# Your first app (guided, end to end)

> A real task list — layout, input, state, navigation and persistence in SQLite.

Source: https://crossfly.dev/docs/get-started/your-first-app — CrossFly documentation. Full docs as one file: https://crossfly.dev/llms-full.txt

# Your first app: task list

Let's build an app that **opens, adds, navigates and persists** — step by step, introducing each
piece in the order it's needed.

## Complete example (copy, paste and run)
A lean, self-contained version of the essentials: a `TextField` + `Button` that adds items to a virtual
list. Paste it into a `MainWindow` class and run with `gradlew :lwjgl3:run`.
```java
package com.exemplo;

import java.util.ArrayList;
import java.util.List;

import dev.crossfly.ui.*;
import dev.crossfly.ui.layout.*;

public class MainWindow extends Window
{
   private final List<String> tarefas = new ArrayList<>();
   private TextField campo;
   private VirtualListView lista;

   @Override
   public void onShow() throws Exception
   {
      super.onShow();
      setLayout(new VerticalLayout());

      // input row: field (absorbs the width) + button (natural size)
      Component entrada = new Component();
      entrada.setLayout(new HorizontalLayout());
      campo = new TextField().caption("Nova tarefa");
      Button add = new Button("Adicionar");
      entrada.add(LAY_FILL, campo);
      entrada.add(LAY_AUTO, add);
      add(LAY_AUTO, entrada);

      // virtual list: each cell is 3x the font height
      lista = new VirtualListView(Relative.ofFont(3f), new DataAdapter()
      {
         @Override public int getCount() { return tarefas.size(); }
         @Override public Component createView() { return new Label(""); }
         @Override public void bindView(Component view, int index)   // view first, then index
         {
            ((Label) view).setText(tarefas.get(index));   // direct cast
         }
      });
      add(LAY_FILL, lista);

      add.onAction(c -> adicionar());
   }

   private void adicionar()
   {
      String texto = campo.getText().trim();
      if (texto.isEmpty())
      {
         Hint.show("Digite uma tarefa", 1500);   // always 2 args
         return;
      }
      tarefas.add(texto);
      campo.clearText();
      lista.notifyDataChanged();   // re-measures and repaints the list
   }
}
```

> The snippets below show **only the core** (what goes inside `onShow()`). To run them, place them in the `MainWindow` skeleton above.

## 1. The screen skeleton (VerticalLayout)
```java
public class TarefasScreen extends Window {
   private final List<Tarefa> tarefas = new ArrayList<>();   // state in Window fields
   private Component listaBox;
   private TextField campo;

   @Override public void onShow() throws Exception {
      super.onShow();
      setLayout(new VerticalLayout());

      // input row (field + button)
      Component entrada = new Component();
      entrada.setLayout(new HorizontalLayout());
      campo = new TextField().caption("Nova tarefa");
      Button add = new Button("Adicionar");
      entrada.add(LAY_FILL, campo);       // field absorbs the width
      entrada.add(LAY_AUTO, add);         // button sized to its content
      add(LAY_AUTO, entrada);

      // list area, which absorbs the rest of the height
      listaBox = new Component();
      listaBox.setLayout(new VerticalLayout());
      add(LAY_FILL, listaBox);

      add.onAction(c -> adicionar());
      carregar();                          // populates from the database
   }
}
```

## 2. Add and reflect in the UI (no reactive binding)
```java
private void adicionar() {
   String texto = campo.getText().trim();
   if (texto.isEmpty()) return;
   Tarefa t = new Tarefa(texto);
   tarefas.add(t);
   salvar(t);              // persists (step 4)
   campo.clearText();
   renderizarLista();      // you update the UI manually
}

private void renderizarLista() {
   listaBox.removeAll();
   for (Tarefa t : tarefas) {
      Button item = new Button(t.titulo);
      item.onAction(c -> new DetalheScreen(t).show());   // navigation (step 3)
      listaBox.add(LAY_AUTO, item);
   }
   listaBox.setValidLayout(false);   // re-measures the layout
   Component.repaint();              // requests a new frame
}
```

## 3. Push/pop navigation
```java
public class DetalheScreen extends Window {
   private final Tarefa tarefa;
   public DetalheScreen(Tarefa t) { this.tarefa = t; }   // data via the constructor
   @Override public void onShow() throws Exception {
      super.onShow();
      setLayout(new VerticalLayout());
      add(LAY_AUTO, new Label(tarefa.titulo));
      Button voltar = new Button("Voltar");
      voltar.onAction(c -> hide());     // pop
      add(LAY_AUTO, voltar);
   }
}
```

## 4. Persistence in SQLite (off the paint thread)
```java
private void carregar() {
   new Thread(() -> {
      List<Tarefa> carregadas = new ArrayList<>();
      try (SqliteDb db = SqliteDb.open("tarefas.db")) {
         db.exec("CREATE TABLE IF NOT EXISTS tarefa(id INTEGER PRIMARY KEY, titulo TEXT)");
         try (SqliteCursor c = db.query("SELECT id, titulo FROM tarefa ORDER BY id")) {
            while (c.next())                                   // getters 0-based
               carregadas.add(new Tarefa(c.getInt(0), c.getString(1)));
         }
      } catch (Exception e) { Debug.handle(e); }
      Component.onUI(() -> {                                   // back to the paint thread
         tarefas.clear(); tarefas.addAll(carregadas);
         renderizarLista();
      });
   }).start();
}

private void salvar(Tarefa t) {
   new Thread(() -> {
      try (SqliteDb db = SqliteDb.open("tarefas.db")) {
         db.execute("INSERT INTO tarefa(titulo) VALUES(?)", t.titulo);   // bind 1-based
      } catch (Exception e) { Debug.handle(e); }
   }).start();
}
```
> **Two golden rules of SQLite:** parameter binding is **1-based**, cursor getters are
> **0-based**; and the database runs **off** the paint thread, with the result coming back via
> `Component.onUI(...)`.

## What you built
An app that **opens, adds, navigates and persists** — and you've already seen the pillars that the
[`learn/` course](/docs/learn) goes deeper into.

## Next steps
→ [Anatomy of a project](/docs/get-started/project-structure)

## See also
- [Layouts](/docs/ui/layouts) · [Navigation](/docs/ui/navigation) · [SQLite](/docs/data-and-services/storage-sqlite)