents

The extensions

Splitties is primarily made of extension functions and properties, to create views with minimal code but maximum flexibility.

Just calling the constructor, then calling needed methods in an apply { ... } block could be enough to use Kotlin instead of xml for your user interfaces, but Splitties Views DSL allows something more readable, more concise, and with a few features, like themes, styles, and seamless AppCompat support, without the boilerplate.

Creating and configuring views

The most generic way: view

The view extension functions are a primitive of Splitties Views DSL. They are generic, so they allow you to instantiate a View of any type.

There are 6 functions named view, because there's 2 overload types, and they are made available for 3 receiver types: Ui, View and Context. One of this overload type is an internal API (more info below).

With respect to efficiency, they are all inline. That means no unnecessary allocation that would slightly decrease performance otherwise.

Both overloads allow the following 3 optional parameters:

  • @IdRes id: Int, the id of the View. Example argument: R.id.input_name, given you declared it in xml, as done in the sample
  • @StyleRes theme: Int, resource of a theme overlay that will be applied to the View. Example argument: R.style.AppTheme_AppBarOverlay
  • initView: V.() -> Unit, a lambda that is like apply for the created View.

The first overload of view takes a required first parameter that is a function taking a Context, and returning a View. Since constructors are also methods in Kotlin, you can directly use a method reference like so: view(::View). The same goes for any other View subclass (e.g. view(::FrameLayout)). You can also use a lambda instead: view({ FrameLayout(it) }). In fact, that's how you should do it while autocomplete for method references is not optimal, then use the IDE quick action (alt/⌥ option + ⏎ enter) to convert it to method reference. You can of course use any custom method reference that is not a reference to a constructor as long as that method takes a Context parameter and returns a View or any subclass of it.

Here's a simple but typical example:

val myView: MyCustomView = view(::MyCustomView, R.id.my_view) {
    backgroundColor = Color.BLACK
}

The second overload of view, which is an internal API takes no required parameter, but relies on explicit (reified) type parameter to work properly. Just view<TextView>() is enough to instantiate a TextView. However, this version relies on a "view factory" that can automatically provide subclasses of the requested type as necessary. If you use the Views DSL AppCompat, you'll automatically receive instances of AppCompatButton with view<Button> thanks to the underlying View factory.

Here's a simple example of this second overload:

val submitBtn = view<Button>(R.id.btn_submit) { // You should use `button { … }` instead though.
    textResource = R.string.submit
}

The most simple and readable way: plain functions

Instead of using view<Button> { … } or view(::Button) { … } to create a Button instance (which uses an internal API), you can use button(…) { … }. The parameters are exactly the same as the view function.

Such methods exist for most Views and ViewGroups included in Android, and there's more in the additional modules.

You can see implementations for Android's Views and ViewGroups.

These methods are a bit more natural to read and to write, but they are really just inline aliases, purely syntactic sugar.

You can define your own if you want. Just make sure to write it first for Context and add two overloads for View and Ui that delegate to the one for Context. Also, remember to make them inline to avoid lambda allocation.

Using styles defined in xml

There are some times where you need to use an xml defined style, such as when using a style defined in AppCompat like Widget_AppCompat_Button_Colored.

Splitties makes it really easy to use xml styles defined in Android, AppCompat and Material Components.

It also gives you the ability to do the same for custom or third-party styles defined in xml.

Using Android styles

Let's say you want to create a horizontal ProgressBar instance. First, cache an instance of AndroidStyles:

private val androidStyles = AndroidStyles(ctx)

Then, use the function defined on the progressBar property:

val progressbar = androidStyles.progressBar.horizontal()

Other styles defined in the Android platform are provided in AndroidStyles. Just let auto-completion guide you.

Note that you have exactly the same optional parameters as view, including the optional lambda.

