v0.3.0 "CSS Grid"

Highlights

• CSS Grid algorithm support
• Style helper functions

See below for details of breaking changes.

New Feature: CSS Grid

We very excited to report that we now have support for CSS Grid layout. This is in addition to the existing Flexbox layout support, and the two modes interoperate. You can set a node to use Grid layout by setting the display property to Display::Grid.

Learning Resources

Taffy implements the CSS Grid specification faithfully, so documentation designed for the web should translate cleanly to Taffy's implementation. If you are interested in learning how to use CSS Grid, we would recommend the following resources:

• CSS Grid Garden. This is an interactive tutorial/game that allows you to learn the essential parts of CSS Grid in a fun engaging way.
• A Complete Guide To CSS Grid by CSS Tricks. This is detailed guide with illustrations and comphrehensive written explanation of the different Grid propertie and how they work.

Supported Features & Properties

In addition to the usual sizing/spacing proerties (size, min_size, padding, margin, etc), the following Grid style properties are supported on Grid Containers:

Property	Explanation
grid-template-columns	The track sizing functions of the grid's explicit columns
grid-template-rows	The track sizing functions of the grid's explicit rows
grid-auto-rows	Track sizing functions for the grid's implicitly generated rows
grid-auto-columns	Track sizing functions for the grid's implicitly generated columns
grid-auto-flow	Whether auto-placed items are placed row-wise or column-wise. And sparsely or densely.
gap	The size of the vertical and horizontal gaps between grid rows/columns
align-content	Align grid tracks within the container in the inline (horizontal) axis
justify-content	Align grid tracks within the container in the block (vertical) axis
align-items	Align the child items within their grid areas in the inline (horizontal) axis
justify-items	Align the child items within their grid areas in the block (vertical) axis

And the following Grid style properties are supported on Grid Items (children):

Property	Explanation
grid-row	The (row) grid line the item starts at (or a span)
grid-column	The (column) grid line the item end at (or a span)
align-self	Align the item within it's grid area in the inline (horizontal) axis. Overrides align-items.
justify-self	Align the item within it's grid area in the block (vertical) axis. Overrides justify-items.

The following properties and features are not currently supported:

• Subgrids
• Masonry grid layout
• Named grid lines
• Named areas: grid-template-areas and grid-area
• grid-template or grid shorthand

Example

See examples/grid_holy_grail.rs for an example using Taffy to implement the so-called Holy Grail Layout. If you want to run this example, the don't forget the enable the CSS Grid cargo feature:

cargo run --example grid_holy_grail --features grid

New Feature: Style Helpers

Ten new helper functions have added to the taffy prelude. These helper functions have short, intuitive names, and have generic return types which allow them to magically return the correct type depending on context. They make defining styles much easier, and means you won't typically need to use types like Dimension or TrackSizingFunction directly.

For example, instead of:

let size : Size<Dimension> = Size { width: Dimension::Points(100.0), height: Dimension::Percent(50.0) };

you can now write

let size : Size<Dimension> = Size { width: points(100.0), height: percent(50.0) };

And that same helper function will work other types like LengthPercentage and MinTrackSizingFunction that also have a Points variant. There are also generic impl's for Size<T>, Rect<T> and Line<T> which means if your node is the same size in all dimensions you can even write

let size : Size<Dimension> = points(100.0);

Available style helpers:

Type(s)	Helpers that work with that type
LengthPercentage	zero()	Generates a Points variant with the value 0.0
	points(val: f32)	Generates a Points variant with the specified value
	percent(val: f32)	Generates a Percent variant with the specified value. Note that the scale of 0-1 not 0-100.
LengthPercentageAuto Dimension	All helpers from LengthPercentage and...
	auto()	Generates an Auto variant
MinTrackSizingFunction	All helpers from LengthPercentageAuto/Dimension and...
	min_content()	Generates an MinContent variant
	max_content()	Generates an MinContent variant
MaxTrackSizingFunction	All helpers from MinTrackSizingFunction and...
	fit_content(limit: LengthPercentage)	Generates a FitContent variant with the specified limit. Nest the points or percent helper inside this function to specified the limit.
	fr(fraction: f32)	Generates a Fraction (fr) variant with the specified flex fraction
NonRepeatingTrackSizingFunction	All helpers from MaxTrackSizingFunction and...
	minmax(min: MinTrackSizingFunction, max: MaxTrackSizingFunction)	Equivalent to CSS minmax() function.
	flex(fraction: f32)	Equivalent to CSS minmax(0px, 1fr). This is likely what you want if you want evenly sized rows/columns.
TrackSizingFunction	All helpers from NonRepeatingTrackSizingFunction and...
	repeat(rep: GridTrackRepetition, tracks: Vec<TrackSizingFunction>)	Equivalent to css repeat() function.
Vec<TrackSizingFunction>	evenly_sized_tracks(count: u16)	Equivalent to CSS repeat(count, minmax(0px, 1fr)
AvailableSpace	auto()	Generates an Auto variant
	min_conte...

