Less painful Android development with Scala

Scaloid is a library that simplifies your Android code. It makes your code easy to understand and maintain by leveraging Scala language.

For example, the code block shown below:

val button = new Button(context)
button.setText("Greet")
button.setOnClickListener(new OnClickListener() {
  def onClick(v: View) {
    Toast.makeText(context, "Hello!", Toast.LENGTH_SHORT).show()
  }
})
layout.addView(button)

is reduced to:

SButton("Greet", toast("Hello!"))

Benefits

• Write elegant Android software
  Scaloid provides a concise and type-safe way of writing Android application.
• Simple to use
  Check our quick start guide
• Compatible with your legacy code
  You can use both Scaloid and plain-old Java Android API. You can gradually improve your legacy code.
• Maintained actively
  Scaloid is a dogfooding software. This is originally created to be used for my own Android apps.

Demos

• Hello world of Scaloid for maven
  Fork this to start a new project.

• Examples for sbt
  Fork this for sbt build.

• Scaloid port of apidemos app
  Learn how Scaloid can be used in action.

• List of projects using Scaloid

• FAQs

  • Why Scala rather than Xtend?
  • Why Scala rather than JRuby?
  • FAQs about Scala on Android

• Contributing

Features

• UI Layout without XML
  • Layout context
  • Styles for programmers
  • Automatic layout converter
• Lifecycle management
• Asynchronous task processing
• Implicit conversions
  • Context as an implicit parameter
  • Activity as an implicit parameter
  • Shorter listeners
• Traits
• Smarter logging
• Improved getters/setters
• Classes
  • Concise dialog builder
  • Beauty ArrayAdapter
  • Dynamically Preferences _{Read in blog}
  • Binding services concisely _{Read in blog}

...and many other things! Check the official Scaloid blog for news and announcements.

UI Layout without XML

Android SDK leverages XML to build UI layouts. However, XML is considered still a bit verbose, and lacks programmability. Scaloid composes UI layout in Scala DSL style, therefore achieve both clarity and programmability. For example, suppose a legacy XML layout as shown below:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical" android:layout_width="match_parent"
        android:layout_height="wrap_content" android:padding="20dip">
    <TextView android:layout_width="match_parent"
            android:layout_height="wrap_content" android:text="Sign in"
            android:layout_marginBottom="25dip" android:textSize="24.5sp"/>
    <TextView android:layout_width="match_parent"
            android:layout_height="wrap_content" android:text="ID"/>
    <EditText android:layout_width="match_parent"
            android:layout_height="wrap_content" android:id="@+id/userId"/>
    <TextView android:layout_width="match_parent"
            android:layout_height="wrap_content" android:text="Password"/>
    <EditText android:layout_width="match_parent"
            android:layout_height="wrap_content" android:id="@+id/password"
            android:inputType="textPassword"/>
    <Button android:layout_width="match_parent"
            android:layout_height="wrap_content" android:id="@+id/signin"
            android:text="Sign in"/>
    <LinearLayout android:orientation="horizontal"
            android:layout_width="match_parent"
            android:layout_height="wrap_content">
        <Button android:text="Help" android:id="@+id/help"
                android:layout_width="match_parent" android:layout_height="wrap_content"/>
        <Button android:text="Sign up" android:id="@+id/signup"
                android:layout_width="match_parent" android:layout_height="wrap_content"/>
    </LinearLayout>
</LinearLayout>

is reduced to:

new SVerticalLayout {
  STextView("Sign in").textSize(24.5 sp).<<.marginBottom(25 dip).>>
  STextView("ID")
  SEditText()
  STextView("Password")
  SEditText() inputType TEXT_PASSWORD
  SButton("Sign in")
  this += new SLinearLayout {
    SButton("Help")
    SButton("Sign up")
  }
}.padding(20 dip)

The layout description shown above is highly programmable. You can easily wire your logic into the layout:

new SVerticalLayout {
  STextView("Sign in").textSize(24.5 sp).<<.marginBottom(25 dip).>>
  STextView("ID")
  val userId = SEditText()
  STextView("Password")
  val pass = SEditText() inputType TEXT_PASSWORD
  SButton("Sign in", signin(userId.text, pass.text))
  this += new SLinearLayout {
    SButton("Help", openUri("http://help.url"))
    SButton("Sign up", openUri("http://signup.uri"))
  }
}.padding(20 dip)

Because a Scaloid layout description is plain Scala code, it is type-safe.

Automatic layout converter

This converter turns an Android XML layout into a Scaloid layout:

http://layout.scaloid.org

Migration tip

Scaloid is fully compatible with legacy xml layout files. You can access a widget described in xml layout as:

onCreate {
  setContentView(R.layout.main)
  val name = find[EditText](R.id.name)
  // do something with `name`
}

Further readings about Scaloid layout

These are highly recommended articles!

