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Deep Dive Into LayoutManager for RecyclerView
This document presents a deep dive into creating custom LayoutManagers for RecyclerViews by Takeshi Hagikura, highlighting important methods and considerations such as onLayoutChildren, fill, and scroll logic. It emphasizes the complexity of building custom layouts compared to custom views, and discusses the recycling mechanism along with item decorations and predictive animations. Additional resources and a thank you note are included at the end.
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Similar to Deep Dive Into LayoutManager for RecyclerView
Deep Dive Into LayoutManager for RecyclerView
- 1. thagikura Deep Dive IntoLayoutManager for RecyclerView
- 2. $ whoami Takeshi Hagikura @thagikura DeveloperRelations @Google
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- 8. Is it possibleto add a custom LayoutManager?
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- 11. Ports Flexbox inCSS to Android
- 12. How to createa custom LayoutManager?
- 13. Think before youbuild it Much harder than custom view Gives you much flexibility on top of RecyclerView features
- 15. At the bareminimum, need to override…
- 16. onLayoutChildren Called on: - initiallayout - adapter changes Lay out all relevant child views
- 19. If this isViewGroup (exceptRecyclerView) All child views on memory
- 20. In RecyclerView + LayoutManager Onlyvisible views should be on memory
- 21. onLayoutChildren 1. Find theanchor position
- 22. Anchor position Starting positionto put views. Start with 0 on initial layout. Remember the first visible view to restore the state
- 23. onLayoutChildren 1. Find theanchor position 2. Calculate how many items should be in visible portion
- 24. onLayoutChildren 1. Find theanchor position 2. Calculate how many items should be in visible portion 3. Fill toward end
- 25. private int fill(RecyclerView.Recyclerrecycler, RecyclerView.State state, LayoutState layoutState) { int start = layoutState.mAvailable; int remainingSpace = layoutState.mAvailable; int consumed = 0; while (remainingSpace > 0 && layoutState.hasMore(state, mFlexLines)) { FlexLine flexLine = mFlexLines.get(layoutState.mFlexLinePosition); layoutState.mPosition = flexLine.mFirstIndex; consumed += layoutFlexLine(flexLine, layoutState); remainingSpace -= flexLine.getCrossSize(); } layoutState.mAvailable -= consumed; return start - layoutState.mAvailable; } Simplified version of fill
- 26. private int fill(RecyclerView.Recyclerrecycler, RecyclerView.State state, LayoutState layoutState) { int start = layoutState.mAvailable; int remainingSpace = layoutState.mAvailable; int consumed = 0; while (remainingSpace > 0 && layoutState.hasMore(state, mFlexLines)) { FlexLine flexLine = mFlexLines.get(layoutState.mFlexLinePosition); layoutState.mPosition = flexLine.mFirstIndex; consumed += layoutFlexLine(flexLine, layoutState); remainingSpace -= flexLine.getCrossSize(); } layoutState.mAvailable -= consumed; return start - layoutState.mAvailable; } mAvailable: Amount of pixels need to be filled mDirection: START | END
- 27. private int fill(RecyclerView.Recyclerrecycler, RecyclerView.State state, LayoutState layoutState) { int start = layoutState.mAvailable; int remainingSpace = layoutState.mAvailable; int consumed = 0; while (remainingSpace > 0 && layoutState.hasMore(state, mFlexLines)) { FlexLine flexLine = mFlexLines.get(layoutState.mFlexLinePosition); layoutState.mPosition = flexLine.mFirstIndex; consumed += layoutFlexLine(flexLine, layoutState); remainingSpace -= flexLine.getCrossSize(); } layoutState.mAvailable -= consumed; return start - layoutState.mAvailable; } Put views and loop until mAvailable < 0
- 28. onLayoutChildren 1. Find theanchor position 2. Calculate how many items should be in visible portion 3. Fill toward end 4. Fill toward start
- 29. Why fill towardboth directions?
