Pragmatic Kotlin on Android Josh Skeen | [email protected] | @mutexkid 

Who am I? Android Developer & Instructor at Big Nerd Ranch Josh Skeen | [email protected] | @mutexkid 

The Story So Far… • Started with a Legacy Java/Android Application (side-project) Code and slides: https://goo.gl/gmFMqS Code and slides: https://goo.gl/gmFMqS 

The Story So Far… • Started with a Legacy Java/Android Application (side-project) • Run Automatic Conversion to Kotlin Code and slides: https://goo.gl/gmFMqS 

The Story So Far… • Started with a Legacy Java/Android Application (side-project) • Run Automatic Conversion to Kotlin • Learn from What Changed Code and slides: https://goo.gl/gmFMqS 

The Story So Far… • Started with a Legacy Java/Android Application (side-project) • Run Automatic Conversion to Kotlin • Learn from What Changed • Improve upon Design w/ Kotlin Features Code and slides: https://goo.gl/gmFMqS 

Running the Automatic Conversion RxFragment.java 

Running the Automatic Conversion RxFragment.java RxFragment.kt abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } } public abstract class RxFragment extends Fragment { private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; private boolean requestInProgress; public boolean isRequestInProgress() { return requestInProgress; } public void setRequestInProgress(boolean requestInProgress) { this.requestInProgress = requestInProgress; } private CompositeDisposable compositeDisposable; @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); compositeDisposable = new CompositeDisposable(); if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false); } } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress); } @Override public void onResume() { super.onResume(); if (isRequestInProgress()) { loadRxData(); } } @Override public void onPause() { super.onPause(); compositeDisposable.clear(); } public abstract void loadRxData(); } 

RxFragment.java 1. Properties Aren’t Fields 

public abstract class RxFragment extends Fragment { private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; private boolean requestInProgress; public boolean isRequestInProgress() { return requestInProgress; } public void setRequestInProgress(boolean requestInProgress) { this.requestInProgress = requestInProgress; } private CompositeDisposable compositeDisposable; @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); compositeDisposable = new CompositeDisposable(); if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false); } } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress); } @Override public void onResume() { super.onResume(); if (isRequestInProgress()) { loadRxData(); } } @Override public void onPause() { super.onPause(); compositeDisposable.clear(); } public abstract void loadRxData(); } Before and After.. abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } } public abstract class RxFragment extends Fragment { private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; private boolean requestInProgress; public boolean isRequestInProgress() { return requestInProgress; } public void setRequestInProgress(boolean requestInProgress) { this.requestInProgress = requestInProgress; } private CompositeDisposable compositeDisposable; @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); compositeDisposable = new CompositeDisposable(); if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false); } } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress); } @Override public void onResume() { super.onResume(); if (isRequestInProgress()) { loadRxData(); } } @Override public void onPause() { abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } } 1. Properties Aren’t Fields 

No Getters & Setters‽ public abstract class RxFragment extends Fragment { private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROG private boolean requestInProgress; public boolean isRequestInProgress() { return requestInProgress; } public void setRequestInProgress(boolean requestInProgress) { this.requestInProgress = requestInProgress; } private CompositeDisposable compositeDisposable; @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); compositeDisposable = new CompositeDisposable(); if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRE } } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress); } abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = Comp override fun onCreate(savedInstanceState: Bundle?) super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getB 1. Properties Aren’t Fields 

Direct Access‽ What About Encapsulation‽ 1. Properties Aren’t Fields private fun Observable.applyRequestStatus(rxFragment: RxFragment): Observable = doOnSubscribe { rxFragment.requestInProgress = true } .doOnTerminate { rxFragment.requestInProgress = false } RxUtil.kt: 

Looking under the Hood… 1. Properties Aren’t Fields var requestInProgress = false 

Looking under the Hood… 1. Properties Aren’t Fields 

Looking under the Hood… var requestInProgress = false generates this… 1. Properties Aren’t Fields 