• Accessing widgets in view class
• Layout context
• In-depth tutorial on styles
• Styles for programmers

Lifecycle management

With Android API, Registering and unregistering BroadcastReceiver can be done as:

var connectivityListener: BroadcastReceiver = null

def onResume() {
  super.onResume()
  // ...
  connectivityListener = new BroadcastReceiver {
    def onReceive(context: Context, intent: Intent) {
     doSomething()
    }
  } 
  registerReceiver(connectivityListener, new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION))
}

def onPause() {
  unregisterReceiver(connectivityListener)
  // ...
  super.onPause()
}

In Scaloid, the directly equivalent code is:

broadcastReceiver(ConnectivityManager.CONNECTIVITY_ACTION) { (context, intent) =>
  doSomething()
}

Scaloid has highly flexible resource register/unregister management architecture. If this code is written in services, registering and unregistering is done in onCreate and onDestroy respectively. If the same code is in activities, registering and unregistering is done in onResume and onPause respectively. This is just a default behavior. You can override a preference that determine when the register/unregister preforms. Overriding it is simple as well:

broadcastReceiver(ConnectivityManager.CONNECTIVITY_ACTION) { (context, intent) =>
  doSomething()
}(this, onStartStop)

Then, the receiver is registered onStart, and unregistered onStop.

Further reading: Refer to a blog post for more details.

Asynchronous task processing

Android API provides runOnUiThread() only for class Activity. Scaloid provides a Scala version of runOnUiThread() for anywhere other than Activity.

Instead of:

activity.runOnUiThread {
  new Runnable() {
    def run() {
      debug("Running only in Activity class")
    }
  }
}

In Scaloid, use it like this:

runOnUiThread(debug("Running in any context"))

Running a job asynchronously and notifying the UI thread is a very frequently used pattern. Although Android API provides a helper class AsyncTask, implementing such a simple idea is still painful, even when we use Scala:

new AsyncTask[String, Void, String] {
  def doInBackground(params: Array[String]) = {
    doAJobTakeSomeTime(params)
  }

  override def onPostExecute(result: String) {
    alert("Done!", result)
  }
}.execute("param")

Using scala.concurrent.future, the asynchronous job shown above can be rewritten like this:

future {
  val result = doAJobTakeSomeTime(params)
  runOnUiThread(alert("Done!", result))
}

When you don't want to build sophisticate UI interactions, but just want to display something by calling a single Scaloid method (e.g. alert, toast, and spinnerDialog), Scaloid handles runOnUiThread for you. Therefore, the code block shown above is reduced to:

future {
  alert("Done!", doAJobTakeSomeTime(params))
}

It is a great win as it exposes your idea clearly.

Just like we thrown away AsyncTask, we can also eliminate all other Java helpers for asynchronous job, such as AsyncQueryHandler and AsyncTaskLoader. Compare with the original Java code and a Scala port of ApiDemos example app.

Using future is just an example of asynchronous task processing in Scaloid. You can freely use any modern task management utilities.

Implicit conversions

Scaloid employs several implicit conversions. Some of the available implicit conversions are shown below:

Uri conversion

String => Uri

The functions such as play ringtones play() or open URIs openUri() takes an instance of Uri as a parameter. However, we frequently have URIs as a String. Scaloid implicitly converts String into Uri. Therefore, you can freely use String when you play a ringtone:

play("content://media/internal/audio/media/50")

, open a URI:

openUri("http://scaloid.org")

, or wherever you want.

Resource IDs

Scaloid provides several implicit conversions that convert from Int type resource ID to CharSequence, Array[CharSequence], Array[String], Drawable and Movie. For example:

def toast(msg:CharSequence) = ...

toast(R.string.my_message) // implicit conversion works!

Although Scaloid provides these conversions implicitly, explicit conversion may be required in some context. In this case, methods r2... are provided for the Int type:

warn("Will display the content of the resource: " + R.string.my_message.r2String)

Currently, r2Text, r2TextArray, r2String, r2StringArray, r2Drawable and r2Movie is provided.

Why implicit conversion of Resource ID is cool?

Android API provides two versions of methods for string resources; One for CharSequence, the other for Int as a resource ID. If you write a function that handles Android resource, you also have to expose methods for every combination of two versions of resources:

def alert(titleId:Int, textId:Int)(implicit context:Context) = {
  alert(context.getText(titleId), context.getText(textId))
}

def alert(titleId:Int, text:CharSequence)(implicit context:Context) = {
  alert(context.getText(titleId), text)
}

def alert(title:CharSequence, textId:Int)(implicit context:Context) = {
  alert(title, context.getText(textId))
}

def alert(title:CharSequence, text:CharSequence) = ...

