Imperative is dead, long live Declarative! - .droidcon ONLINE - Webinar series

Nowadays, the mobile world has started to see larger adoption of the declarative style to build UI as opposed to the (not so) old imperative style. This pattern is borrowed from the web world, from frameworks like React and Vue.js and it started to appear in the mobile world first with React Native, then with Flutter and finally, it captured the attention of the “native world” with Jetpack Compose and Swift UI.
In this talk, we will explore the declarative style of building UI compared with the imperative one. We will try to enter in this mindset by finding the differences and by looking at some examples of these patterns.

Transcript

1. Imperative is dead, long live Declarative .droidcon ONLINE - Marco Gomiero
2. Marco Gomiero 👨💻 Tech Lead @ revelop.app 🇮🇹 🇩🇪 🇺🇸 
 🍻 Co-Lead @ GDG Venezia .droidcon ONLINE - @marcoGomier > Twitter: @marcoGomier 
 > Github: prof18 
 > Blog: marcogomiero.com
3. Hacker News Client Loading State .droidcon ONLINE - @marcoGomier
4. Hacker News Client Error State .droidcon ONLINE - @marcoGomier Loading State
5. Hacker News Client Success State .droidcon ONLINE - @marcoGomier Loading State Error State
6. 👷 How to built it 👑? 🚧 .droidcon ONLINE - @marcoGomier
7. 👷 How to built it 👑? 🚧 .droidcon ONLINE - @marcoGomier 1. Define the view
8. XML or Code 1. Define the view <androidx.constraintlayout.widget.ConstraintLayout ... /> <ProgressBar android:id="@+id/progress_bar" android:layout_width="wrap_content" android:layout_height="wrap_content" ... /> <TextView android:id="@+id/error_message" android:layout_width="wrap_content" android:layout_height="wrap_content" android:visibility="gone" ... /> <Button android:id="@+id/error_button" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Refresh" android:visibility="gone" ... /> <androidx.recyclerview.widget.RecyclerView android:id="@+id/recycler_view" android:layout_width="match_parent" android:layout_height="match_parent" android:visibility="gone" ... /> < / androidx.constraintlayout.widget.ConstraintLayout> .droidcon ONLINE - @marcoGomier
9. 👷 How to built it 👑? 🚧 .droidcon ONLINE - @marcoGomier 1. Define the view 2. Retrieve the views instances
10. 1. Define the view 2. Retrieve the views instances val progressBar = findViewById<ProgressBar>(R.id.progress_bar) .droidcon ONLINE - @marcoGomier
11. 👷 How to built it 👑? 🚧 .droidcon ONLINE - @marcoGomier 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list
12. 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list RecyclerView.Adapter class NewsAdapter(var items: List<News> = listOf()) : RecyclerView.Adapter<NewsAdapter.ViewHolder>() { ... } .droidcon ONLINE - @marcoGomier
13. 👷 How to built it 👑? 🚧 .droidcon ONLINE - @marcoGomier 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list 4. Add some logic
14. Kotlin 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list 4. Add some logic when (appState.newsState) { is NewsState.Loading -> { progressBar.visibility = View.VISIBLE ... } is NewsState.Error -> { progressBar.visibility = View.GONE errorMessage.visibility = View.VISIBLE ... } is NewsState.Success -> { progressBar.visibility = View.GONE errorMessage.visibility = View.GONE recyclerView.visibility = View.VISIBLE ... } } .droidcon ONLINE - @marcoGomier
15. 👑 Imperative .droidcon ONLINE - @marcoGomier • Lots of boilerplate and boring code • Errors and bugs prone • Not a single source of truth • Hard to maintain
16. 👑 Imperative .droidcon ONLINE - @marcoGomier
17. “ a programming paradigm that uses statements that change a program's state ” .droidcon ONLINE - @marcoGomier - Wikipedia
18. state A .droidcon ONLINE - @marcoGomier
19. state A state B
20. state A .droidcon ONLINE - @marcoGomier 🧙
21. state B .droidcon ONLINE - @marcoGomier 🧙
22. 📣 Declarative .droidcon ONLINE - @marcoGomier
23. “ what the program should accomplish without specifying how the program should achieve the result. ” .droidcon ONLINE - @marcoGomier - Wikipedia
