Mobile development cheat sheet (Android)

1. 1Kotlin essentialsThe Kotlin 2 that Android code relies on: null safety, data and sealed classes, lambdas, collections, coroutines and Flow, plus the Gradle build.10 concepts

   1. 1val, var and type inferenceval is a read-only reference, var can be reassigned, and the compiler infers types when they are obvious.
   2. 2Null safetyTypes are non-null by default; String? may be null.
   3. 3Functions, lambdas and extensionsFunctions support default and named arguments.
   4. 4Data classesA data class generates equals, hashCode, toString, copy and destructuring from its constructor properties.
   5. 5Sealed classes and whenA sealed class or interface has a fixed set of subtypes known at compile time.
   6. 6Collections and their operationsKotlin separates read-only List, Set and Map from MutableList and friends, and offers functions such as map, filter, groupBy, associateBy and…
   7. 7Coroutines and suspend functionsA suspend function can pause without blocking a thread.
   8. 8Dispatchers and withContextA dispatcher decides which threads a coroutine runs on: Dispatchers.Main for UI, Dispatchers.IO for blocking I/O and Dispatchers.Default for CPU work.
   9. 9Flow and StateFlowA Flow is a cold stream of values produced by a suspend block and read with collect.
   10. 10Gradle and version catalogsGradle builds Android apps.

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2. 2UI and layoutTurning a design into screens: small composable pieces, layout containers, long lists, forms, theming and fitting every screen size.10 concepts

   1. 1Declarative UIYou describe what the screen should look like for the current state, and the framework works out what to change on screen.
   2. 2ModifiersA Modifier is an ordered chain that decorates a composable with size, padding, background, click handling, semantics and more.
   3. 3Recomposition and side effectsWhen state a composable reads changes, Compose runs that composable again (recomposition).
   4. 4Rows, columns and stacksThe basic layout containers place children horizontally, vertically or on top of each other, with spacing and alignment options.
   5. 5Lazy listsA lazy list only builds the rows currently on screen, plus a few ahead, and recycles them as you scroll.
   6. 6Text input and formsA text field shows a value from state and reports every change back.
   7. 7Theming and dark modeA theme defines colours, typography and shapes in one place.
   8. 8Platform design guidelinesMaterial Design 3 is Google's design system for Android, and the Human Interface Guidelines are Apple's rules for iOS.
   9. 9Safe areas and screen sizesSafe areas, or insets, are the parts of the screen not covered by the notch, status bar, camera cutout or gesture bar.
   10. 10Hot reload and previewsTools that show code changes in a running app or an editor preview within a second or two, without rebuilding the whole app.

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3. State and navigationWhere state lives, how screens share it and how users move between screens, including tabs, sheets and deep links.9 conceptsOpens with credits
4. Networking and local dataCalling APIs, storing data on the device and keeping the app useful on a slow or missing connection, with tokens kept safe.9 conceptsOpens with credits
5. Device features and notificationsWhat makes a phone a phone: the app lifecycle, permissions, camera, location, notifications, background work and calling native code.8 conceptsOpens with credits
6. Testing, performance and accessibilityProving the app works and keeps working: unit and UI tests, profiling a release build, accessibility with real screen readers and a security check.10 conceptsOpens with credits
7. Publishing and releasesGetting the app to real users: IDs and versions, release builds and signing, beta testing, privacy declarations, store review, crash reporting and rollouts.10 conceptsOpens with credits
8. 8Glossary, A to ZEvery concept in the sheet and the smaller terms it uses, in one list.145 terms

Apps for Android and iOS.