Imperative is dead, long live Declarative! - S.T.A.Y. Home #4

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 S.T.A.Y. Home #4 - Marco Gomiero
2. Marco Gomiero Tech Lead @ Uniwhere Co-Lead @ GDG Venezia S.T.A.Y. Home #4 - @marcoGomier > Twitter: @marcoGomier > Github: prof18 > Website: marcogomiero.com
3. Hacker News Client Loading State S.T.A.Y. Home #4 - @marcoGomier
4. Hacker News Client Error State S.T.A.Y. Home #4 - @marcoGomier Loading State
5. Hacker News Client Success State S.T.A.Y. Home #4 - @marcoGomier Loading State Error State
6. How to built it now? S.T.A.Y. Home #4 - @marcoGomier
7. How to built it now? S.T.A.Y. Home #4 - @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> S.T.A.Y. Home #4 - @marcoGomier
9. Storyboard or Code 1. Define the view S.T.A.Y. Home #4 - @marcoGomier
10. How to built it now? S.T.A.Y. Home #4 - @marcoGomier 1. Define the view 2. Retrieve the views instances
11. 1. Define the view 2. Retrieve the views instances val progressBar = findViewById<ProgressBar>(R.id.progress_bar) S.T.A.Y. Home #4 - @marcoGomier
12. 1. Define the view 2. Retrieve the views instances @IBOutlet weak var loader: UIActivityIndicatorView! S.T.A.Y. Home #4 - @marcoGomier
13. How to built it now? S.T.A.Y. Home #4 - @marcoGomier 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list
14. 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>() { ""... } S.T.A.Y. Home #4 - @marcoGomier
15. 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 S.T.A.Y. Home #4 - @marcoGomier
16. How to built it now? S.T.A.Y. Home #4 - @marcoGomier 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list 4. Add some logic
17. 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 ""... } } S.T.A.Y. Home #4 - @marcoGomier
18. How to built it now? S.T.A.Y. Home #4 - @marcoGomier 1. Define the view 2. Retrieve the views instances 3. Add some code to handle a list 4. Add some logic
19. Imperative S.T.A.Y. Home #4 - @marcoGomier • Lots of boilerplate and boring code • Errors and bugs prone • Hard to maintain • Not a single source of truth
20. Imperative S.T.A.Y. Home #4 - @marcoGomier
21. “ a programming paradigm that uses statements that change a program's state ” S.T.A.Y. Home #4 - @marcoGomier - Wikipedia
22. What if I can declare the views and forget the update step? S.T.A.Y. Home #4 - @marcoGomier
23. Declarative S.T.A.Y. Home #4 - @marcoGomier
24. “ what the program should accomplish without specifying how the program should achieve the result. ” S.T.A.Y. Home #4 - @marcoGomier - Wikipedia
25. state A S.T.A.Y. Home #4 - @marcoGomier
26. state A state B
27. S.T.A.Y. Home #4 - @marcoGomier
28. state A S.T.A.Y. Home #4 - @marcoGomier
29. state B S.T.A.Y. Home #4 - @marcoGomier
30. S.T.A.Y. Home #4 - @marcoGomier
31. UI = f( S.T.A.Y. Home #4 - @marcoGomier state)
32. S.T.A.Y. Home #4 - @marcoGomier state • Describes the UI right now, i.e. what to show • Independent of the previous state[s] • If changes, the UI is redrawn
33. S.T.A.Y. Home #4 - @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]
34. S.T.A.Y. Home #4 - @marcoGomier Changes are handled by the magician system not by the developer
35. How the magician system works S.T.A.Y. Home #4 - @marcoGomier 1. Creates an abstract representation of the UI and renders it
36. How the magician system works S.T.A.Y. Home #4 - @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
37. How the magician system works S.T.A.Y. Home #4 - @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]
38. How it works in details? S.T.A.Y. Home #4 - @marcoGomier
39. It depends! S.T.A.Y. Home #4 - @marcoGomier
40. React [Native] S.T.A.Y. Home #4 - @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!' } } } }
41. React [Native] S.T.A.Y. Home #4 - @marcoGomier • Virtual DOM as representation of the UI • Diff comparison using reconciliation, an O(n) heuristic algorithm
42. React [Native] S.T.A.Y. Home #4 - @marcoGomier • Virtual DOM as representation of the UI • Diff comparison using reconciliation, an O(n) heuristic algorithm reactjs.org/docs/reconciliation.html
