SpannedGridLayoutManager

SpannedGridLayouManager is a layout manager that will resize and reorder views based on a provided SpanSize.

IMPORTANT: as the goal of this LayoutManager is to fill all gaps if possible, views may be placed in an order which may not be related to their positions. That is, if View #9 can fill a gap before View #8, it will be placed in an previous position, ignoring the normal ordering.

Usage

Gradle dependency:

dependencies {
 implementation 'com.arasthel:spannedgridlayoutmanager:3.0.2'
}

When you create a new SpannedGridLayoutManager you must provide:

• An Orientation: Orientation.VERTICAL or Orientation.HORIZONTAL.
• The span count to divide the layout.

Kotlin Example:

val spannedGridLayoutManager = SpannedGridLayoutManager(
                orientation = SpannedGridLayoutManager.Orientation.VERTICAL, 
                spans = 4)

Java Example:

SpannedGridLayoutManager spannedGridLayoutManager = new SpannedGridLayoutManager(
   SpannedGridLayoutManager.Orientation.VERTICAL, 4);

To set the SpanSize for each view, you need to use SpannedGridLayoutManager.SpanSizeLookup. This class has a lambda that receives an adapter position and must return an SpanSize:

Kotlin:

spannedGridLayoutManager.spanSizeLookup = SpannedGridLayoutManager.SpanSizeLookUp { position -> 
    SpanSize(2, 2)
}
recyclerview.layoutManager = spannedGridLayoutManager

Java:

// If your project has Java 8 support for lambdas
spannedGridLayoutManager.setSpanSizeLookup(new SpannedGridLayoutManager.SpanSizeLookup({ position ->
    new SpanSize(2, 2);
}));

// If your project uses Java 6 or 7. Yup, it's ugly as hell
spannedGridLayoutManager.setSpanSizeLookup(new SpannedGridLayoutManager.SpanSizeLookup(new Function1<Integer, SpanSize>(){
  @Override public SpanSize invoke(Integer position) {
    return new SpanSize(2, 2);
  }
}));

recyclerview.setLayoutManager(spannedGridLayoutManager);

If the logic needed to calculate this values is very complex, they can be cached by setting spanSizeLookup.usesCache = true or spanSizeLookup.setUsesCache(true). If you need to invalidate them, just call spanSizeLookup.invalidateCache().

About restoring scroll position:

As this LayoutManager may change the order in screen of its items, it has some issues with restoring scroll position when the sizes of the items change drastically. To work around this, restoring the scroll position when recreating this LayoutManager is disabled by default.

If you are fairly sure that the position of the items won't change much, you can enable it again using:

spannedLayoutManager.itemOrderIsStable = true

Migrating from 1.X to 2.X

Due to critical layout issues, the API for using SpanSizes had to change. The only changes you should have to do in your code are:

    val width = 1
    val height = 2

    // OLD
    // holder.itemView.layoutParams = RecyclerView.LayoutParams(width, height)

    // NEW
    holder.itemView.layoutParams = SpanLayoutParams(SpanSize(width, height))

Just use the new SpanLayoutParams instead of generic RecyclerView.LayoutParams.

Migrating from 2.X to 3.X

Sadly, due to more limitations of the initial design and bugs, most of the LayoutManager's layouting and recycling process had to be re-written. This depended on some breaking API changes since otherwise there would have been lots of unnecessary recycling of views and performance loss when the spans are calculated.

Because of this, while you would set the SpanSizes using SpanLayoutParams, these are no longer needed. You can safely delete those from your adapter.

Instead of that, you must use the SpannedGridLayoutManager.SpanSizeLookup like you would do with a GridLayoutManager. You can find more info on the Usage section.

Animations

To have animations as shown in the sample, you must:

• Use setHasStableIds(true) in your adapter.
• Override getItemId method to return a stable id - that is, it won't change for the same position.
• Use adapter.notifyDatasetChanged() to trigger the layout process.

Implementation details

This library uses Rects to find the empty gaps and choose where a view will be placed. The underlying algorithm is explained in this paper.

• Initially, there will be 1 free-space Rect (0, 0, spanCount, Int.MAX_VALUE) for Orientation.VERTICAL or (0, 0, Int.MAX_VALUE, spanCount) for Orientation.HORIZONTAL.
• When a view must added, it will search for the free rects that are adjacent to the view's Rect or that intersects it.
• It will iterate over these rects looking for those that are adjacent to the view and don't contain it, which will be stored. If a rect doesn't match this criteria, it will be removed from the list of free rects and divided in 4 possible sub-rects - left, top, right, bottom - like this:

• For each of that subrects, if none of the previously stored adjacent free rects and none of the rects in the list of free rects intersects with it, it will be added to this list as a valid free rect.

You can see this code in SpannedGridLayoutManager's subtract(Rect) method.

In version 3.0 however, to ensure no layout issues happened, the whole rects are pre-calculated and cached on every relayout of the whole LayoutManager. These are some elapsed time results that I got from testing:

Android emulator: 
    - 500 items: 10ms
    - 1000 items: 18ms
    - 10000 items: 35ms
    - 50000 items: 128ms
    
OnePlus 3T:
    - 500 items: 12ms
    - 1000 items: 20ms
    - 10000 items: 135ms
    - 50000 items: 530ms
    
OnePlus 6T:
    - 500 items: 5ms
    - 1000 items: 10ms
    - 10000 items: 55ms
    - 50000 items: 200ms

They aren't that bad, but they're also not great for huge amounts of items and could cause performance issues if you have infinite scrolling. I will try to improve this and calculate rects by batches, although this might not give exact results in the placement of the views.

License - MIT

MIT License

Copyright © 2017 Jorge Martín Espinosa

Permission is hereby granted, free of charge, to any person obtaining a copy
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