# FAQ — Frequently Asked Questions

> Short answers to the most common CrossFly questions, each with a link to the canonical page.

Source: https://crossfly.dev/docs/faq/perguntas-frequentes — CrossFly documentation. Full docs as one file: https://crossfly.dev/llms-full.txt

# FAQ — Frequently Asked Questions

Answers in one or two sentences, always with a link to the source. To **learn**, see
[`learn/`](/docs/learn); to **fix an error**, see [`troubleshooting/`](/docs/troubleshooting).

## General
**What is CrossFly?** A UI framework in **Java on top of libGDX/OpenGL** — it draws everything via the GPU, with no
native widgets, so the look is identical on every platform. (Ch. 1)

**Do I need to know libGDX?** Not for day-to-day work — you work with `Window`/`Component`.
(Ch. 4)

## Bootstrap and lifecycle
**How does the app start?** The `Main` instantiates the `MainWindow` **via reflection** (using the *classname*).
(Ch. 4)

**My `MainWindow` doesn't load.** It must **extend `Window`** and have a **public no-args
constructor**. ([Project anatomy](/docs/get-started/project-structure))

**Where do I build the UI?** In **`onShow()`** (with `super.onShow()`), never in the constructor.
(Ch. 7)

## Threading and rendering
**Can I touch the UI from another thread?** **No** — come back with `Component.onUI(r)` /
`Main.runOnPaintThread(r)`. ([The paint thread](/docs/concepts/the-paint-thread))

**I changed something and the screen doesn't update.** Rendering is **on demand**: call `Component.repaint()` (and
`setValidLayout(false)` if you changed size/position). (Ch. 4 §4.6)

**A long operation without freezing?** Run it on a `Thread` and come back with `onUI`.
([Background work](/docs/data-and-services/background-work))

## Layout
**What do the numbers in `add()` mean?** `LAY_FILL = -1f` fills the remaining space; `LAY_AUTO = 0f` = content
size; `>0` = **fraction of the container** (not pixels). (Ch. 12)

**Padding and margin?** `ingap` (inner) and `outgap` (outer), in the style.
([Styling components](/docs/ui/theming/styling-components))

**Responsive UI?** Use `Relative` + `LAY_FILL`/`LAY_AUTO`, never pixels.
([Responsiveness](/docs/best-practices/responsividade))

**`GridLayout`?** Pass **all the widths, then all the components**: `add(w0,w1,…,c0,c1,…)`.
([GridLayout](/docs/ui/layouts/grid-layout))

## Navigation
**How do I navigate between screens?** `novaWindow.show()` = push, `hide()` = pop (it's a **stack of `Window`**).
([Navigation](/docs/ui/navigation/navigate-between-screens))

**Pass data to another screen?** Through the **constructor** of the destination `Window` — there are no named routes.

**Receive a result?** `picker.show(onAnswer)` for pickers; regular screens use a callback through the
constructor; `Alert` responds via `buttonAction`. ([Returning values](/docs/ui/navigation/returning-results))

## Style, theme, and colors
**Switch light/dark?** `Styles.setDarkMode(true/false)` / `toggleDarkMode()` — it re-skins everything on its own.
([Light/dark mode](/docs/ui/theming/dark-mode))

**Change the Material/iOS "look"?** `Skin.set(new MaterialSkin()|new IOSSkin()|new FlutterSkin())`.
([Skins](/docs/ui/theming/skins))

**Define the colors?** Generate a `ColorScheme` from a seed color.
([Color schemes](/docs/ui/theming/color-schemes))

**My color became invisible.** It's an `int` ARGB `0xAARRGGBB` — without `FF` in the alpha it disappears. Use `0xFF…`.
(Ch. 11)

## Data and persistence
**Save simple preferences?** **There are no more built-in prefs** (`Settings.appSettings` was removed) —
use SQLite or files in `Device.homeFolder`. ([Files and preferences](/docs/data-and-services/files-and-preferences))

**Use a database?** `dev.crossfly.sqlite` (`SqliteDb`/`SqliteStmt`/`SqliteCursor`), with SQLite3MC encryption, off
the paint thread. ([Persistence with SQLite](/docs/data-and-services/storage-sqlite))

**Is there a ready-made HTTP/sync client?** **No** — use `java.net`/OkHttp; JSON with Jackson.
([Networking and HTTP](/docs/data-and-services/networking-http))

**Text i18n?** The framework does **not** translate — it only formats dates/numbers by locale (`LocaleFormats`);
the translation is yours. ([Internationalization](/docs/best-practices/internacionalizacao))

**Large list slow?** Use `VirtualListView`/`PagedDataAdapter`.
([Performance and large lists](/docs/best-practices/performance-e-listas-grandes))

## Platforms and device
**Which platforms does it run on?** Android, iOS, Desktop (Windows/macOS/Linux via LWJGL3) and Web (TeaVM).
([Platform matrix](/docs/platforms/overview))

**Camera/GPS/Bluetooth/biometrics?** The `dev.crossfly.device` package; Bluetooth is **asynchronous BLE/GATT**.
(Ch. 24)

**Thermal printer?** **Support removed** — talk directly via BLE/GATT.
([Bluetooth BLE](/docs/data-and-services/device-bluetooth-ble))

**UI stutters on an old device?** Check `Screen.slowGPU` and reduce shadows/gradients/glass.
(Ch. 8)

**Test the UI automatically?** The `dev.crossfly.test` framework (`TestCase` + screenshot comparison).
([Testing](/docs/best-practices/testes))

**The Javadoc contradicts the behavior?** Trust the **code and this documentation**.
([How the reference is organized](/docs/reference/overview))

## See also
- Canonical source: Chapter 34 — FAQ · [`troubleshooting/`](/docs/troubleshooting) · [`reference/`](/docs/reference)