Looking under the Hood… private boolean requestInProgress; public final boolean getRequestInProgress() { return this.requestInProgress; } public final void setRequestInProgress(boolean var1) { this.requestInProgress = var1; } var requestInProgress = false generates this… 1. Properties Aren’t Fields 

2. Var and Val mean “writable” and “read-only” 

var/val syntax: var var requestInProgress = false private boolean requestInProgress; public final boolean getRequestInProgress() { return this.requestInProgress; } public final void setRequestInProgress(boolean var1) { this.requestInProgress = var1; } private boolean requestInProgress; public final boolean getRequestInProgress() { return this.requestInProgress; } public final void setRequestInProgress(boolean var1) { this.requestInProgress = var1; } var 2. Var and Val mean “writable” and “read-only” 

var/val syntax: val val requestInProgress = false private final boolean requestInProgress; public final boolean getRequestInProgress() { return this.requestInProgress; } public final void setRequestInProgress(boolean var1) { this.requestInProgress = var1; } Read-only, not immutable! val 2. Var and Val mean “writable” and “read-only” 

Proof: val != immutable! private val notImmutable: Int get() { return (Math.random() * 10).toInt() } 2. Var and Val mean “writable” and “read-only” 

Proof: val != immutable! private val notImmutable: Int get() { return (Math.random() * 10).toInt() } public final int getNotImmutable() { return (int)(Math.random() * (double)10); } 2. Var and Val mean “writable” and “read-only” 

Related: No Primitives? 2. Var and Val mean “writable” and “read-only” 

Related: No Primitives? 2. Var and Val mean “writable” and “read-only” private boolean requestInProgress; 

3. No Static Keyword, Use File-Level or Object Instead 

public abstract class RxFragment extends Fragment { private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; private boolean requestInProgress; public boolean isRequestInProgress() { return requestInProgress; } public void setRequestInProgress(boolean requestInProgress) { this.requestInProgress = requestInProgress; } private CompositeDisposable compositeDisposable; @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); compositeDisposable = new CompositeDisposable(); if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false); } } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress); } @Override public void onResume() { super.onResume(); if (isRequestInProgress()) { loadRxData(); } } @Override public void onPause() { super.onPause(); compositeDisposable.clear(); } public abstract void loadRxData(); } Before and After.. abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } } public abstract class RxFragment extends Fragment { private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; private boolean requestInProgress; public boolean isRequestInProgress() { return requestInProgress; } public void setRequestInProgress(boolean requestInProgress) { this.requestInProgress = requestInProgress; } private CompositeDisposable compositeDisposable; @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); compositeDisposable = new CompositeDisposable(); if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false); } } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress); } @Override public void onResume() { super.onResume(); if (isRequestInProgress()) { loadRxData(); } } @Override public void onPause() { super.onPause(); compositeDisposable.clear(); } public abstract void loadRxData(); } abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } } 3. No Static Keyword 

What Changed: Where’d static go?! private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } 3. No Static Keyword 

Object’s not java.lang.Object! private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } 3. No Static Keyword 

What Changed: Where’s static‽ private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } 3. No Static Keyword 

What Changed: Where’s static‽ private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } 3. No Static Keyword 

companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) compositeDisposable = CompositeDisposable() if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } What Changed: Where’s static‽ 3. No Static Keyword 

Looking under the Hood… companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) compositeDisposable = CompositeDisposable() if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } 3. No Static Keyword 

Looking under the Hood… private static final String EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS"; public static final class Companion { private final String getEXTRA_RX_REQUEST_IN_PROGRESS() { return RxFragment.EXTRA_RX_REQUEST_IN_PROGRESS; } private Companion() { } // $FF: synthetic method public Companion(DefaultConstructorMarker $constructor_marker) { this(); } } 3. No Static Keyword 

Related: Java Interop class Foo { companion object { val QAZ = "STATIC CLING" } } public static void main(String[] args) { Foo.Companion.getQAZ(); } 3. No Static Keyword 