Using Material Components styles

Since Material Components styles are not included by default inside the theme, you need to load them first. This is simply done with the following code:

private val materialStyles = MaterialComponentsStyles(ctx)

You can then use styles using the MaterialComponentsStyles instance. Here's an example:

val bePoliteBtn = materialStyles.button.outlined {
    textResource = R.string.be_polite
}
Using AppCompat styles

Since AppCompat styles are not included by default inside the theme, you need to load them first. This is simply done with the following code:

private val appCompatStyles = AppCompatStyles(ctx)

You can then use styles using the AppCompatStyles instance. Here's an example:

val bePoliteBtn = appCompatStyles.button.colored {
    textResource = R.string.be_rude
}
Using any other xml style

The colored, horizontal and other properties you can find in the AndroidStyles and the AppCompatStyles have the XmlStyle type. It is easy to instantiate it and support any xml style after the style is loaded into the current theme, but before we see how it's done, let's see what is this type.

The XmlStyle inline class has:

  • A type parameter, for the target View type.
  • A single Int value, a theme attribute (@AttrRes, not @StyleRes).

As you can see, its constructor doesn't expect a style resource (e.g. R.style.Widget_AppCompat_ActionButton), but a theme attribute resource (e.g. R.attr.Widget_AppCompat_ActionButton). This is because of a limitation in Android where you can programmatically only use xml styles that are inside a theme. That doesn't mean that you will have to pollute all the themes you're using with styles definitions though.

Android allows you to combine multiple themes with the applyStyle(…) method that you can call on any theme, which any Context has. That way, you can apply a theme that already includes references the xml styles you need with only one line of code. This is what the AppCompatStyles(ctx) function mentioned does under the hood.

Here's a short example:

ctx.theme.applyStyle(R.style.ThirdPartyStyles, false)
val clapButton = XmlStyle<Button>(R.attr.Widget_ThirdParty_FancyButton)(ctx) {
    imageResource = R.drawable.ic_clap_white_24dp
}

The first line makes sure the theme associated to the Context named ctx can resolve all the style attributes defined into R.style.ThirdPartyStyles, such as R.attr.Widget_ThirdParty_FancyButton.

To make this work, you have to do the following:

  1. Declare ThirdPartyStyles in xml (usually in a file named styles.xml)
  2. Declare the Widget_ThirdParty_FancyButton attribute (usually in a file named attrs.xml)
  3. Declare Widget_ThirdParty_FancyButton into ThirdPartyStyles and make sure it references the style target resource (named Widget_ThirdParty_FancyButton too). Using the same name for the target style and the attribute is a recommendation (for clarity), but not a requirement.

After this is done, you can make a class to group related styles, as done in the Views DSL AppCompat split, so you get type inference, and a nicer syntax.

View extensions

For even more expressive UI code, Splitties Views DSL has a transitive dependency on the Views split that provides a useful set of Kotlin-friendly extension functions and properties dedicated to Views and some of their subclasses.

Inflating existing xml layouts

Splitties Views DSL works well with xml layouts too!

The inflate extension functions is a variant to the view function mentioned earlier in this guide which has an additional first parameter: the layout resource id you want to inflate.

Also, if the xml layout defines an id for the root view, it will be kept, unless you specified an explicit id (including View.NO_ID).

Just like view, inflate is defined for Context, View and Ui.

Here's a short example:

val mySecretFancyView = inflate(R.layout.my_fancy_layout) {
    isVisible = false
}

Laying out the views

ViewGroup.add(…), an alias to ViewGroup.addView(…)

To add a View to a ViewGroup in code, you can use View.addView(…). However, this can become quite redundant to have View repeated over and over when it's already obvious that you are in a UI centric class that passes a parameter that is clearly a View.

That's why this split has an inline alias to it named just add(…) for ViewGroup. It has the extra benefit of returning the passed View, which can be handy in some situations.

The ViewGroup.add(…) function requires an instance of ViewGroup.LayoutParams, see how Splitties helps instantiating it with minimal, yet explicit code.

ViewGroups extension functions to instantiate LayoutParams

Splitties provides several methods named lParams(…) { … } for the 2 Android's built-in ViewGroups: LinearLayout and FrameLayout. You can find support for additional ViewGroups in the additional modules.

These methods make it easy to instantiate LayoutParams with typesafe and readable code (unlike xml).