0.2.2

0.2.2

Fixes

• Border or padding on the horizontal axis could, in some cases, increase the height of nodes.

0.2.1

0.2.1

Fixes

• In case of conflicts, min_size now overrides max_size which overrides size (#261). This is the behaviour specified in the CSS specification, and was also the behaviour in Taffy v0.1.0, but a regression was introduced in Taffy v0.2.0.
• taffy::compute_layout has been made public allowing Taffy to be used with custom storage (#263)

0.2.0

0.2.0

New features

Flexbox "gap" and AlignContent::SpaceEvenly

The gap property is now supported on flex containers. This can make it much easier to create even spacing or "gutters" between nodes.

Additionally we have a SpaceEvenly variant to the AlignContent enum to support evenly spaced justification in the cross axis (equivalent to align-content: space-evenly in CSS)

Debug module and cargo feature

Two debugging features have been added:

• taffy::debug::print_tree(&Taffy, root) - This will print a debug representation of the computed layout of an entire node tree (starting at root), which can be useful for debugging layouts.
• A cargo feature debug. This enabled debug logging of the layout computation process itself (this is probably mainly useful for those working taffy itself).

Performance improvements

A number of performance improvements have landed since taffy 0.1:

• Firstly, our custom taffy::forest storage implementation was ripped out and replaced with a much simpler implementation using the slotmap crate. This led to performance increases of up to 90%.
• Secondly, the caching implementation was improved by upping the number of cache slots from 2 to 4 and tweaking how computed results are allocated to chache slots to better match the actual usage patterns of the flexbox layout algorithm. This had a particularly dramatic effect on deep hierachies (which often involve recomputing the same results repeatedly), fixing the exponential blowup that was previously exhibited on these trees and improving performance by over 1000x in some cases!

Benchmarks vs. Taffy 0.1

Benchmark	Taffy 0.1	Taffy 0.2	% change (0.1 -> 0.2)
wide/1_000 nodes (2-level hierarchy)	699.18 µs	445.01 µs	-36.279%
wide/10_000 nodes (2-level hierarchy)	8.8244 ms	7.1313 ms	-16.352%
wide/100_000 nodes (2-level hierarchy)	204.48 ms	242.93 ms	+18.803%
deep/4000 nodes (12-level hierarchy))	5.2320 s	2.7363 ms	-99.947%
deep/10_000 nodes (14-level hierarchy)	75.207 s	6.9415 ms	-99.991%
deep/100_000 nodes (17-level hierarchy)	-	102.72 ms	-
deep/1_000_000 nodes (20-level hierarchy)	-	799.35 ms	-

(note that the table above contains multiple different units (milliseconds vs. microseconds vs. nanoseconds))

As you can see, we have actually regressed slightly in the "wide" benchmarks (where all nodes are siblings of a single parent node). Although it should be noted our results in these benchmarks are still very fast, especially on the 10,000 node benchmark which we consider to be the most realistic size where the result is measured in microseconds.

However, in the "deep" benchmarks we see dramatic improvements. The previous version of Taffy suffered from exponential blowup in the case of deeply nested hierachies. This has resulted in somewhat silly improvements like the 10,000 node (14-level) hierachy where Taffy 0.2 is a full 1 million times faster than Taffy 0.1. We've also included results with larger numbers of nodes (although you're unlikely to need that many) to demonstrate that this scalability continues up to even deeper levels of nesting.

Benchmarks vs. Yoga

Yoga benchmarks run via it's node.js bindings (the yoga-layout-prebuilt npm package), they were run a few times manually and it was verified that variance in the numbers of each run was minimal. It should be noted that this is using an old version of Yoga.

Benchmark	Yoga	Taffy 0.2
yoga/10 nodes (1-level hierarchy)	45.1670 µs	33.297 ns
yoga/100 nodes (2-level hierarchy)	134.1250 µs	336.53 ns
yoga/1_000 nodes (3-level hierarchy)	1.2221 ms	3.8928 µs
yoga/10_000 nodes (4-level hierarchy)	13.8672 ms	36.162 µs
yoga/100_000 nodes (5-level hierarchy)	141.5307 ms	1.6404 ms

(note that the table above contains multiple different units (milliseconds vs. microseconds vs. nanoseconds))

While we're trying not to get too excited (there could easily be an issue with our benchmarking methodology which make this an unfair comparison), we are pleased to see that we seem to be anywhere between 100x and 1000x times faster depending on the node count!