Related: Java Interop class Foo { @JvmField companion object { val QAZ = "STATIC CLING" } } public static void main(String[] args) { println(Foo.Companion.getQAZ()); println(Foo.QAZ); } 3. No Static Keyword (don’t cling to static) 

4. Automatic Migration Makes Mistakes! 

Automatic Migration Mistakes private const val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" abstract class RxFragment : Fragment() { … companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } 4. Automatic Migration Makes Mistakes! - companion object often unneeded 

Automatic Migration Mistakes private const val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" abstract class RxFragment : Fragment() { … File level definition - totally normal in Kotlin! 4. Automatic Migration Makes Mistakes! - companion object often unneeded 

Clean Up: Use File Level Constants private const val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" abstract class RxFragment : Fragment() { … 4. Automatic Migration Makes Mistakes! - Use File Level Constant if possible 

Cleaning up Automatic Migration: Nullability override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } 4. Automatic Migration Makes Mistakes! - nullability is often mistranslated 

Cleaning up Auto Migration: Nullability public void onSaveInstanceState(Bundle outState) { } 4. Automatic Migration Makes Mistakes! “Nullability?” - nullability is often mistranslated Fragment.java 

Cleaning up Auto Migration: Nullability public void onSaveInstanceState(Bundle outState) { } Don’t see a @NonNullable annotation either… Assume it’s nullable. 4. Automatic Migration Makes Mistakes! - nullability is often mistranslated Fragment.java 

Cleaning up Auto Migration: Nullability override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } 4. Automatic Migration Makes Mistakes! - nullability is often mistranslated 

1. Properties Aren’t Fields 1. Manages field, a getter and a setter as needed 2. No direct access to creating Fields in Kotlin. 3. Even more powerful when used with Extensions! 4. No primitives, but translated to primitives in byte code 5. Override getter/setter operators for the property, don’t adds functions Takeaways 

Takeaways 1. Properties Aren’t Fields 2. Var and Val mean “readable/writable” and “read-only” 1. val != immutable! 2. For vals and vars, can’t specify primitives for type (but they are correctly “unboxed” in bytecode) 

1. Properties Aren’t Fields 2. Var and Val mean “writable” and “read-only” 3. No Static Keyword 1. ‘object’ (singleton instance) or file level values & functions now 2. Favor file-level values/functions unless ne 3. Properties and functions at the file level are 100% normal in Kotlin 4. Java Interop annotations support generating static fields that match Java style Takeaways 

1. Properties Aren’t Fields 2. Var and Val mean “writable” and “read-only” 3. No Static Keyword 4. Automatic Migration Makes Mistakes! 1. Often gets nullability wrong when generating Kotlin from Java, since anything goes in Java. Relies on @Null/@NonNull annotations to generate 2. Companion objects are often not needed 3. not quite idiomatic Kotlin…but, a good start! Takeaways 

Improve Upon Design w/ Kotlin Features 

1. Let/Apply to Drop Null Checks & Config Objects 

abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } } abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" } } Code Smells: != null Checks 1. Let/Apply to Drop Null Checks & Config Objects RxFragment.kt 

First thought: Make sure it actually needs to be nullable! abstract class RxFragment : Fragment() { var requestInProgress = false val compositeDisposable: CompositeDisposable = CompositeDisposable() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle?) { super.onSaveInstanceState(outState) outState!!.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } override fun onResume() { super.onResume() if (requestInProgress) { loadRxData() } } override fun onPause() { super.onPause() compositeDisposable.clear() } abstract fun loadRxData() companion object { private val EXTRA_RX_REQUEST_IN_PROGRESS = "EXTRA_RX_REQUEST_IN_PROGRESS" 1. Let/Apply to Drop Null Checks & Config Objects RxFragment.kt 

Fix: Use Let with Safe Call Operator override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) savedInstanceState?.let { requestInProgress = it.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } 1. Let/Apply to Drop Null Checks & Config Objects RxFragment.kt 

