Handsome Codes with Kotlin

Presented in GDG DevFest Istanbul 2017, GDG DevFest Izmir 2017

Kotlin is a very flexible programming language. You have a chance to write more readable and good-looking code. The only limit is your imagination and creativity. I'll show you some examples which are really handsome :)

- Extension Functions
- Higher-order Functions
- Operator Overloading
- Infix Notations
- Delegate Properties
- Reified Types
- Semantic Validations
- Standard.kt (let, apply, also, with...)
- DSLs
- Elegant DiffUtil
- Android Runtime Permissions in Kotlin way

Transcript

1. Handsome Codes with Kotlin Semih Bozdemir Android Developer @ Monitise MEA
2. Handsome Codes with Kotlin Semih Bozdemir Android Developer @ Monitise MEA Sexy
3. #dfist #Devfest17 What will I tell? Some tricks and features to write more readable and good-looking code in Kotlin
4. #dfist #Devfest17 Extension Functions
5. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG if (shouldVisible) { view.setVisibility(View.VISIBLE); } else { view.setVisibility(View.GONE); } Java
6. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG view.setVisible(shouldVisible) Kotlin
7. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG fun View.setVisible(visible: Boolean) { visibility = if (visible) View.VISIBLE else View.GONE } Views.kt Kotlin
8. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG fun View.setVisible(visible: Boolean, falseVisibility: Int = View.GONE) { visibility = if (visible) View.VISIBLE else falseVisibility } Views.kt Kotlin
9. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG if (view.getVisibility() == View.VISIBLE) { // do stuff } Java
10. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG if (view.isVisible()) { // do stuff } Kotlin
11. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG fun View.isVisible() = visibility == View.VISIBLE Views.kt Kotlin
12. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG textView.setTextColor(ContextCompat.getColor(context, R.color.blue)); Java
13. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG textView.setTextColor(colorRes(R.color.blue)) Kotlin
14. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG fun Context.colorRes(@ColorRes colorRes: Int): Int { return ContextCompat.getColor(this, colorRes) } Contexts.kt Kotlin
15. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG Intent launchIntent = getPackageManager().getLaunchIntentForPackage(packageName); if (launchIntent == null) { String playStoreUrl = "http://play.google.com/store/apps/details?id=" + packageName; launchIntent = new Intent(Intent.ACTION_VIEW, Uri.parse(playStoreUrl)); } launchIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK); startActivity(launchIntent); Java
16. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG launchApp("com.instagram.android") Kotlin
17. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG fun Context.launchApp(packageName: String) { val launchIntent = packageManager.getLaunchIntentForPackage(packageName) ?: Intent( Intent.ACTION_VIEW, Uri.parse("http://play.google.com/store/apps/details?id=$packageName") ) launchIntent.flags = Intent.FLAG_ACTIVITY_NEW_TASK startActivity(launchIntent) } Intents.kt Kotlin
18. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG Picasso.with(context).load(imageUrl).into(imageView); Java
19. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG imageView.setImageUrl(imageUrl) Kotlin
20. #dfist #Devfest17 Extension Functions ENTER FILENAME/LANG fun ImageView.setImageUrl(url: String) { Picasso.with(context).load(url).into(this) } ImageViews.kt Kotlin
21. #dfist #Devfest17 Higher Order Functions
22. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG Picasso.with(context) .load(imageUrl) .placeholder(R.drawable.ic_placeholder) .into(imageView); Java
23. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG imageView.setImageUrl(imageUrl) { placeholder(R.drawable.ic_placeholder) } Kotlin
24. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG fun ImageView.setImageUrl( url: String, init: (RequestCreator.() -> RequestCreator)? = null) { val requestCreator = Picasso.with(context).load(url) if (init != null) requestCreator.init() requestCreator.into(this) } ImageViews.kt Kotlin
25. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG SharedPreferences.Editor editor = prefs.edit(); editor.putInt(KEY_ID, 1); editor.putString(KEY_FOO, "bar"); editor.apply(); Java
26. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG prefs.edit { putInt(KEY_ID, 1) putString(KEY_FOO, "bar") } Kotlin
27. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG prefs.edit { put(KEY_ID to 1) put(KEY_FOO to "bar") } Kotlin
28. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG inline fun SharedPreferences.edit( body: SharedPreferences.Editor.() -> Unit) { val editor = edit() editor.body() editor.apply() } SharedPrefs.kt Kotlin
29. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG fun SharedPreferences.Editor.put(pair: Pair<String, Any>) { val key = pair.first val value = pair.second when(value) { is String -> putString(key, value) is Int -> putInt(key, value) is Boolean -> putBoolean(key, value) is Long -> putLong(key, value) is Float -> putFloat(key, value) else -> error("Only primitive types can be stored with put() function.") } } SharedPrefs.kt Kotlin
30. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG Snackbar.make(rootView, "Hello!", Snackbar.LENGTH_LONG) .setAction("Bye!", new View.OnClickListener() { @Override public void onClick(View view) { sayBye(); } }) .show(); Java
31. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG snackbar("Hello!") { action("Bye!") { sayBye() } } Kotlin
32. #dfist #Devfest17 Higher-order Functions ENTER FILENAME/LANG fun Activity.snackbar(message: CharSequence, view: View = findViewById(android.R.id.content), duration: Int = Snackbar.LENGTH_LONG, init: (Snackbar.() -> Unit)? = null) { val snackbar = Snackbar.make(view, message, duration) with(snackbar) { init?.invoke(snackbar) show() } } Snackbars.kt Kotlin
33. #dfist #Devfest17 Operator Overloading
34. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG View childView = viewGroup.getChildAt(i); Java
35. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG val childView = viewGroup[i] Kotlin
36. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG operator fun ViewGroup.get(i: Int): View = getChildAt(i) Views.kt Kotlin
37. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG if (viewGroup.indexOfChild(view) != -1) { doSomething(view); } Java
38. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG if (view in viewGroup) { doSomething(view) } Kotlin
39. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG operator fun ViewGroup.contains(view: View): Boolean { return indexOfChild(view) != -1 } Views.kt Kotlin
40. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG viewGroup.addView(view); Java
41. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG viewGroup += view Kotlin
42. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG viewGroup.addView(view); viewGroup.removeView(view); Java
43. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG viewGroup += view viewGroup -= view Kotlin
44. #dfist #Devfest17 Operator Overloading ENTER FILENAME/LANG operator fun ViewGroup.plusAssign(view: View) = addView(view) operator fun ViewGroup.minusAssign(view: View) = removeView(view) Views.kt Kotlin
45. #dfist #Devfest17 Infix Notations
46. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG if (checked && visible) { doSomething(); } Java
47. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG if (checked and visible) { doSomething() } Kotlin
48. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG if (checked || visible) { doSomething(); } Java
49. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG if (checked or visible) { doSomething() } Kotlin
50. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG public infix fun and(other: Boolean): Boolean public infix fun or(other: Boolean): Boolean public infix fun xor(other: Boolean): Boolean Boolean.kt Kotlin
51. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG when(foo()).thenReturn("bar"); Java
52. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG When calling foo() itReturns "bar" Kotlin
53. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG prefs.edit { put(KEY_ID to 1) put(KEY_FOO to "bar") } Kotlin
54. #dfist #Devfest17 Infix Notations ENTER FILENAME/LANG public infix fun <A, B> A.to(that: B): Pair<A, B> = Pair(this, that) Kotlin
55. #dfist #Devfest17 Delegate Properties
56. #dfist #Devfest17 Delegate Properties ENTER FILENAME/LANG val foo by lazy { intent.getStringExtra("foo") } Kotlin
57. #dfist #Devfest17 Delegate Properties ENTER FILENAME/LANG var items: MutableList<String> by Delegates.observable(mutableListOf()) { p, old, new -> adapter.notifyDataSetChanged() } val adapter = RecyclerViewAdapter(items) // somewhere in code items = newItems Kotlin
58. #dfist #Devfest17 Delegate Properties ENTER FILENAME/LANG SharedPreferences prefs = getSharedPreferences("prefs", Context.MODE_PRIVATE); boolean isToolTipShown = prefs.getBoolean("isToolTipShown", false); if (!isToolTipShown) { showToolTip(); prefs.edit().putBoolean("isToolTipShown", true).apply(); } Java
59. #dfist #Devfest17 Delegate Properties ENTER FILENAME/LANG var isToolTipShown by sharedPref("isToolTipShown", false) //... if (!isToolTipShown) { showToolTip() isToolTipShown = true } Kotlin
60. #dfist #Devfest17 Delegate Properties ENTER FILENAME/LANG class SharedPrefDelegate<T>(private val context: Context, private val key: String, private val defaultValue: T, private val fileName: String, private val mode: Int) { private val prefs: SharedPreferences by lazy { context.getSharedPreferences(fileName, mode) } operator fun getValue(thisRef: Any?, property: KProperty<*>): T { return prefs.get(key, defaultValue) } operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) { prefs.edit { put(key to value) } } } SharedPrefs.kt Kotlin
61. #dfist #Devfest17 Delegate Properties ENTER FILENAME/LANG fun <T> Context.sharedPref(key: String, defaultValue: T, fileName: String = "prefs", mode: Int = Context.MODE_PRIVATE): SharedPrefDelegate<T> { return SharedPrefDelegate(this, key, defaultValue, fileName, mode) } SharedPrefs.kt Kotlin
62. #dfist #Devfest17 Reified Types
63. #dfist #Devfest17 Reified Types ENTER FILENAME/LANG Intent intent = new Intent(this, PostActivity.class); startActivity(intent); Java
64. #dfist #Devfest17 Reified Types ENTER FILENAME/LANG start<PostActivity>() Kotlin
65. #dfist #Devfest17 Reified Types ENTER FILENAME/LANG inline fun <reified T : Activity> Context.start() { startActivity(Intent(this, T::class.java)) } Intents.kt Kotlin
66. #dfist #Devfest17 Reified Types ENTER FILENAME/LANG Intent intent = new Intent(this, PostActivity.class); intent.putExtra("fooString", "bar"); intent.putExtra("fooInt", 2); startActivity(intent); Java
67. #dfist #Devfest17 Reified Types ENTER FILENAME/LANG start<PostActivity>( "fooString" to "bar", "fooInt" to 2 ) Kotlin
68. #dfist #Devfest17 Reified Types ENTER FILENAME/LANG inline fun <reified T : Activity> Context.start(vararg params: Pair<String, Any>) { val intent = Intent(this, T::class.java).apply { params.forEach { val value = it.second when (value) { is Int -> putExtra(it.first, value) is String -> putExtra(it.first, value) //... else -> throw IllegalArgumentException("Wrong param type!") } return@forEach } } startActivity(intent) } Intents.kt Kotlin
69. #dfist #Devfest17 Semantic Validations
70. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG public <T> T getFifthElement(T[] array) { if (array == null || array.length < 5) { throw new IllegalArgumentException( "array should be non-null and have at least 5" ); } return array[4]; } Java
71. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG fun <T> getFifthElement(array: Array<T>): T { require(array.size >= 5) { "array should have at least five elements, " + "but has ${array.size}!" } return array[4] } Kotlin
72. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG inline fun require(value: Boolean, lazyMessage: () -> Any): Unit { if (!value) { val message = lazyMessage() throw IllegalArgumentException(message.toString()) } } Preconditions.kt Kotlin
73. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG public void doSomething() { if (!isInitialized()) { throw new IllegalStateException( "Helper is not initialized!" ); } // do something } Java
74. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG fun doSomething() { check(!isInitialized()) { "Helper is not initialized" } // do something } Kotlin
75. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG inline fun check(value: Boolean, lazyMessage: () -> Any): Unit { if (!value) { val message = lazyMessage() throw IllegalStateException(message.toString()) } } Preconditions.kt Kotlin
76. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG fun doSomething(param: Int?) { requireNotNull(param) { "Param must not be null" } // do something } Kotlin
77. #dfist #Devfest17 Semantic Validations ENTER FILENAME/LANG inline fun <T:Any> requireNotNull(value: T?, lazyMessage: () -> Any): T { if (value == null) { val message = lazyMessage() throw IllegalArgumentException(message.toString()) } else { return value } } Preconditions.kt Kotlin
78. #dfist #Devfest17 let, apply, also, with... Standard.kt
79. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG if (foo != null) { foo.doSomething(); foo.doSomethingDifferent(); } Java
80. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG foo?.let { it.doSomething() it.doSomethingDifferent() } Kotlin
81. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG inline fun <T, R> T.let(block: (T) -> R): R = block(this) Standard.kt Kotlin
82. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG val foo = getFoo().apply { doSomething() doSomethingDifferent() } foo.doSomething() Kotlin
83. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG inline fun <T> T.apply(block: T.() -> Unit): T { block() return this } Standard.kt Kotlin
84. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG val foo = getFoo().also { doSomethingWithFoo(it) } foo.doSomethingDifferent() Kotlin
85. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG inline fun <T> T.also(block: (T) -> Unit): T { block(this) return this } Standard.kt Kotlin
86. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG with(foo) { doSomething() doSomethingDifferent() } Kotlin
87. #dfist #Devfest17 Standard.kt ENTER FILENAME/LANG inline fun <T, R> with(receiver: T, block: T.() -> R): R { return receiver.block() } Standard.kt Kotlin
88. #dfist #Devfest17 More tricks?
89. #dfist #Devfest17 DSLs
90. #dfist #Devfest17 DSLs ENTER FILENAME/LANG Picasso.with(context).load(url).into(imageView, new Callback() { @Override public void onSuccess() { Log.d(TAG, "Yay!"); } @Override public void onError() { // no need } }); Java
91. #dfist #Devfest17 DSLs ENTER FILENAME/LANG Picasso.with(context).load(url).into(imageView) { onSuccess { Log.d(TAG, "Yay!") } } Kotlin
92. #dfist #Devfest17 DSLs class __Callback : com.squareup.picasso.Callback { private var _onSuccess: (() -> Unit)? = null private var _onError: (() -> Unit)? = null fun onSuccess(func: () -> Unit) { _onSuccess = func } fun onError(func: () -> Unit) { _onError = func } override fun onSuccess() { _onSuccess?.invoke() } override fun onError() { _onError?.invoke() } } Picassos.kt Kotlin
93. #dfist #Devfest17 DSLs ENTER FILENAME/LANG fun RequestCreator.into(target: ImageView, func: __Callback.() -> Unit) { val callback = __Callback() callback.func() into(target, callback) } Picassos.kt Kotlin
94. #dfist #Devfest17 Elegant DiffUtil
