Imperative is dead, long live Declarative! - AppDevCon

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 Appdevcon - Marco Gomiero
2. Marco Gomiero ! Tech Lead @ Uniwhere " # $ Co-Lead @ GDG Venezia Appdevcon - @marcoGomier > Twitter: @marcoGomier > Github: prof18 > Website: marcogomiero.com
3. Hacker News Client Success State Appdevcon - @marcoGomier Loading State Error State
4. Appdevcon - @marcoGomier % How to built it now? &
5. Appdevcon - @marcoGomier % How to built it now? & 1. Define the view
6. 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> Appdevcon - @marcoGomier
7. Storyboard or Code 1. Define the view Appdevcon - @marcoGomier
8. Appdevcon - @marcoGomier % How to built it now? & 1. Define the view 2. Retrieve the views instances
9. 1. Define the view 2. Retrieve the views instances val progressBar = findViewById<ProgressBar>(R.id.progress_bar) Appdevcon - @marcoGomier
10. 1. Define the view 2. Retrieve the views instances @IBOutlet weak var loader: UIActivityIndicatorView! Appdevcon - @marcoGomier
11. Appdevcon - @marcoGomier % How to built it now? & 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>() { ""# } Appdevcon - @marcoGomier
13. 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list extension MainViewController: UITableViewDelegate, UITableViewDataSource { ""# } Delegate and a DataSource Appdevcon - @marcoGomier
14. Appdevcon - @marcoGomier % How to built it now? & 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list 4. Add some logic
15. 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 ""# } } Appdevcon - @marcoGomier
16. ' Imperative Appdevcon - @marcoGomier • Lots of boilerplate and boring code • Errors and bugs prone • Not a single source of truth • Hard to maintain
17. Appdevcon - @marcoGomier ' Imperative
18. Appdevcon - @marcoGomier “ a programming paradigm that uses statements that change a program's state ” - Wikipedia
19. Appdevcon - @marcoGomier ( What if I can declare the views and forget the update step?
20. Appdevcon - @marcoGomier ) Declarative
21. Appdevcon - @marcoGomier “ what the program should accomplish without specifying how the program should achieve the result. ” - Wikipedia
22. Appdevcon - @marcoGomier state A
23. state A state B
24. Appdevcon - @marcoGomier *
25. state A * Appdevcon - @marcoGomier
26. Appdevcon - @marcoGomier state B *
27. Appdevcon - @marcoGomier +
28. Appdevcon - @marcoGomier UI = f(state)
29. Appdevcon - @marcoGomier state • Describes the UI right now, i.e. what to show • Independent of the previous state[s] • If changes, the UI is redrawn
30. Appdevcon - @marcoGomier Changes are handled by the magician system not by the developer *
31. How the magician system works Appdevcon - @marcoGomier 1. Creates an abstract representation of the UI and renders it
32. How the magician system works Appdevcon - @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]
33. Appdevcon - @marcoGomier ( How it works in details?
34. Appdevcon - @marcoGomier , It depends!
35. React [Native] Appdevcon - @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!' } } } }
36. React [Native] Appdevcon - @marcoGomier • Virtual DOM as representation of the UI • Diff comparison using reconciliation, an O(n) heuristic algorithm
37. React [Native] Appdevcon - @marcoGomier • Virtual DOM as representation of the UI • Diff comparison using reconciliation, an O(n) heuristic algorithm reactjs.org/docs/reconciliation.html