- 30. In some casesanchor position may be at the bottom
- 31. 1 2 3 4 5 67 8 9 10 11 scrollToPosition(50) 42 43 44 45 46 47 48 49 50 Anchor position
- 32. scrollVerticallyBy Interact with scrollby the user
- 33. @Override public boolean canScrollVertically(){ return true; } Could be false depending on attributes. E.g. Orientation in LinearLayoutManager
- 34. @Override public int scrollVerticallyBy(intdy, RecyclerView.Recycler recycler, RecyclerView.State state) { if (isMainAxisDirectionHorizontal()) { int scrolled = handleScrollingCrossAxis(dy, recycler, state); mViewCache.clear(); return scrolled; } else { int scrolled = handleScrollingMainAxis(dy); mAnchorInfo.mPerpendicularCoordinate += scrolled; mSubOrientationHelper.offsetChildren(-scrolled); return scrolled; } }
- 35. @Override public int scrollVerticallyBy(intdy, RecyclerView.Recycler recycler, RecyclerView.State state) { if (isMainAxisDirectionHorizontal()) { int scrolled = handleScrollingCrossAxis(dy, recycler, state); mViewCache.clear(); return scrolled; } else { int scrolled = handleScrollingMainAxis(dy); mAnchorInfo.mPerpendicularCoordinate += scrolled; mSubOrientationHelper.offsetChildren(-scrolled); return scrolled; } } Distance to scroll in pixels
- 36. @Override public int scrollVerticallyBy(intdy, RecyclerView.Recycler recycler, RecyclerView.State state) { if (isMainAxisDirectionHorizontal()) { int scrolled = handleScrollingCrossAxis(dy, recycler, state); mViewCache.clear(); return scrolled; } else { int scrolled = handleScrollingMainAxis(dy); mAnchorInfo.mPerpendicularCoordinate += scrolled; mSubOrientationHelper.offsetChildren(-scrolled); return scrolled; } } The actual distance scrolled
- 37. Small dy Only shiftexisting views mRecyclerView.offsetChildrenVertical(dy);
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- 39. Large dy Recycle viewsno longer visible
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- 41. What does Recyclemean?
- 42. Two types ofcache in RecyclerView 1. Recycle 2. Scrap
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- 46. RecyclerRecycler RecycledViewPool Can be sharedacross multiple RecyclerViews
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- 50. How to retrieveaView?
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- 52. Case1. Found inscrap Case2. Found in Recycled Pool Case3. Not found in cache
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- 54. LayoutManager Adapter Recycler Case2. Foundin Recycled Pool adapter.bindViewHolder
- 55. LayoutManager Adapter Recycler Case2. Foundin Recycled Pool
- 56. LayoutManager Adapter Recycler Case3. Notfound in cache adapter.createViewHolder
- 57. LayoutManager Adapter Recycler Case3. Notfound in cache
- 58. Use scrap ifyou re-attach in a same layout pass Use recycle for views no longer needed
- 59. scrollHorizontallyBy Same with verticalexcept for the direction
- 60. Now you implementbasic functions. Is it enough?
- 61. Could be useable...but NO.
- 62. scrollTo(position) smoothScrollTo(position) Fast Scrolling Item Decorations Itemprefetch Predictive Animations There are a lot more…
- 63. scrollTo(position) smoothScrollTo(position) Fast Scrolling Item Decorations Itemprefetch Predictive Animations There are a lot more…
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- 67. @Override public boolean supportsPredictiveItemAnimations(){ return true; }
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- 69. In pre-layout Lay outall the views currently visible and additional views that are appearing (or disappearing) after the animation
- 70. Pre-layout Remaining Appearing Disappearing To be inserted atthis positionVisible area
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- 74. Now you knowthe basics of building a custom LayoutManager
- 75. Resources Source of defaultLayoutManagers github.com/google/flexbox-layout wiresareobsolete.com/2014/09/building-a-recyclerview- layoutmanager-part-1/
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