Here's the contract that every lParams or alike function must respect:

  1. The receiver is the type of the target ViewGroup subclass.
  2. The function returns the LayoutParams for the target ViewGroup.
  3. The first parameter is width and defaults to wrapContent, unless otherwise noted.
  4. The second parameter is height and defaults to the same value as width, unless otherwise noted.
  5. The width or height parameters may be missing in case they shall always have the same value for this target ViewGroup or for this function.
  6. There may be additional parameters, with default values if possible.
  7. The last parameter is a lambda with LayoutParams as a receiver and is executed exactly once, last (i.e. after any logic that the lParams implementation may have).
  8. If the lParams function targets a ViewGroup that has a superclass that also has its own LayoutParams, and its own lParams function, it should be named defaultLParams instead to prevent any overload resolution ambiguity. A great example is AppBarLayout that is a child class of LinearLayout and has such extension functions for LayoutParams.
  9. In case the function is specialized for non default use case (e.g. adding an AppBarLayout into a CoordinatorLayout), it can have a custom name, but should always end with LParams (e.g. appBarLParams).
WARNING regarding lParams and defaultLParams usage:

lParams and similar functions are resolved based on the type of their receiver. However, unless you prepend lParams or defaultLParams call with this., the received is picked implicitly, and can be indirect, possibly causing the wrong lParams method to be used.

Here's a short, example:

You're in a FrameLayout (because you're writing a subclass of it, or because of a lambda receiver, like inside frameLayout { … }). You call constraintLayout { … } and start adding views inside it, but when you call lParams, you may use the implementation for FrameLayout, and wonder why the ConstraintLayout.LayoutParams properties and extensions are not available.

To highlight such errors, you can prepend this. to your suspicious lParams calls, and if they are in red, then you used the wrong one for the ViewGroup you're in. The IDE should quickly fix it, adding the proper import at this point.

After this is done, you can then safely remove the this. prefix.

To avoid this issue, you can be alert when you're typing/auto-completing lParams and defaultLParams and make sure that you're selecting the extension for the type of the ViewGroup you're in (direct parent of the child View you are adding).

Other extensions for ViewGroup

  • wrapContent and matchParent inline extensions properties on ViewGroup are convenience aliases to ViewGroup.LayoutParams.WRAP_CONTENT and ViewGroup.LayoutParams.MATCH_PARENT.
  • horizontalMargin, verticalMargin and margin for convenient margins definition in layout parameters (ViewGroup.MarginLayoutParams which is the base of nearly all LayoutParams).
  • startMargin and endMargin which are compatible below API 17 (using LTR) and fix the inconsistent name ordering (leftMargin, but marginStart?).

The interface for user interfaces, named Ui

This section doesn't just writes so many words about how the Ui interface has only 2 properties. It explains why it is useful, how to use it the right way, and the possibilities it offers.

FYI, the declaration of this interface looks like this:

interface Ui {
    val ctx: Context
    val root: View
}

Why this interface

As said above, you can put your UI code directly in an Activity or a Fragment, but the fact you can doesn't mean you should. Mixing UI code with business logic, data storage code, network calls and miscellaneous boilerplate in the same "god" class will quickly make further work (like feature additions and maintenance) very hard, because you're likely not a god programmer, and even if you are, your coworkers, or successors, are likely not.

Xml layouts alleviate this issue by forcing you to put most of your UI code into a separate xml file, but you often need complementary code (e.g. to handle transitions, dynamic visibility), and this is often put into a Fragment or an Activity, which makes things worse, as you now have your UI code spread over at least two places that are tightly coupled.

What it is made of

With Splitties Views DSL, there's an optional interface named Ui, whose implementations are meant to contain your UI code.

It has a ctx property because in Android, a Context is needed to create a View. It has a root property because you need a View to display in the end.

Since you're using Kotlin code, you can put all the UI related logic in it too, in a single place this time.

Also, since Ui is an interface, you can get creative by creating sub-interfaces or sub-classes to have different implementations of the same UI, which is nice for A/B testing, user preferences (different styles that the user can pick), configuration (like screen orientation), and more.

Implementing the Ui interface

When writing a Ui implementation, override the ctx property as the first constructor parameter (e.g. class MainUi(override val ctx: Context) : Ui {), and override the root parameter as a property with a backing field by assigning it a View (e.g. override val root = coordinatorLayout { ... }).