24. 🧐 .droidcon ONLINE - @marcoGomier
25. UI = f( .droidcon ONLINE - @marcoGomier state)
26. .droidcon ONLINE - @marcoGomier state • Describes the UI right now, i.e. what to show • Independent of the previous state[s] • If changes, the UI is redrawn
27. .droidcon ONLINE - @marcoGomier state • Describes the UI right now, i.e. what to show • Independent of the previous state[s] • If changes, the UI is redrawn [with optimizations]
28. .droidcon ONLINE - @marcoGomier Changes are handled by the magician system not by the developer 🧙
29. How the magician system works .droidcon ONLINE - @marcoGomier 1. Creates an abstract representation of the UI and renders it
30. How the magician system works .droidcon ONLINE - @marcoGomier 1. Creates an abstract representation of the UI and renders it 2. When a change is made, it creates a new representation 3. Computes the differences between the two representations 4. Renders the differences
31. How the magician system works .droidcon ONLINE - @marcoGomier 1. Creates an abstract representation of the UI and renders it 2. When a change is made, it creates a new representation 3. Computes the differences between the two representations 4. Renders the differences [if any]
32. 🤔 How it works in details? .droidcon ONLINE - @marcoGomier
33. 🤷 It depends! .droidcon ONLINE - @marcoGomier
34. React [Native] .droidcon ONLINE - @marcoGomier • Virtual DOM as representation of the UI <button class='button button-blue'> <b> OK! </ b> </ button> { type: 'button', props: { className: 'button button-blue', children: { type: 'b', props: { children: 'OK!' } } } }
35. React [Native] .droidcon ONLINE - @marcoGomier • Virtual DOM as representation of the UI • Diff comparison using reconciliation, an O(n) heuristic algorithm
36. React [Native] .droidcon ONLINE - @marcoGomier • Virtual DOM as representation of the UI • Diff comparison using reconciliation, an O(n) heuristic algorithm reactjs.org/docs/reconciliation.html
37. Flutter .droidcon ONLINE - @marcoGomier • Element Tree as representation of the UI
38. Flutter .droidcon ONLINE - @marcoGomier • Element Tree as representation of the UI • Not a tree-diffing algorithm • For each element, the child list is examined independently 
 (O(n) algorithm)
39. Flutter .droidcon ONLINE - @marcoGomier • Element Tree as representation of the UI • Not a tree-diffing algorithm • For each element, the child list is examined independently 
 (O(n) algorithm) flutter.dev/docs/resources/inside-flutter
40. Jetpack Compose .droidcon ONLINE - @marcoGomier • Slot table as representation of the UI
41. Jetpack Compose .droidcon ONLINE - @marcoGomier • Every node is cached with a key
42. Jetpack Compose .droidcon ONLINE - @marcoGomier • Reuse the node unless it changed
43. Jetpack Compose .droidcon ONLINE - @marcoGomier • Reuse the node unless it changed youtube.com/watch?v=6BRlI5zfCCk
44. .droidcon ONLINE - @marcoGomier 👨🎨 How to define a piece of UI?
45. .droidcon ONLINE - @marcoGomier Component 👨🎨 How to define a piece of UI?
46. Component .droidcon ONLINE - @marcoGomier Component export default class NewsCard extends React.Component { constructor(props) { super(props); } render() { const { news } = this.props; if (news) { return ( <View style={styles.container}> <TouchableOpacity . . > <Text .. > <Text .. > </ TouchableOpacity> </ View> ); } return null; } }
47. .droidcon ONLINE - @marcoGomier Component 👨🎨 How to define a piece of UI? Widget
48. .droidcon ONLINE - @marcoGomier Component Widget Widget class NewsCardWidget extends StatelessWidget { final News news; NewsCardWidget({this.news}); @override Widget build(BuildContext context) { return Padding( padding: const EdgeInsets.all(6.0), child: Card( elevation: 8.0, shape: RoundedRectangleBorder( borderRadius: BorderRadius.circular(16.0), ), child: ... , ), ); } }
49. .droidcon ONLINE - @marcoGomier Component Widget composable function 👨🎨 How to define a piece of UI?