This is not a smart way. Write just one method that defines the logic:

def alert(title:CharSequence, text:CharSequence) = ...

Then Scaloid implicit conversions will take care about these resource type conversions.

Unit conversion

Units dip and sp can be converted into the pixel unit.

val inPixel:Int = (32 dip)
val inPixel2:Int = (22 sp)

Runnable

(_ => Any) => Runnable

Runnable also covered with rich and prefixed classes.

There are more implicit conversions available. Check the source code as needed.

IntentFilter

String can be converted into IntentFilter:

implicit string2IntentFilter(str: String) = new IntentFilter(str)

Context as an implicit parameter

Many methods in the Android API require an instance of a class Context. Providing this for every method call results in clumsy code. We employ an implicit parameter to eliminate this. Just declare an implicit value that represents current context:

implicit val ctx = ...

or just extend trait SContext, which defines it for you. Then the code that required Context becomes much simpler, for example:

Intent

new Intent(context, classOf[MyActivity])

is reduced to:

SIntent[MyActivity]

Toast

Toast.makeText(context, "hi, there!", Toast.LENGTH_SHORT).show()

is reduced to:

toast("hi, there!")

If you want a longer toast:

longToast("long toast")

Dialog

ProgressDialog.show(context, "Dialog", "working...", true)

is reduced to:

spinnerDialog("Dialog", "working...")

When you call toast, longToast or spinnerDialog from non-UI thread, you don't have to mind about threading. The toast example shown above is equivalent to the following Java code:

activity.runOnUiThread(new Runnable() {
    public void run() {
        Toast.makeText(activity, "hi, there!", Toast.LENGTH_SHORT).show();
    }
});

Pending intent

PendingIntent.getActivity(context, 0, new Intent(context, classOf[MyActivity]), 0)
PendingIntent.getService(context, 0, new Intent(context, classOf[MyService]), 0)

is reduced to:

pendingActivity[MyActivity]
pendingService[MyService]

DefaultSharedPreferences

PreferenceManager.getDefaultSharedPreferences(context)

is reduced to:

defaultSharedPreferences

Play ringtones

Just play the default notification ringtone:

play()

specify ringtone resources as a String:

play("content://media/internal/audio/media/50")

or specify a resource Uri:

play(alarmSound)

Open URIs

This opens a web browser (or another view assigned to the http protocol).

openUri("http://scaloid.org")

System services

Getting system service objects become much simpler. The following legacy code:

val vibrator = context.getSystemService(Context.VIBRATOR_SERVICE).asInstanceOf[Vibrator]
vibrator.vibrate(500)

is reduced to:

vibrator.vibrate(500)

Under the hood, Scaloid defines a function vibrator like this:

def vibrator(implicit ctx: Context) = ctx.getSystemService(Context.VIBRATOR_SERVICE).asInstanceOf[Vibrator]

All the system service accessors available in Android API level 8 are defined (e.g. audioManager, alarmManager, notificationManager, etc.). The name of a system service accessor is the same as its class name, except that the first character is lowercased.

Enriched Implicit classes

Suppose an Android class Foo, for example, Scaloid defines an implicit conversion Foo => RichFoo. The class RichFoo defines additional methods for more convenient access to Foo. This is a common pattern in Scala to extend existing API (see pimp-my-library pattern). This section describes various features added on existing Android API classes.

Listeners

Android API defines many listener interfaces for callback notifications. For example, View.OnClickListener is used to be notified when a view is clicked:

find[Button](R.id.search).setOnClickListener(new View.OnClickListener {
  def onClick(v:View) {
    openUri("http://scaloid.org")
  }
})

Scaloid provides a shortcut that dramatically reduces the length of the code:

find[Button](R.id.search).onClick(openUri("http://scaloid.org"))

All other listener-appending methods such as .onKey(), .onLongClick(), and .onTouch() are defined.

Some conventions we employed for method naming are:

• We omit set..., add..., and ...Listener from the method name, which is less significant.
  For example, .setOnKeyListener() becomes .onKey().
• Every method has two versions of parameters overridden. One is a lazy parameter, and another is a function which has full parameters defined in the original Android API. For example, these two usages are valid:

button.onClick(info("touched"))
button.onClick((v:View) => info("touched a button "+v))

• Methods add... is abbreviated with a method += if it is not a listener-appender.
  For example, layout.addView(button) becomes layout += button.