95. #dfist #Devfest17 Elegant DiffUtil ENTER FILENAME/LANG fun notifyChanges(oldList: List<Item>, newList: List<Item>) { val diff = DiffUtil.calculateDiff(object : DiffUtil.Callback() { override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean { return oldList[oldItemPosition].id == newList[newItemPosition].id } override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean { return oldList[oldItemPosition] == newList[newItemPosition] } override fun getOldListSize() = oldList.size override fun getNewListSize() = newList.size }) diff.dispatchUpdatesTo(this) } Kotlin
96. #dfist #Devfest17 Elegant DiffUtil ENTER FILENAME/LANG fun <T> RecyclerView.Adapter<*>.autoNotify(oldList: List<T>, newList: List<T>, compare: (T, T) -> Boolean) { val diff = DiffUtil.calculateDiff(object : DiffUtil.Callback() { override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean { return compare(oldList[oldItemPosition], newList[newItemPosition]) } override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean { return oldList[oldItemPosition] == newList[newItemPosition] } override fun getOldListSize() = oldList.size override fun getNewListSize() = newList.size }) diff.dispatchUpdatesTo(this) } RecyclerViews.kt Kotlin
97. #dfist #Devfest17 Elegant DiffUtil ENTER FILENAME/LANG adapter.autoNotify(oldItems, newItems) { oldItem, newItem -> oldItem.id == newItem.id } Kotlin
98. #dfist #Devfest17 Android Runtime Permissions in Kotlin way
99. #dfist #Devfest17 Permissions ENTER FILENAME/LANG if (ContextCompat.checkSelfPermission(this, Manifest.permission.READ_CONTACTS) != PackageManager.PERMISSION_GRANTED) { if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.READ_CONTACTS)) { Toast.makeText(this, "You better grant permission!", Toast.LENGTH_LONG).show(); } else { ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_CONTACTS}, REQUEST_READ_CONTACTS); } } else { doSomething() } Java
100. #dfist #Devfest17 Permissions ENTER FILENAME/LANG @Override public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) { switch (requestCode) { case REQUEST_READ_CONTACTS: { if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) { doSomething(); } else { Log.d(TAG, "Permission Denied!"); } return; } } } Java
101. #dfist #Devfest17 Permissions ENTER FILENAME/LANG doIfGranted(Manifest.permission.READ_CONTACTS, REQUEST_READ_CONTACTS) { doSomething() } Kotlin
102. #dfist #Devfest17 Permissions ENTER FILENAME/LANG override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<out String>, grantResults: IntArray) { when (requestCode) { REQUEST_READ_CONTACTS -> handlePermissionsResult(permissions, grantResults) { doSomething() } } } Kotlin
103. #dfist #Devfest17 Permissions ENTER FILENAME/LANG inline fun Activity.doIfGranted(permissions: Array<String>, requestCode: Int, onGranted: () -> Unit) { val permissionsNeeded = permissions.filter { !isPermissionGranted(it) } if (permissionsNeeded.isNotEmpty()) { ActivityCompat.requestPermissions(this, permissionsNeeded.toTypedArray(), requestCode) } else { onGranted() } } Permissions.kt Kotlin
104. #dfist #Devfest17 Permissions ENTER FILENAME/LANG inline fun Activity.doIfGranted(permission: String, requestCode: Int, onGranted: () -> Unit) { doIfGranted(arrayOf(permission), requestCode, onGranted) } fun Activity.isPermissionGranted(permission: String): Boolean { return ContextCompat.checkSelfPermission(this, permission) == PackageManager.PERMISSION_GRANTED } Permissions.kt Kotlin
105. #dfist #Devfest17 Permissions ENTER FILENAME/LANG inline fun Activity.handlePermissionsResult( permissions: Array<out String>, grantResults: IntArray, noinline onDenied: ((List<String>) -> Unit)? = null, onGranted: (() -> Unit)) { val deniedPermissions = permissions.indices .filter { grantResults[it] == PackageManager.PERMISSION_DENIED } .map { permissions[it] } if (deniedPermissions.isEmpty()) { onGranted() } else { onDenied?.invoke(deniedPermissions.toList()) } } Permissions.kt Kotlin
106. #dfist #Devfest17 Permissions ENTER FILENAME/LANG Full version of Permissions is here: https://gist.github.com/sembozdemir/ce8bebc7000adef04360b4f47ce478fd Permissions.kt
107. #dfist #Devfest17 http://bit.ly/handsome-codes-with-kotlin Presentation available
108. Thank you! Questions? Questions @sembozdemir www.sembozdemir.com