43. Flutter S.T.A.Y. Home #4 - @marcoGomier • Element Tree as representation of the UI
44. Flutter S.T.A.Y. Home #4 - @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)
45. Flutter S.T.A.Y. Home #4 - @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
46. Jetpack Compose S.T.A.Y. Home #4 - @marcoGomier • Slot table as representation of the UI
47. Jetpack Compose S.T.A.Y. Home #4 - @marcoGomier • Every node is cached with a key
48. Jetpack Compose S.T.A.Y. Home #4 - @marcoGomier • Reuse the node unless it changed
49. Jetpack Compose S.T.A.Y. Home #4 - @marcoGomier • Reuse the node unless it changed youtube.com/watch?v=6BRlI5zfCCk
50. S.T.A.Y. Home #4 - @marcoGomier How to define a piece of UI?
51. S.T.A.Y. Home #4 - @marcoGomier Component How to define a piece of UI?
52. Component S.T.A.Y. Home #4 - @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; } }
53. S.T.A.Y. Home #4 - @marcoGomier Component How to define a piece of UI? Widget
54. S.T.A.Y. Home #4 - @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: ""... , ), ); } }
55. S.T.A.Y. Home #4 - @marcoGomier Component Widget composable function How to define a piece of UI?
56. S.T.A.Y. Home #4 - @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) ""... } } }
57. S.T.A.Y. Home #4 - @marcoGomier Component Widget composable function struct How to define a piece of UI?
58. S.T.A.Y. Home #4 - @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) ""... } } } }
59. S.T.A.Y. Home #4 - @marcoGomier Component Widget composable function struct How to define a piece of UI?
60. S.T.A.Y. Home #4 - @marcoGomier Component Widget composable function struct How to define a piece of UI? With a function!
61. S.T.A.Y. Home #4 - @marcoGomier Component Widget composable function struct How to define a piece of UI? With a function (of the state) !
62. S.T.A.Y. Home #4 - @marcoGomier Pure Function
63. S.T.A.Y. Home #4 - @marcoGomier fun sum(a: Int, b: Int): Int { return a + b }
64. S.T.A.Y. Home #4 - @marcoGomier fun sum(a: Int, b: Int): Int { return a + b } fun addExpense(account: BackAccount, amount: Int) { account.balance = account.balance - amount }
65. S.T.A.Y. Home #4 - @marcoGomier fun sum(a: Int, b: Int): Int { return a + b } fun addExpense(account: BackAccount, amount: Int) { account.balance = account.balance - amount }
66. Pure Function S.T.A.Y. Home #4 - @marcoGomier • Returns the same value given the same input • Has no side effects
67. S.T.A.Y. Home #4 - @marcoGomier Take some inputs and return a piece of UI
68. UI = f(state) S.T.A.Y. Home #4 - @marcoGomier
70. S.T.A.Y. Home #4 - @marcoGomier fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection "-> settings.update(selection) } ) }
71. 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) } ) } S.T.A.Y. Home #4 - @marcoGomier SwitchComponent isChecked State Holder
72. fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection "-> settings.update(selection) } ) } S.T.A.Y. Home #4 - @marcoGomier SwitchComponent State Holder onSelectionChange
73. S.T.A.Y. Home #4 - @marcoGomier SwitchComponent isChecked fun checkComponent(isChecked: Boolean): MaterialSwitch { return MaterialSwitch ( selected = isChecked, onSelectionChange = { selection "-> settings.update(selection) } ) }
74. S.T.A.Y. Home #4 - @marcoGomier Properties goes down Events goes up
75. S.T.A.Y. Home #4 - @marcoGomier
76. S.T.A.Y. Home #4 - @marcoGomier Composition
77. Inheritance S.T.A.Y. Home #4 - @marcoGomier Composition over
78. S.T.A.Y. Home #4 - @marcoGomier Inheritance fun Container() : View() { ".. } fun NewsCard() : View() { ".. } fun NewsCardContainer() : "?? { ".. }
79. S.T.A.Y. Home #4 - @marcoGomier Composition fun NewsCardContainer() { NewsCard { Container { ""... } } } fun Container() { ".. } fun NewsCard() { ".. }
80. Imperative vs Declarative S.T.A.Y. Home #4 - @marcoGomier
81. S.T.A.Y. Home #4 - @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() } }
82. S.T.A.Y. Home #4 - @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
83. S.T.A.Y. Home #4 - @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() } }
84. S.T.A.Y. Home #4 - @marcoGomier https://github.com/prof18/imperative-vs-declarative
85. Conclusions S.T.A.Y. Home #4 - @marcoGomier
86. Conclusions S.T.A.Y. Home #4 - @marcoGomier • Describe WHAT to see NOT HOW • Less code to write → Fewer bugs • Totally new paradigm, it requires time to get used to
87. 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
88. Thank you! > Twitter: @marcoGomier > Github: prof18 > Website: marcogomiero.com S.T.A.Y. Home #4 - Marco Gomiero