Multiple method listeners

Methods beforeTextChanged(), onTextChanged(), and afterTextChanged() are defined in RichTextView, which can be implicitly converted from TextView. It is more convenient than using TextWatcher directly. For example:

inputField.beforeTextChanged(saveTextStatus())

is equivalent to:

inputField.addTextChangedListener(new TextWatcher {
  def beforeTextChanged(s: CharSequence, start: Int, before: Int, count: Int) {
    saveTextStatus()
  }

  def onTextChanged(p1: CharSequence, p2: Int, p3: Int, p4: Int) {}

  def afterTextChanged(p1: Editable) {}
})

Also, we override beforeTextChanged() with full parameters defined in the original listener:

inputField.beforeTextChanged((s:CharSequence, _:Int, _:Int) => saveText(s))

Other listeners in Android API can also be accessed in this way.

Layout context

In Android API, layout information is stored into a View object via the method View.setLayoutParams(ViewGroup.LayoutParams). A specific type of parameter passing into that method is determined by a the type of ...Layout object which contains the View object. For example, let us see some Java code shown below:

LinearLayout layout = new LinearLayout(context);
Button button = new Button(context);
button.setText("Click");
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams();
params.weight = 1.0f;  // sets some value
button.setLayoutParams(params);
layout.addView(button);

Because the button is appended into the LinearLayout, the layout parameter must be LinearLayout.LayoutParams, otherwise a runtime error might be occurred. Meanwhile, Scaloid eliminate this burden, while still preserving rigorous typing of LayoutParams. The code shown below is equivalent to the previous Java code:

val layout = new SLinearLayout {
  SButton("Click").<<.Weight(1.0f).>>
}

In the anonymous constructor of 'SLinearLayout', Scaloid provides an implicit function called "layout context". This affects a return type of the method << defined in the class SButton. If we use SFrameLayout as a layout context, the method << returns FrameLayout.LayoutParams, which does not have Weight method. Therefore, the code below results a syntax error.

val layout = new SFrameLayout {
  SButton("Click").<<.Weight(1.0f).>>   // Syntax error on Weight()
}

Compared with XML layout description, Scaloid layout is simple and type-safe.

The method << is overloaded with parameters <<(width:Int, height:Int) which assigns the size of the view component. For example:

SButton("Click").<<(40 dip, WRAP_CONTENT)

Operator new and method apply

Usually, View components are referenced multiple times in an Activity. For example:

var button: SButton = null
override def onCreate(savedInstanceState: Bundle) {
  // ...
  new SLinearLayout {
    button = new SButton() text "Click"
    this += button
  }
  // ...
}
// ... uses the button somewhere in other methods (e.g. changing text or adding listeners)

Prefixed classes in Scaloid (e.g. SButton) have a companion object that implements apply methods that create a new component. These methods also append the component to the layout context that enclose the component. Therefore, the code block from the above example:

button = new SButton() text "Click"
this += button

is equivalent to:

button = SButton("Click")

Because the apply methods access to the layout context, it cannot be called outside of the layout context. In this case, use the new operator instead.

Method >>

As we noted, the method << returns an object which is a type of ViewGroup.LayoutParams:

val params = SButton("Click").<<   // type LayoutParams

This class provides some setters for chaining:

val params = SButton("Click").<<.marginBottom(100).marginLeft(10)   // type LayoutParams

if we want use the SButton object again, Scaloid provides >> method returning back to the object:

val button = SButton("Click").<<.marginBottom(100).marginLeft(10).>>   // type SButton

Nested layout context

When the layout context is nested, inner-most layout's context is applied:

val layout = new SFrameLayout {
  this += new SLinearLayout {
    SButton("Click").<<.Weight(1.0f).>>   // in context of SLinearLayout
  }
}

Methods fill and wrap

When we get a LayoutParams from <<, the default values of width and height properties are width = FILL_PARENT and height = WRAP_CONTENT. You can override this when you need it:

SButton("Click").<<(FILL_PARENT, FILL_PARENT)

This is a very frequently used idiom. Therefore we provide further shorthand:

SButton("Click").<<.fill

If you want the View element to be wrapped,

SButton("Click").<<(WRAP_CONTENT, WRAP_CONTENT)

This is also shortened as:

SButton("Click").<<.wrap

Styles for programmers

Naming conventions

Scaloid follows the naming conventions of XML attributes in the Android API with some improvements.