38. Flutter Appdevcon - @marcoGomier • Element Tree as representation of the UI
39. Flutter Appdevcon - @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)
40. Flutter Appdevcon - @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
41. Jetpack Compose Appdevcon - @marcoGomier • Slot table as representation of the UI
42. Jetpack Compose Appdevcon - @marcoGomier • Every node is cached with a key
43. Jetpack Compose Appdevcon - @marcoGomier • Reuse the node unless it changed
44. Jetpack Compose Appdevcon - @marcoGomier • Reuse the node unless it changed youtube.com/watch?v=6BRlI5zfCCk
45. Appdevcon - @marcoGomier - How to define a piece of UI?
46. Appdevcon - @marcoGomier Component - How to define a piece of UI?
47. Component 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; } } Appdevcon - @marcoGomier
48. Appdevcon - @marcoGomier Component - How to define a piece of UI? Widget
49. 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: ""# , ), ); } } Appdevcon - @marcoGomier
50. Appdevcon - @marcoGomier Component Widget composable function - How to define a piece of UI?
51. Component Widget composable function composable function @Composable fun NewsCard(news: News) { Card( shape = RoundedCornerShape(16.dp), elevation = 8.dp, modifier = "' ) { Column { Text(news.title) ""# } } } Appdevcon - @marcoGomier
52. Appdevcon - @marcoGomier Component Widget composable function struct - How to define a piece of UI?
53. Component Widget composable function struct struct struct NewsCard: View { let news: News var body: some View { HStack { VStack(alignment: .leading) { Text(self.news.title) ""# } } } } Appdevcon - @marcoGomier
54. Appdevcon - @marcoGomier Component Widget composable function struct - How to define a piece of UI?
55. Appdevcon - @marcoGomier Component Widget composable function struct - How to define a piece of UI? With a function!
56. Appdevcon - @marcoGomier Component Widget composable function struct - How to define a piece of UI? With a function (of the state) !
57. Appdevcon - @marcoGomier Pure Function
58. Appdevcon - @marcoGomier fun sum(a: Int, b: Int): Int { return a + b } .
59. Appdevcon - @marcoGomier . fun sum(a: Int, b: Int): Int { return a + b } fun addExpense(account: BackAccount, amount: Int) { account.balance = account.balance - amount }
60. Appdevcon - @marcoGomier . fun sum(a: Int, b: Int): Int { return a + b } fun addExpense(account: BackAccount, amount: Int) { account.balance = account.balance - amount } /
61. Pure Function Appdevcon - @marcoGomier • Returns the same value given the same input • Has no side effects
62. Appdevcon - @marcoGomier Take some inputs and return a piece of UI
63. Appdevcon - @marcoGomier UI = f(state)
65. Appdevcon - @marcoGomier fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection "& settings.update(selection) } ) }
66. 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) } ) } SwitchComponent isChecked State Holder Appdevcon - @marcoGomier
67. fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection "& settings.update(selection) } ) } SwitchComponent State Holder onSelectionChange Appdevcon - @marcoGomier
68. SwitchComponent isChecked fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection "& settings.update(selection) } ) } Appdevcon - @marcoGomier
69. Appdevcon - @marcoGomier Properties goes down Events goes up
70. Appdevcon - @marcoGomier 0
71. Appdevcon - @marcoGomier 0 Composition
72. Inheritance Appdevcon - @marcoGomier 0 Composition over
73. Appdevcon - @marcoGomier 0 Inheritance fun Container() : View() { "' } fun NewsCard() : View() { "' } fun NewsCardContainer() : "( { "' }
74. Appdevcon - @marcoGomier 0 Composition fun NewsCardContainer() { NewsCard { Container { ""# } } } fun Container() { "' } fun NewsCard() { "' }
75. Appdevcon - @marcoGomier Imperative vs Declarative
76. Appdevcon - @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() } }
77. Appdevcon - @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
78. Appdevcon - @marcoGomier when (appState")newsState) { is 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() } }
79. Appdevcon - @marcoGomier https://github.com/prof18/imperative-vs-declarative
80. Appdevcon - @marcoGomier Conclusions
81. Appdevcon - @marcoGomier Conclusions • Describe WHAT to see NOT HOW • Less code to write → Fewer bugs • Totally new paradigm, it requires time to get used to
82. 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
83. Appdevcon - @marcoGomier Thank you! > Twitter: @marcoGomier > Github: prof18 > Website: marcogomiero.com