Then create your views (usually putting them as final properties, like root), and add them to the ViewGroups they belong to, so they are direct, or indirect children of root (in the likely case where you have multiple views in your UI and root is therefore a ViewGroup).

Using Ui implementations

To use a Ui implementation from an Activity subclass, just call setContentView(ui). To use it from any other place, just get the root property. In a Fragment subclass, that will mean returning it from onCreateView(…).

You can also use any function or property you've declared in your sub-interface or implementation.

Here are two examples:

  • Using a public property of type Button to set it an OnClickListener in the place where the Ui is used (like an Activity or a Fragment that connects your UI to a ViewModel and any other components).
  • Call a method called animateGoalReached().

Simple examples

See concrete examples in MainUi and DemoUi with their respective Activities MainActivity and DemoActivity.

Possibilities brought by the Ui interface

IDE Preview

You can preview Ui implementations in the IDE. See the Views DSL IDE preview split.

Modular user interface contracts

While having a dedicated class for user interface, that is agnostic from where it will be used (Activity, Fragment, IDE Preview…), is a great first step to a modular user interface code, you can go further.

Instead of exposing your Ui implementation directly to the Activity or Fragment, you can decide to write several interfaces that define a contract that your Activity, Fragment, ViewModel (beware of leaks), or whatever will need, and implement all of these with one or more classes.

For example, let's say you are developing an email app. You write two interfaces: InboxUi and ComposeUi that both extend the Ui interface. You add to the interfaces all the functions (including any suspend fun), properties and other symbols you may need to expose to the Activity, Fragment, ViewModel or whatever. Then you implement these two interfaces, with either one class or two, depending on whether you want to display them separately or not.

Easier multi form factors support

Modular UI contracts open the door to a great benefit: an easier way to support multiple form factors (smartphones, smartwatches, tablets, laptops, cars…).

In the previous example, we highlighted the fact that you could have multiple interfaces that expose the needed symbols, and then decide to implement these interfaces in one, or multiple classes.

This can help you support different form factors with zero, or only a few changes in non-UI code as it is no longer relies on a specific implementation.

It is planned to add such examples in the samples of this repository. If you want to have them faster, please open an issue so the examples can be discussed. Also, maybe you, or someone you know, can contribute.

Multiplatform user interface contracts

Here's an example of how you may write multiplatform user interface contracts:

In Kotlin common code, you would write an interface that is platform agnostic but declares the needed symbols that all platforms can share:

Continuing our email app example, you would write these two interfaces:

interface InboxUiContract {
    // Whatever you need
}

interface ComposeUiContract {
    // Whatever you need
}

Then write to sub-interfaces for each platform you want to support, Android and iOS in this example:

interface AndroidInboxUi : InboxUiContract, Ui
interface IOSInboxUi : InboxUiContract {
    val root: UIView
}

And you may finally implement them for each platform, still supporting multiple form-factors and platform variants if needed.

The two common interfaces (InboxUiContract and ComposeUiContract) could be replaced by abstract classes in case you need to have backing fields, final declarations or final implementations, as long as they don't reference Splitties Ui interface and no platform specific code.

Testing

Having your user interface as an interface can make it easy to mock it, and simulate user interactions for testing purposes.

Redesign

If you expect an interface for the user interface, then it becomes easy to replace an implementation by another one in case you're redesigning your app. You can also split your UI contracts (the interfaces) into smaller subsets before starting a redesign if needed, this can be helpful if you want to move some UI controls to another area of the application, or just organize things differently.

A/B Testing

When you have multiple UI interfaces implementations, you can then swap them at runtime for A/B testing, allowing you to test which UI works the best for what you determined.

Additional modules

There are additional splits for extended support. Views DSL…

  • AppCompat provides proper styling to Button, TextView, EditText and other widgets. views like coloredFlatButton.
  • ConstraintLayout provides support for ConstraintLayout.LayoutParams. ViewGroups and bottom sheets.
  • IDE preview provides the ability to preview your user interfaces right from the IDE.
  • Material provides extensions for Material Components
  • RecyclerView provides extensions to have scrollbars and proper itemView layout parameters.

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