Code Smells: == null abstract class SingleFragmentActivity : AppCompatActivity() { protected abstract fun createFragment(): Fragment public override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState == null) { supportFragmentManager.beginTransaction() .add(R.id.fragment_container, createFragment()) .commit() } } } 1. Let/Apply to Drop Null Checks & Config Objects SingleFragmentActivity.kt 

Fix: Use Let with Null Coalesce Operator if (savedInstanceState == null) { supportFragmentManager.beginTransaction() .add(R.id.fragment_container, createFragment()) .commit() } savedInstanceState ?: let { supportFragmentManager.beginTransaction() .add(R.id.fragment_container, createFragment()) .commit() } 1. Let/Apply to Drop Null Checks & Config Objects SingleFragmentActivity.kt 

Smell: Init a Class and Configure lots of Methods on it ProgressDialog.kt (legacy) 1. Let/Apply to Drop Null Checks & Config Objects override fun onCreateDialog(savedInstanceState: Bundle?): Dialog { val progressDialog = ProgressDialog(context) progressDialog.setMessage(arguments.getString(ARG_MESSAGE)) progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER) return progressDialog } 

Fix: Use an Apply Block ProgressDialog.kt (legacy) 1. Let/Apply to Drop Null Checks & Config Objects override fun onCreateDialog(savedInstanceState: Bundle?): Dialog { val progressDialog = ProgressDialog(context) progressDialog.setMessage(arguments.getString(ARG_MESSAGE)) progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER) return progressDialog } override fun onCreateDialog(savedInstanceState: Bundle?): Dialog { return ProgressDialog(context).apply { setMessage(arguments.getString(ARG_MESSAGE)) setProgressStyle(ProgressDialog.STYLE_SPINNER) } } 

Cleaning up: Drop Unneeded Object object DialogUtils { private const val TAG_DIALOG_PROGRESS = "dialog_progress" internal fun showProgressDialog(fragmentManager: FragmentManager, message: String) { if (fragmentManager.findFragmentByTag(DialogUtils.TAG_DIALOG_PROGRESS) != null) { hideProgressDialog(fragmentManager) } val dialog = ProgressDialogFragment.newInstance(message) dialog.isCancelable = false dialog.show(fragmentManager, TAG_DIALOG_PROGRESS) } internal fun hideProgressDialog(fragmentManager: FragmentManager) { val fragment = fragmentManager.findFragmentByTag(TAG_DIALOG_PROGRESS) (fragment as? ProgressDialogFragment)?.dismissAllowingStateLoss() } } 1. Let/Apply to Drop Null Checks & Config Objects 

2. Refactor to Extensions 

Extensions: Quick Example fun String.addEnthusiasm() = this + "!!!!" fun main(args: Array) { "Android Summit is Awesome".addEnthusiasm() } 2. Refactor to Extensions 

More Involved Example: Building Lambda Elvis val mightBeNull: String? = null val definitelyNotNull: String = mightBeNull ?: "default" “If the thing on the left is null, do the thing on the right” 2. Refactor to Extensions 

infix fun T?.orDefault(default: T?.(T?) -> T) = 
 this ?: default() More Involved Example: Building Lambda Elvis 2. Refactor to Extensions “For all Types? Add a function called orDefault that accepts a lambda. The lambda must return a non-nullable version of the type. If the instance is null, call the lambda” 

More Involved Example: Building Lambda Elvis val displayName = cachedDisplayName orDefault { val formatted = formatName(user.first, user.last) cache.displayName = formatted formatted } 2. Refactor to Extensions 

Question: Where Will An Extension Help? 2. Refactor to Extensions var requestInProgress = false override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) if (savedInstanceState != null) { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } } override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) } RxFragment.kt 

Refactoring RxFragment with Extensions private var Bundle.requestInProgress: Boolean get() = getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) set(inProgress) = putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, inProgress) 2. Refactor to Extensions 

Refactoring RxFragment with Extensions private var Bundle.requestInProgress: Boolean get() = getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) set(inProgress) = putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, inProgress) 2. Refactor to Extensions 