For XML attributes, layout related properties are prefixed with layout_ and as you might have guessed, Scaloid does not need it. For boolean attributes, the default is false. However, Scaloid flags it as true when the attribute is declared explicitly without any parameter. For example:

new SRelativeLayout {
  STextView("hello").<<.centerHorizontal.alignParentBottom.>>
}

Scaloid omits unnecessary ="true" for the attribute centerHorizontal. Equivalent XML layout description for TextView is:

<TextView
    android:id="@+id/helloText"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_alignParentBottom="true"
    android:text="hello"/>

For layout methods named with four directions (e.g. ...Top, ...Right, ...Bottom and ...Left), Scaloid provides additional methods that specifies all properties at once. For example, Because Android XML layout defines margin... properties(marginTop(v:Int), marginRight(v:Int), marginBottom(v:Int) and marginLeft(v:Int)), Scaloid provides additional margin(top:Int, right:Int, bottom:Int, left:Int) and margin(amount:Int) methods that can be used as:

STextView("hello").<<.margin(5 dip, 10 dip, 5 dip, 10 dip)

or

STextView("hello").<<.margin(10 sp)  // assigns the same value for all directions

Android SDK introduced styles to reuse common properties on XML layout. We repeatedly pointed out that XML is verbose. To apply styles in Scaloid, you do not need to learn any syntax or API library, because Scaloid layout is an ordinary Scala code. Just write a code that work as styles.

Basic: Assign it individually

Suppose the following code that repeats some properties:

SButton("first").textSize(20 dip).<<.margin(5 dip).>>
SButton("prev").textSize(20 dip).<<.margin(5 dip).>>
SButton("next").textSize(20 dip).<<.margin(5 dip).>>
SButton("last").textSize(20 dip).<<.margin(5 dip).>>

Then we can define a function that applies these properties:

def myStyle = (_: SButton).textSize(20 dip).<<.margin(5 dip).>>
myStyle(SButton("first"))
myStyle(SButton("prev"))
myStyle(SButton("next"))
myStyle(SButton("last"))

Still not satisfying? Here we have a shorter one:

def myStyle = (_: SButton).textSize(20 dip).<<.margin(5 dip).>>
List("first", "prev", "next", "last").foreach(title => myStyle(SButton(title)))

Advanced: CSS-like stylesheet

Scaloid provides SViewGroup.style(View => View) method to provide more generic component styling. The parameter is a function which receives a view requested for styling, and returns a view which is finished applying the style. Then the example in the previous subsection becomes:

style {
  case b: SButton => b.textSize(20 dip).<<.margin(5 dip).>>
}

SButton("first")
SButton("prev")
SButton("next")
SButton("last")

Note that individually applying myStyle is reduced. Let us see another example:

style {
  case b: SButton => b.textColor(Color.RED).onClick(toast("Bang!"))
  case t: STextView => t.textSize(10 dip)
  case v => v.backgroundColor(Color.YELLOW)
}

STextView("I am 10 dip tall")
STextView("Me too")
STextView("I am taller than you").textSize(15 dip) // overriding
SEditText("Yellow input field")
SButton("Red alert!")

Similar to CSS, you can assign different styles for each classes using Scala pattern matching. Unlike Android XML styles or even CSS, Scaloid can assign some actions to the component (see onclick(toast(...))), or can do anything that you imagine. Also, you can easily override the property individually, as shown in the example above.

Last thing that you may missed: These are type-safe. If you made a mistake, compiler will check it for you.

Further readings:

• Accessing widgets in view class
• In-depth tutorial on styles

Traits

Trait UnregisterReceiver

When you register BroadcastReceiver with Context.registerReceiver() you have to unregister it to prevent memory leak. Trait UnregisterReceiver handles these chores for you. All you need to do is append the trait to your class.

class MyService extends SService with UnregisterReceiver {
  def func() {
    // ...
    registerReceiver(receiver, intentFilter)
    // Done! automatically unregistered at UnregisterReceiverService.onDestroy()
  }
}

Trait SContext

Trait SContext includes several shortcuts for frequently used android idioms, and inherits TagUtil.

Starting and stopping service

startService(new Intent(context, classOf[MyService]))
stopService(new Intent(context, classOf[MyService]))

is reduced to:

startService[MyService]
stopService[MyService]

Starting activity

startActivity(new Intent(context, classOf[MyActivity]))

is reduced to:

startActivity[MyActivity]

Trait SActivity

Instead of

findViewById(R.id.login).asInstanceOf[Button]

use a shorthand:

find[Button](R.id.login)

Although we provide this shorthand, Scaloid recommends programmatically laying out UI, not with XML.

Activity as an implicit parameter

Similar to the implicit context, an Activity typed implicit parameter is also required for some methods. Therefore, you have to define an activity as an implicit value:

implicit val ctx: Activity = ...

Because the class Activity is a subclass of Context, it can also be an implicit context. When you extend SActivity, object this is assigned as the implicit activity by default.

