Lazy Layouts in Compose - Overview

Speaker: Simona Stojanovic and Mihai Popa

Topics Covered:

• Introduction to Lazy Layouts in Compose
• Basic and advanced use cases
• Implementation of lazy lists and grids
• Adding items and animating content
• New and upcoming features
• Optimization tips and performance insights

Basics of Lazy Layouts:

• Concept: Render a scrollable list of items as they become visible on the screen, enhancing app performance and responsiveness.
• Components:
  • Lazy Column: Vertically scrolling list.
  • Lazy Row: Horizontally scrolling list.
  • Lazy Grids: Support both vertical and horizontal scrolling.

Differences from RecyclerView:

• RecyclerView requires more code (adapter, view holder, XML layout, etc.).
• Lazy Layouts in Compose require fewer lines of code, making them easier and simpler to manage.
• Code Comparison: A significant reduction in code when using Lazy Lists.

Adding Items to Lazy Lists:

• Use lazyListScope DSL block to add items.
• Two ways to insert items:
  • item Block: For a single item.
  • items Block: For multiple items.
• Can use a combination to describe list content.
• Use itemsIndexed block to access item indexes for customizations (e.g., different background for even/odd items).

Customizing List Appearance:

• **Padding: **
  • Use the padding modifier for overall indentation.
  • Use contentPadding parameter to set padding that scrolls content within list bounds without clipping.
• Spacing: Use Arrangement.spacedBy to add spacing between items.

Observing and Reacting to Scroll Events:

• LazyListState: Stores scroll position and contains useful list information.
• Use rememberLazyListState to hoist state.
• Provides firstVisibleItemIndex and firstVisibleItemScrollOffset properties for scroll reactions.
• To optimize unnecessary recompositions, use remember and derivedStateOf.
• LayoutInfo Object: Provides info like visible items and total item count.

Practical Example - Scroll-to-Top Button:

• Use scrollToItem for snapping to an item.
• Use animateScrollToItem for smooth transitions.
• Both functions are suspend functions and need to be called within a coroutine scope.

Lazy Grids:

• Rework and Graduation: From experimental to stable in Compose 1.2.
• Usage: LazyVerticalGrid and LazyHorizontalGrid composables.
  • Vertically scrollable container with multiple columns (LazyVerticalGrid).
  • Horizontally scrollable container (LazyHorizontalGrid).
• Grid Description: Similar to lazy columns but with added column descriptions.
• Adaptive Sizing: Use GridCells.Adaptive API for varying screen sizes.
• Custom Column Sizing: Implement GridCells for custom widths, catering to specific requirements.
• Custom Column Spans: Specify via span parameter for headers or highlighted items.

Advanced Customizations:

• LazyLayout API: Introduced for creating custom lazy layouts.
• Use Cases: E.g., Wear OS-specific lists.
• Example: Staggered grids using LazyLayout API.

Animating List Item Changes:

• Item Reordering Animations: Available in Compose 1.1 for lists and 1.2 for grids.
• Use animateItemPlacement modifier for item content animations.
• Ensure items have keys for accurate reordering.

Optimization Tips:

• Avoid 0-pixel Sized Items: Retain default sizing for accurate viewport calculations.
• Nesting Components: Avoid nesting scrollable components in the same direction as it can negatively impact performance.
• Multiple Elements in One Item: Avoid for performance reasons. Valid use cases include dividers.
• Custom Arrangements: Use for specific layout requirements and viewport positioning.
• Composition Reusing: Reuse compositions of similar structure, specify contentType for better performance.
• Prefetching: Compose and measure items earlier to avoid jank.

Conclusion:

• Lazy layouts provide performant lists and grids with fewer lines of code.
• Encourage to try out new features.

References:

• Further information and exploration opportunities provided.

Acknowledgments:

• Thanks to the speakers Simona Stojanovic and Mihai Popa for the detailed presentation.

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