Fragment は、Activity内に存在する再利用可能なUIの一部分であり、独自のライフサイクルを持つモジュール化された自己完結型のUIコンポーネントです。Fragmentは柔軟で再利用可能なUIを実現し、現代的なシングルActivityアーキテクチャの中心となるものです。
Fragmentとは何か
A FRAGMENT = a reusable, modular piece of UI hosted inside an Activity:
→ has its OWN lifecycle (tied to but distinct from the host activity's)
→ multiple fragments can be combined in one activity; reused across activities
→ represents a portion of the screen (or a whole screen's content)
→ Think: an activity can host one or more fragments that make up its UI.
FragmentとActivityの違い
ACTIVITY → a top-level screen/entry point; heavier; managed by the OS; declared in manifest
FRAGMENT → a UI portion WITHIN an activity; lighter, reusable, modular; managed by the
activity's FragmentManager; NOT declared in the manifest
→ Fragments are more flexible/reusable; activities are the containers/entry points.