50. .droidcon ONLINE - @marcoGomier Component Widget composable function composable function @Composable fun NewsCard(news: News) { Card( shape = RoundedCornerShape(16.dp), elevation = 8.dp, modifier = .. ) { Column { Text(news.title) .. . } } }
51. .droidcon ONLINE - @marcoGomier Component Widget composable function struct 👨🎨 How to define a piece of UI?
52. .droidcon ONLINE - @marcoGomier Component Widget composable function struct struct struct NewsCard: View { let news: News var body: some View { HStack { VStack(alignment: .leading) { Text(self.news.title) .. . } } } }
53. .droidcon ONLINE - @marcoGomier Component Widget composable function struct 👨🎨 How to define a piece of UI?
54. .droidcon ONLINE - @marcoGomier Component Widget composable function struct 👨🎨 How to define a piece of UI? With a function!
55. .droidcon ONLINE - @marcoGomier Component Widget composable function struct 👨🎨 How to define a piece of UI? With a function (of the state) !
56. .droidcon ONLINE - @marcoGomier Pure Function
57. Pure Function .droidcon ONLINE - @marcoGomier • Returns the same value given the same input • Has no side effects
58. .droidcon ONLINE - @marcoGomier fun sum(a: Int, b: Int): Int { return a + b } 👍
59. .droidcon ONLINE - @marcoGomier 👍 fun sum(a: Int, b: Int): Int { return a + b } fun addExpense(account: BackAccount, amount: Int) { account.balance = account.balance - amount }
60. .droidcon ONLINE - @marcoGomier 👍 fun sum(a: Int, b: Int): Int { return a + b } fun addExpense(account: BackAccount, amount: Int) { account.balance = account.balance - amount } 👎
61. .droidcon ONLINE - @marcoGomier Take some inputs and return a piece of UI
62. UI = f(state) .droidcon ONLINE - @marcoGomier
64. .droidcon ONLINE - @marcoGomier fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection -> settings.update(selection) } ) }
65. fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection -> settings.update(selection) } ) } fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection -> settings.update(selection) } ) } .droidcon ONLINE - @marcoGomier SwitchComponent isChecked State Holder
66. fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection -> settings.update(selection) } ) } .droidcon ONLINE - @marcoGomier SwitchComponent State Holder onSelectionChange
67. .droidcon ONLINE - @marcoGomier SwitchComponent isChecked fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection -> settings.update(selection) } ) }
68. .droidcon ONLINE - @marcoGomier State goes down Events goes up Unidirectional Data Flow
69. .droidcon ONLINE - @marcoGomier 🧩
70. .droidcon ONLINE - @marcoGomier 🧩 Composition
71. Inheritance .droidcon ONLINE - @marcoGomier 🧩 Composition over
72. .droidcon ONLINE - @marcoGomier 🧩 Inheritance fun Container() : View() { .. } fun NewsCard() : View() { .. } fun NewsCardContainer() : ?? { .. }
73. .droidcon ONLINE - @marcoGomier 🧩 Composition fun NewsCardContainer() { NewsCard { Container { ... } } } fun Container() { .. } fun NewsCard() { .. }
74. Imperative vs Declarative .droidcon ONLINE - @marcoGomier
75. .droidcon ONLINE - @marcoGomier Imperative when (appState.newsState) { is NewsState.Loading -> { progressBar.visibility = View.VISIBLE recyclerView.visibility = View.GONE errorMessage.visibility = View.GONE errorButton.visibility = View.GONE } is NewsState.Error -> { val errorState = appState.newsState as NewsState.Error recyclerView.visibility = View.GONE errorButton.visibility = View.VISIBLE errorMessage.visibility = View.VISIBLE errorMessage.text = errorState.reason errorButton.setOnClickListener { viewModel.loadData() } } ->