Here we show some example cases of using the implicit activity:

Automatically allocate a unique View ID

Often, Views are required to have an ID value. Although Android API document specifies that the ID need not be unique, allocating unique ID is virtually mandatory in practice. Scaloid provides a package scope function getUniqueId, which returns Int type ID that is not allocated by any existing View components for given implicit activity.

val newUniqueIdForCurrentActivity = getUniqueId

Using this, Scaloid also extended View class to add a method uniqueId, that assigns a new unique ID if it is not already allocated.

val uniqueIdOfMyView = myView.uniqueId

One of the good use case of uniqueId is SRelativeLayout. Some of the methods in this layout context, such as below, above, leftOf and rightOf, takes another View object as an anchor:

new SRelativeLayout {
  val btn = SButton("Hi")
  SButton("There").<<.below(btn)
}

Here we show the implementation of the below function:

def below(anchor: View)(implicit activity: Activity) = {
  addRule(RelativeLayout.BELOW, anchor.uniqueId)
  this
}

A new unique ID is assigned to the anchor if it is not assigned already, and passes it to addRule function.

Logging

Unlike other logging frameworks, Android Logging API requires a String tag for every log call. We eliminate this by introducing an implicit parameter. Define an implicit value type of LoggerTag as shown:

implicit val tag = LoggerTag("MyAppTag")

or, extend trait TagUtil or SContext which defines the tag by default. Then you can simply log like this:

warn("Something happened!")

Other functions for every log level (verbose(), debug(), info(), warn(), error() and wtf()) are available.

info("hello " + world)

A String parameter passed with info() is a by-name parameter, so it is evaluated only if the logging is possible. Therefore, the example shown above is equivalent to:

val tag = "MyAppTag"
if(Log.isLoggable(tag, Log.INFO)) Log.i(tag, "hello " + world)

Scala getters and setters

You can use any of the setters listed below:

• obj.setText("Hello") Java bean style
• obj.text = "Hello" Assignment style
• obj text "Hello" DSL style
• obj.text("Hello") Method calling style

Compared to Java style getters and setters, for example:

new TextView(context) {
  setText("Hello")
  setTextSize(15)
}

that of Scala style clearly reveals the nature of the operations as shown below:

new STextView {
  text = "Hello"
  textSize = 15
}

Or, you can also chain the setters:

new STextView text "Hello" textSize 15

which is a syntactic sugar for:

new STextView.text("Hello").textSize(15)

We recommend "assignment style" and "DSL style". Use assignment style when you emphasize that you are assigning something, or use DSL style when the code length of the assignee is short and needs to be chained.

Note: Using .apply(String) method on object STextView, you can further reduce the code above like this:

STextView("Hello") textSize 15

Return value of setters

Unlike most setters in the Android API, our setters return the object itself. This feature can be used as a syntactic sugar when setters need to be chained or a function returning some object. For example, a snippet of Java code from ApiDemos that is shown below:

public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
  TextView textView = getGenericView();
  textView.setText(getGroup(groupPosition).toString());
  return textView;
}

is reduced to:

def getGroupView(groupPosition: Int, isExpanded: Boolean, convertView: View, parent: ViewGroup): View =
  getGenericView.text = getGroup(groupPosition).toString

Design considerations on returning values: In C or Java, the assignment operator = returns a right hand side object. However, chaining assignment operator is very rarely used in these languages. Assigning the same value to multiple variables might means that your code is badly designed (except some context such as involving intensive mathematical computations). However, in Scala DSLs, setters return a left hand side object, and chaining setters are more frequent. For example:

getGenericView text "hello" maxHeight 8

Prefixed classes

If you want to use scala style getters/setters, implicit conversions do the magic on native Android objects:

val v: TextView = ...
v.text = "Hello"    // Valid code. Implicit conversion handles this.

However, if you use it in constructors, the compiler will not find the correct implicit conversion:

def getInstance = new TextView(context) {
  text = "Hello"    // Compilation Error.
}

Therefore, we extended Android classes with the same name prefixed with the 'S' character:

def getInstance = new STextView {
  text = "Hello"    // OK.
}

These classes explicitly provide the extra methods that was provided implicitly.

Aditionally, prefixed classes support implicit context value and additional syntactic sugar. For example, many classes have .apply(...) methods for creating a new instance:

STextView("Hello")
SButton("title", onClickBehavior())
SIntent[MyActivity]

Design considerations on making prefixed classes: In modern programming language, using packages (or namespaces) are preferred than prefixing. However, when we use both classes from Android API and Scaloid, using a package name is more verbose than prefixing the class name itself (compare with common.Button and SButton) and can be confused when you use both classes at the same code. We choose pragmatism rather than discipline.

