XML Views

The legacy, imperative way of building UIs in Android relies on XML layouts and View classes (e.g., TextView, RecyclerView, ConstraintLayout).

Key XML Concepts

1. View Hierarchies

XML layouts are structured as trees.

• Avoid deep hierarchies as they negatively impact measurement and drawing performance.
• ConstraintLayout is preferred to flatten the view hierarchy, allowing complex layouts without nesting view groups.

2. ViewBinding

Replaces findViewById, providing null safety and type safety.

• It generates a binding class for each XML layout.
• Directly links to the views via their IDs, avoiding runtime crashes.

3. RecyclerView

Essential for displaying lists.

• Avoids memory exhaustion by recycling off-screen ViewHolder views.
• Requires an Adapter (creates ViewHolders and binds data) and a LayoutManager (positions items).
• Use ListAdapter with DiffUtil for calculating list changes efficiently on a background thread and animating the differences.

4. Custom Views

You can extend existing views (TextView) or View/ViewGroup directly.

• Override onMeasure() to dictate the view's size.
• Override onDraw() using a Canvas for custom graphics.
• Extract custom attributes into attrs.xml for XML configuration.

Best Practices

1. Accessibility: Always use android:contentDescription on ImageViews for TalkBack support. Use minHeight="48dp" for touch targets.
2. Styles and Themes: Do not hardcode colors in XML. Use ?attr/colorPrimary or @style/MyCustomText. Extract common styles into styles.xml.
3. Strings: Never hardcode text strings. Use @string/my_string to support localization.

Interview Questions

Q: What is the difference between View.GONE and View.INVISIBLE? Answer: View.INVISIBLE hides the view, but it still takes up space in the layout hierarchy. View.GONE completely removes the view from the layout calculations, collapsing the space it previously occupied.

Q: Explain how DiffUtil works. Answer: DiffUtil is a utility class that calculates the difference between two lists and outputs a list of update operations that converts the first list into the second. It determines exactly which items were added, removed, or changed, drastically improving RecyclerView update performance compared to notifyDataSetChanged().

Đọc sâu và thực hành

Custom View: MeasureSpec, Canvas và accessibility.