76. .droidcon ONLINE - @marcoGomier recyclerView.visibility = View.GONE errorButton.visibility = View.VISIBLE errorMessage.visibility = View.VISIBLE errorMessage.text = errorState.reason errorButton.setOnClickListener { viewModel.loadData() } } is NewsState.Success -> { val successState = appState.newsState as NewsState.Success progressBar.visibility = View.GONE errorMessage.visibility = View.GONE errorButton.visibility = View.GONE recyclerView.visibility = View.VISIBLE adapter.items = successState.news adapter.notifyDataSetChanged() } } Imperative
77. .droidcon ONLINE - @marcoGomier when (appState ?. newsState) { NewsState.Loading - > { LoadingView() } is NewsState.Success -> { val successState = appState ?. newsState as NewsState.Success NewsList(newsList = successState.news) } is NewsState.Error -> { val errorState = appState ? . newsState as NewsState.Error ErrorView( reason = errorState.reason, onRefreshClick = { viewModel.loadData() }) } } Declarative when (appState.newsState) { is NewsState.Loading -> { progressBar.visibility = View.VISIBLE recyclerView.visibility = View.GONE errorMessage.visibility = View.GONE errorButton.visibility = View.GONE } is NewsState.Error -> { val errorState = appState.newsState as NewsState.Error recyclerView.visibility = View.GONE errorButton.visibility = View.VISIBLE errorMessage.visibility = View.VISIBLE errorMessage.text = errorState.reason errorButton.setOnClickListener { viewModel.loadData() } } is NewsState.Success -> { val successState = appState.newsState as NewsState.Success progressBar.visibility = View.GONE errorMessage.visibility = View.GONE errorButton.visibility = View.GONE recyclerView.visibility = View.VISIBLE adapter.items = successState.news adapter.notifyDataSetChanged() } }
78. .droidcon ONLINE - @marcoGomier https://github.com/prof18/imperative-vs-declarative
79. Conclusions .droidcon ONLINE - @marcoGomier
80. Conclusions .droidcon ONLINE - @marcoGomier • Describe WHAT to see NOT HOW • Less code to write → Fewer bugs • Totally new paradigm, it requires time to get used to
81. Bibliography / Useful Links • https: // www.youtube.com/watch?v=zaSn_qA7Nb4 • https: // www.youtube.com/watch?v=4EFjDSijAZU • https: // www.youtube.com/watch?v=x7cQ3mrcKaY • https: // www.youtube.com/watch?v=1uRC3hmKQnM • https: // www.youtube.com/watch?v=eR4LjL1h6cE • https: // overreacted.io/react-as-a-ui-runtime/ • https: // rauchg.com/2015/pure-ui • http: / / intelligiblebabble.com/compose-from-first-principles/ • https: // www.youtube.com/watch?v=6BRlI5zfCCk • https: // www.youtube.com/watch?v=WqnR_XhEiVI • https: // reactjs.org/docs/reconciliation.html • https: // flutter.dev/docs/resources/inside-flutter • https: // www.youtube.com/watch?v=rshXbwrrUxY • https: // www.youtube.com/watch?v=_1_XkeFUUME • https: // www.youtube.com/watch?v=aH7oWxfxpJY • https: // medium.com/androiddevelopers/understanding-jetpack-compose-part-1-of-2-ca316fe39050 • https: // medium.com/androiddevelopers/under-the-hood-of-jetpack-compose-part-2-of-2-37b2c20c6cdd • https: // developer.android.com/jetpack/compose/state
82. Thank you! .droidcon ONLINE - @marcoGomier > Twitter: @marcoGomier 
 > Github: prof18 
 > Blog: marcogomiero.com revelop.app