Sweet-little sugar

If the setter ends with ...Enabled, Scaloid adds functions named enable... and disable.... For example:

new SLinearLayout().disableVerticalScrollBar

is equivalent to:

new SLinearLayout().verticalScrollBarEnabled(false)

Because setting the property orientation = VERTICAL for SLinearLayout is frequently used, we provide a shorthand:

new SVerticalLayout()

that is equivalent to:

new SLinearLayout().orientation(LinearLayout.VERTICAL)

Classes

Class AlertDialogBuilder

A Scala-style builder for AlertDialog.

new AlertDialogBuilder(R.string.title, R.string.message) {
  neutralButton()
}.show()

This displays an alert dialog with given string resources. We provide an equivalent shortcut:

alert(R.string.title, R.string.message)

Also you can build a more complex dialog:

new AlertDialogBuilder("Exit the app", "Do you really want to exit?") {
  positiveButton("Exit", finishTheApplication())
  negativeButton("Cancel")
}.show()

The code above is equivalent to:

new AlertDialog.Builder(context)
  .setTitle("Exit the app")
  .setMessage("Do you really want to exit?")
  .setPositiveButton("Exit", new DialogInterface.OnClickListener {
    def onClick(dialog: DialogInterface, which: Int) {
      finishTheApplication()
    }
  })
  .setNegativeButton("Cancel", new DialogInterface.OnClickListener {
    def onClick(dialog: DialogInterface, which: Int) {
      dialog.cancel()
    }
  }).show()

When you call show() or alert from non-UI thread, you don't have to mind about threading.

Class SArrayAdapter

Suppose you want to let the user selects a string from spinner, and larger font should be displayed in the dropdown list. Then the plain-old Android code is consisted of a chunk of XML and its wiring:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    style="?android:attr/spinnerDropDownItemStyle"
    android:id="@+id/spinner_textview"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
	android:textSize="25 dip" />

val adapter = new ArrayAdapter(context, android.R.layout.simple_spinner_item, Array("One", "Two", "Three"))
adapter.setDropDownViewResource(R.layout.spinner_dropdown)

In Scaloid, a directly equivalent code is:

SArrayAdapter("One", "Two", "Three").dropDownStyle(_.textSize(25 dip))

If you want to let the text color in the spinner be blue, use the style method:

SArrayAdapter("Quick", "Brown", "Fox").style(_.textColor(Color.BLUE))

Can it be simpler?

Class LocalService

Android Developer Guide on service binding says that we have to write more than 60 lines of code to define and bind an in-process service. With Scaloid, you can concisely define and access local service as shown below:

class MyService extends LocalService {
  private val generator = new Random()

  def getRandomNumber() = generator.nextInt(100)
}

class Activity extends SActivity {
  val random = new LocalServiceConnection[MyService]
 
  def onButtonClick(v:View) {
    if(random.connected) toast("number: " + random.service.getRandomNumber())
  }
}

Further reading: Refer to a blog post to see why this is awesome in compared with the existing method.

Class Preferences

Instead of accesing SharedPreference directly:

val ec = pref.getInt("executionCount", 0)
val editor = pref.edit()
editor.putInt("executionCount", ec + 1)
editor.commit()

You can rewrite it as shown below:

val ec = pref.executionCount(0)
pref.executionCount = ec + 1

Further reading:

• In-depth look at Scaloid Preferences
• A simple example: Prompt user to rate your app

Extending View class

Often we need to define a custom view widget for a specific requirement. To do this, we define a class that inherits android.widget.View class or its subclass (e.g. TextView and Button). To enable Scaloid extensions for this custom widget, you can define a class as follows:

class MyView(implicit ctx: Context) extends View(ctx) with TraitView[MyView] {
  def basis = this
  
  // custom code for MyView here
}

Static fields on protected interfaces

Android API has some protected interfaces which has static fields, and inherited it in public classes. For example android.provider.ContactsContract.Contacts inherits a protected interface android.provider.ContactsContract.ContactsColumns, which defines a static field ContactsColumns.DISPLAY_NAME. In Java code, you can access to it with Contacts.DISPLAY_NAME. However, Scala does not support accessing in this way (please refer this and this). It is bad news for an Android-Scala programmer. So we provide a workaround implementation for this problem. Declare import org.scaloid.Workarounds._. Then you can use the interfaces publicly which is originally defined as protected.

Quick start

Starting a new project

For maven:

Fork Hello world of Scaloid project.

For sbt:

Fork Scaloid examples for sbt

Add Scaloid into your existing project

Before start using Scaloid, check your project is properly configured with Scala language.

1. Import Scaloid to your project
2. Declare import org.scaloid.common._ in your code.
3. Modify the signature of your classes

• If your class inherits Activity, change it to SActivity
• If your class inherits Service, change it to SService
• If your class (indirectly) inherits Context, add trait SContext with LoggerTag
• Otherwise, setting an implicit value is required
  implicit val ctx: Context = ...

Then, you are ready to use Scaloid.