Carrying the Refactor Forward: Let no longer Needed savedInstanceState?.let { requestInProgress = savedInstanceState.getBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, false) } requestInProgress = savedInstanceState?.requestInProgress ?: false onCreate: 2. Refactor to Extensions RxFragment.kt 

Carrying the Refactor Forward override fun onSaveInstanceState(outState: Bundle) { super.onSaveInstanceState(outState) outState.putBoolean(EXTRA_RX_REQUEST_IN_PROGRESS, requestInProgress) outState.requestInProgress = requestInProgress } onSaveInstanceState: 2. Refactor to Extensions RxFragment.kt 

private val TAG_DIALOG_PROGRESS = "dialog_progress" object DialogUtils { internal fun showProgressDialog(fragmentManager: FragmentManager, message: String) { fragmentManager.findFragmentByTag(TAG_DIALOG_PROGRESS)?.let { hideProgressDialog(fragmentManager) } ProgressDialogFragment.newInstance(message).apply { isCancelable = true show(fragmentManager, TAG_DIALOG_PROGRESS) } } internal fun hideProgressDialog(fragmentManager: FragmentManager) { val fragment = fragmentManager.findFragmentByTag(TAG_DIALOG_PROGRESS) if (fragment is ProgressDialogFragment) { fragment.dismissAllowingStateLoss() } } } 2. Refactor to Extensions DialogUtils.Kt Another Candidate for Refactoring To Extensions 

Another Candidate for Refactoring To Extensions private const val TAG_DIALOG_PROGRESS = “dialog_progress" private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment internal fun showProgressDialog(fragmentManager: FragmentManager, message: String) { if (fragmentManager.findFragmentByTag(DialogUtils.TAG_DIALOG_PROGRESS) != null) { hideProgressDialog(fragmentManager) } val dialog = ProgressDialogFragment.newInstance(message) dialog.isCancelable = false dialog.show(fragmentManager, TAG_DIALOG_PROGRESS) } internal fun hideProgressDialog(fragmentManager: FragmentManager) { val fragment = fragmentManager.findFragmentByTag(TAG_DIALOG_PROGRESS) (fragment as? ProgressDialogFragment)?.dismissAllowingStateLoss() } 2. Refactor to Extensions DialogUtils.kt 

Refactoring to an Extension Property private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun showProgressDialog(fragmentManager: FragmentManager, message: String) { if (fragmentManager.findFragmentByTag(DialogUtils.TAG_DIALOG_PROGRESS) != null) { hideProgressDialog(fragmentManager) } fragmentManager.progressDialog?.hideProgressDialog(fragmentManager) val dialog = ProgressDialogFragment.newInstance(message) dialog.isCancelable = false dialog.show(fragmentManager, TAG_DIALOG_PROGRESS) } 2. Refactor to Extensions DialogUtils.kt 

private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun showProgressDialog(fragmentManager: FragmentManager, message: String) { if (fragmentManager.findFragmentByTag(DialogUtils.TAG_DIALOG_PROGRESS) != null) { hideProgressDialog(fragmentManager) } fragmentManager.progressDialog?.hideProgressDialog(fragmentManager) val dialog = ProgressDialogFragment.newInstance(message) dialog.isCancelable = false dialog.show(fragmentManager, TAG_DIALOG_PROGRESS) ProgressDialogFragment.newInstance(message).apply { isCancelable = false show(fragmentManager, TAG_DIALOG_PROGRESS) } } 2. Refactor to Extensions Refactoring to an Extension Property DialogUtils.kt 

private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun hideProgressDialog(fragmentManager: FragmentManager) { val fragment = fragmentManager.findFragmentByTag(TAG_DIALOG_PROGRESS) (fragment as? ProgressDialogFragment)?.dismissAllowingStateLoss() fragmentManager.progressDialog?.dismissAllowingStateLoss() } 2. Refactor to Extensions Carrying the Refactor Forward DialogUtils.kt 

Carrying the Refactor Forward private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun hideProgressDialog(fragmentManager: FragmentManager) = fragmentManager.progressDialog?.dismissAllowingStateLoss() 2. Refactor to Extensions DialogUtils.kt 

