1. The CrossFly big picture

What you’ll learn: the mental model that makes every upcoming choice in the course make sense. Prerequisites: having run the Hello World.

Throughout this course we build a single app — a to-do list with persistence — that grows with each page, from a static screen to a tested, responsive app ready to package.

The ideas that hold everything together

• Java + OpenGL. CrossFly is a Java app that draws via the GPU (OpenGL/WebGL). The look is identical across every platform — there are no native widgets.
• Single paint thread. All the UI and rendering happen on a single thread. Heavy work goes to a background thread and comes back with Component.onUI(...).
• Render on demand. The screen only redraws when you call repaint() — it saves battery.
• Everything is a Component. There’s no “widget vs container” hierarchy: any Component becomes a container when it receives a Layout.
• No imposed architecture. There’s no reactive binding and no dependency injection — you mutate objects and update the UI by hand.

The map of the pieces

Recap

CrossFly is imperative and object-oriented: create objects once, keep references, mutate, repaint(). Threading is explicit; the look is your GPU drawing, the same everywhere.

Next steps

→ Windows and navigation

See also

• Concepts: The layered architecture · Everything is a Component