If you want to see how Scaloid can be used in action, check a Scala port of apidemos app.

Import Scaloid to your project

Scaloid is released to the central maven repository.

For maven:

<dependency>
    <groupId>org.scaloid</groupId>
    <artifactId>scaloid</artifactId>
    <version>1.1_8_2.10</version>
</dependency>

For sbt:

libraryDependencies += "org.scaloid" % "scaloid" % "1.1_8_2.10"

Version number

Version number of Scaloid is consisted of three parts, separated by _ characters. The first part is the version of Scaloid, the second is the level of Android API, and the last one is the version of Scala.

Please note that Android API provides backward compatibility. Therefore you can use a Scaloid artifact targeted to API level 8 for the Android application using the level 8 or above. In other side, Scala does not provide binary compatibility. The Scaloid artifact uploaded to the central repo is compiled with Scala 2.10. If you use other Scala versions, build the artifact as shown in the following subsection.

Build the source

1. Clone the git repository
2. If needed, change version of Android API or Scala in pom.xml
3. Issue mvn package

• Scaloid can be built with Android API level 8 or higher and Scala version 2.10.0 or higher.

FAQs

Why Scala rather than Xtend?

Xtend natively supports 1) converting Java bean style getter/setters into the assignment style 2) automatically shorten the clutters when calling one-method callback interface by converting it into closure-like style. Because these are language features, Xtend users can enjoy these features without any wrapper library. We hope that Scala also adopt these benefits soon.

However, We think that Scala is a better alternative for Android platform, because Scala is mature than Xtend and has these advanced features:

• Implicit conversion
  Check implicit conversions section to see how Scaloid leverage it.

• Implicit parameter
  Hiding some common values (e.g. context object, tag for logging, and layout context) from the parameter list is possible by the Scala's implicit parameters.

• Advanced type system
  Scaloid layout parameter is simple, intuitive, and type-safe.

• Traits
  Many useful features such as automatic unregistering receivers are implemented as traits which permits multiple inheritance.

Why Scala rather than JRuby?

• Type-safety

• Runtime performance (and your precious battery)
  See a benchmark

FAQs about Scala on Android

Because programming in Scala on Android is not a widely known practice yet, many people asks me basic questions about it. Here are some frequently asked questions:

How big is the compiled apk?

For Scala + Android projects, using proguard is mandatory. After the library is reduced by proguard, overhead caused by the Scala standard library is about several hundred kilobytes, although it depends on how much you used the library in your code.

How much slow the application?

According to a benchmark, runtime performance of Scala is a little worse than that of Java. However, because most of the code using Scaloid is writing UI and core logic, these performance difference is nearly not noticeable. Still, the display will consume most of the battery life, not Scala.

How much slow the compilation?

Compiling Scala source code and applying proguard takes some time. However, if you have a machine with a multi-core CPU and SSD, it would be a matter of few seconds.

Is it hard to setup a Scala + Android project?

It's not hard. There is an maven project template or sbt project template.

Let's make it together!

This project is in its early stages, and I will grow it constantly. If you have any idea to improve Scaloid, feel free to open issues or post patches.

Check the official Scaloid blog for news and announcements.

Pull Requests

• Create topic branches. Don't ask us to pull from your master branch.

• One pull request per feature. If you want to do more than one thing, send multiple pull requests.

• Document any change in behaviour. Make sure the README and any other relevant documentation are kept up-to-date.

• Send coherent history. Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please squash them before sending them to us.

Sub projects of Scaloid

• Scaloid common package - This project
• Support-v4 package - Scaloid port of support-v4 compatibility library
• Scaloid-stamper - A wrapper generator

License

This software is licensed under the Apache 2 license.

List of projects using Scaloid

• Soundcorset metronome & tuner

Share your experience of using Scaloid by blogging about it and let me know the URL of the post and the name of your Android application via [email protected]. Then I will add a link to your post here.

Roadmap

• Cover full Android API versions
  Currently, only API level 8 is supported. Scaloid may not be compiled with below that API level, and new features introduced above that level are not covered. Some of the features to be covered are:

  1. Fragment
  2. New system services
  3. Action bar

• Build an example Android app
  Finish a Scala port of apidemos app and try another.

• Build a dedicated website

• Write a complete API document

• Write the Beginner's guide

• Build an example of laying out multiple UI

• Write a converter that turns an XML layout into a Scaloid code
  A simple web application is demonstrated. Providing this functionality as an Eclipse or Intellij plugin would also be great.

• WYSIWYG layout builder

• Cover full listener shortcuts

• Cover OpenGL ES and renderscript

• Automatically unregister SensorEventListener onStop()

• Support Google services
  Including Google Cloud Messaging (GCM)

• iOS?