Pragmatic Kotlin on Android

presented at Android Summit: http://androidsummit.org/

In this talk Josh will share how Kotlin isn’t just syntactic sugar, it’s a better way to build Android apps. We will start with a modern java-based Android project and walk through a complete migration to Kotlin.In the process, you will see objective examples of how the Android concepts you're already familiar with are expressed in Kotlin more accurately, with less code, and with far fewer chances for bugs!

code example: http://github.com/mutexkid/stockwatcher

newIntent/newFragment pattern as extension:
https://github.com/mutexkid/kotlin-newintent-experiment/

Transcript

1. Pragmatic Kotlin on Android Josh Skeen | josh@bignerdranch.com | @mutexkid
2. Who am I? Android Developer & Instructor at Big Nerd Ranch Josh Skeen | josh@bignerdranch.com | @mutexkid
3. 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
4. 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
5. 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
6. 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
7. Running the Automatic Conversion RxFragment.java
8. 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(); }
9. RxFragment.java 1. Properties Aren’t Fields
10. 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
11. 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
12. Direct Access‽ What About Encapsulation‽ 1. Properties Aren’t Fields private fun <T> Observable<T>.applyRequestStatus(rxFragment: RxFragment): Observable<T> = doOnSubscribe { rxFragment.requestInProgress = true } .doOnTerminate { rxFragment.requestInProgress = false } RxUtil.kt:
13. Looking under the Hood… 1. Properties Aren’t Fields var requestInProgress = false
14. Looking under the Hood… 1. Properties Aren’t Fields
15. Looking under the Hood… var requestInProgress = false generates this… 1. Properties Aren’t Fields
16. 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
17. 2. Var and Val mean “writable” and “read-only”
18. 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”
19. 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”
20. Proof: val != immutable! private val notImmutable: Int get() { return (Math.random() * 10).toInt() } 2. Var and Val mean “writable” and “read-only”
21. 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”
22. Related: No Primitives? 2. Var and Val mean “writable” and “read-only”
23. Related: No Primitives? 2. Var and Val mean “writable” and “read-only” private boolean requestInProgress;
24. 3. No Static Keyword, Use File-Level or Object Instead
25. 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
26. 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
27. 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
28. 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
29. 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
30. 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
31. 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
32. 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
33. Related: Java Interop class Foo { companion object { val QAZ = "STATIC CLING" } } public static void main(String[] args) { Foo.Companion.getQAZ(); } 3. No Static Keyword
34. 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)
35. 4. Automatic Migration Makes Mistakes!
36. 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
37. 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
38. 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
39. 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
40. Cleaning up Auto Migration: Nullability public void onSaveInstanceState(Bundle outState) { } 4. Automatic Migration Makes Mistakes! “Nullability?” - nullability is often mistranslated Fragment.java
41. 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
42. 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
43. 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
44. 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)
45. 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
46. 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
47. Improve Upon Design w/ Kotlin Features
48. 1. Let/Apply to Drop Null Checks & Config Objects
49. 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
50. 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
51. 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
52. 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
53. 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
54. 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 }
55. 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) } }
56. 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
57. 2. Refactor to Extensions
58. Extensions: Quick Example fun String.addEnthusiasm() = this + "!!!!" fun main(args: Array<String>) { "Android Summit is Awesome".addEnthusiasm() } 2. Refactor to Extensions
59. 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
60. infix fun <T> 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”
61. 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
62. 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
63. 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
64. 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
65. 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
66. 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
67. 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
68. 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
69. 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
70. 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
71. 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
72. 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
73. 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
74. 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
75. 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
76. 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
77. Before and After: Call Site RxUtils.kt private fun <T> Observable<T>.showLoadingDialog(rxFragment: RxFragment): Observable<T> = doOnSubscribe { DialogUtils.showProgressDialog(rxFragment.fragmentManager, LOADING_MESSAGE) } .doOnTerminate { DialogUtils.hideProgressDialog(rxFragment.fragmentManager) } private fun <T> Observable<T>.showLoadingDialog(fragmentManager: FragmentManager): Observable<T> = doOnSubscribe { fragmentManager.showProgressDialog(LOADING_MESSAGE) } .doOnTerminate { fragmentManager.hideProgressDialog() } 2. Refactor to Extensions
78. 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
79. 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
80. 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
81. Thanks! Josh Skeen | josh@bignerdranch.com | @mutexkid