Pushing the Extension Even Further… private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun hideProgressDialog(fragmentManager: FragmentManager) = fragmentManager.progressDialog?.dismissAllowingStateLoss() fun showProgressDialog(fragmentManager: FragmentManager, message: String) { fragmentManager.progressDialog?.let { hideProgressDialog(fragmentManager) } ProgressDialogFragment.newInstance(message).apply { isCancelable = false show(fragmentManager, TAG_DIALOG_PROGRESS) } } 2. Refactor to Extensions DialogUtils.kt 

Pushing the Extension Further… private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun hideProgressDialog(fragmentManager: FragmentManager) = fragmentManager.progressDialog?.dismissAllowingStateLoss() fun FragmentManager.hideProgressDialog() = progressDialog?.dismissAllowingStateLoss() DialogUtils.kt 2. Refactor to Extensions 

private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun FragmentManager.hideProgressDialog() = progressDialog?.dismissAllowingStateLoss() fun FragmentManager.showProgressDialog(message: String) { hideProgressDialog() ProgressDialogFragment.newInstance(message).apply { isCancelable = false }.show(this, TAG_DIALOG_PROGRESS) } Pushing the Extension Further… DialogUtils.kt 2. Refactor to Extensions 

Before and After DialogUtils.kt private val FragmentManager.progressDialog: ProgressDialogFragment? get() = findFragmentByTag(TAG_DIALOG_PROGRESS) as? ProgressDialogFragment fun FragmentManager.hideProgressDialog() = progressDialog?.dismissAllowingStateLoss() fun FragmentManager.showProgressDialog(message: String) { hideProgressDialog() ProgressDialogFragment.newInstance(message).apply { isCancelable = false }.show(this, TAG_DIALOG_PROGRESS) } object DialogUtils { internal fun showProgressDialog(fragmentManager: FragmentManager, message: String) { fragmentManager.findFragmentByTag(TAG_DIALOG_PROGRESS)?.let { hideProgressDialog(fragmentManager) } ProgressDialogFragment.newInstance(message).apply { isCancelable = true show(fragmentManager, TAG_DIALOG_PROGRESS) } } internal fun hideProgressDialog(fragmentManager: FragmentManager) { val fragment = fragmentManager.findFragmentByTag(TAG_DIALOG_PROGRESS) if (fragment is ProgressDialogFragment) { fragment.dismissAllowingStateLoss() } } } 2. Refactor to Extensions 

Before and After: Call Site RxUtils.kt private fun Observable.showLoadingDialog(rxFragment: RxFragment): Observable = doOnSubscribe { DialogUtils.showProgressDialog(rxFragment.fragmentManager, LOADING_MESSAGE) } .doOnTerminate { DialogUtils.hideProgressDialog(rxFragment.fragmentManager) } private fun Observable.showLoadingDialog(fragmentManager: FragmentManager): Observable = doOnSubscribe { fragmentManager.showProgressDialog(LOADING_MESSAGE) } .doOnTerminate { fragmentManager.hideProgressDialog() } 2. Refactor to Extensions 

1. Let/Apply to Drop Null Checks & Config Objects 1. Let with safe call operator (.?) is good for dropping a != null comparison 2. Let with null coalesce operator (?:) is good for dropping a == null comparison 3. Apply is good for configuring objects with many methods to call Takeaways 

1. Let/Apply to Drop Null Checks & Config Objects 2. Refactor to Extensions 1. Extensions give you “this” for free, allows you to drop an argument to a function commonly 2. Let with null coalesce operator is good for dropping a == null comparison 3. Apply is good for configuring objects with many methods to call Takeaways 

Resources •Kotlin track on Exercism: http://exercism.io/languages/kotlin/about •Kotlin Language Design https://discuss.kotlinlang.org/c/language-design •Kotlin koans: https://kotlinlang.org/docs/tutorials/koans.html •Big Nerd Ranch Kotlin course coming november 

Thanks! Josh Skeen | [email protected] | @mutexkid