Breaking API changes

Node creation changes

• taffy::Node is now unique only to the Taffy instance from which it was created.
• Renamed Taffy.new_node(..) -> Taffy.new_with_children(..)
• Renamed Taffy.new_leaf() -> Taffy.new_leaf_with_measure()
• Added taffy::node::Taffy.new_leaf() which allows the creation of new leaf-nodes without having to supply a measure function

Error handling/representation improvements

• Renamed taffy::Error -> taffy::error::TaffyError
• Replaced taffy::error::InvalidChild with a new InvalidChild variant of taffy::error::TaffyError
• Replaced taffy::error::InvalidNode with a new InvalidNode variant of taffy::error::TaffyError
• The following method new return Err(TaffyError::ChildIndexOutOfBounds) instead of panicking:
  • taffy::Taffy::remove_child_at_index
  • taffy::Taffy::replace_child_at_index
  • taffy::Taffy::child_at_index
• Taffy::remove now returns a Result<usize, Error>, to indicate if the operation was sucessful (and if it was, which ID was invalidated).

Some uses of Option<f32> replaced with a new AvailableSpace enum

A new enum Taffy::layout::AvailableSpace has been added.

The definition looks like this:

/// The amount of space available to a node in a given axis
pub enum AvailableSpace {
    /// The amount of space available is the specified number of pixels
    Definite(f32),
    /// The amount of space available is indefinite and the node should be laid out under a min-content constraint
    MinContent,
    /// The amount of space available is indefinite and the node should be laid out under a max-content constraint
    MaxContent,
}

This enum is now used instead of Option<f32> when calling Taffy.compute_layout (if you previously passing Size::NONE to compute_layout, then you will need to change this to Size::MAX_CONTENT).

And a different instance of it is passed as a new second parameter to MeasureFunc. MeasureFuncs may choose to use this parameter in their computation or ignore it as they see fit. The canonical example of when it makes sense to use it is when laying out text. If MinContent has been passed in the axis in which the text is flowing (i.e. the horizontal axis for left-to-right text), then you should line-break at every possible opportunity (e.g. all word boundaries), whereas if MaxContent has been passed then you shouldn't line break at all..

Builder methods are now const where possible

• Several convenience constants have been defined: notably Style::DEFAULT
• Size<f32>.zero() is now Size::<f32>::ZERO
• Point<f32>.zero() is now Point::<f32>::ZERO
• Size::undefined() is now Size::NONE

Removals

• Removed taffy::forest::Forest. taffy::node::Taffy now handles it's own storage using a slotmap (which comes with a performance boost up to 90%).
• Removed taffy::number::Number. Use Option<f32> is used instead
  • the associated public MinMax and OrElse traits have also been removed; these should never have been public
• Removed unused dependencies hashbrown, hash32, and typenum. slotmap is now the only required dependency (num_traits and arrayvec are also required if you wish to use taffy in a no_std environment).

Fixes

• Miscellaneous correctness fixes which align our implementation with Chrome:

  • Nodes can only ever have one parent
  • Fixed rounding of fractional values to follow latest Chrome - values are now rounded the same regardless of their position
  • Fixed computing free space when using both flex-grow and a minimum size
  • Padding is now only subtracted when determining the available space if the node size is unspecified, following section 9.2.2 of the flexbox spec
  • MeasureFunc (and hence NodeData and hence Forest and hence the public Taffy type) are now Send and Sync, enabling their use in async and parallel applications

• Taffy can now be vendored using cargo-vendor (README.md is now included in package).

0.1.0

0.1.0

This is the first release of Taffy, but Taffy is a continuation of an older abandoned library stretch

0.1.0 Changed

• the order field of Layout is now public, and describes the relative z-ordering of nodes
• renamed crate from stretch2 to taffy
• updated to the latest version of all dependencies to reduce upstream pain caused by duplicate dependencies
• renamed stretch::node::Strech -> taffy::node::Taffy

0.1.0 Fixed

• fixed feature strategy for alloc and std: these can now be compiled together, with std's types taking priority

0.1.0 Removed

• removed Javascript / Kotlin / Swift bindings
  • the maintainer team lacks expertise to keep these working
  • more serious refactors are planned, and this will be challenging to keep working through that process
  • if you are interested in helping us maintain bindings to other languages, get in touch!
• the serde_camel_case and serde_kebab_case features have been removed: they were poorly motivated and were not correctly additive (if both were enabled compilation would fail)
• removed the Direction and Overflow structs, and the corresponding direction and overflow fields from Style
  • these had no effect in the current code base and were actively misleading

stretch2 0.4.3

A version of this library was also released as stretch2. The following notes describe the differences between this release and stretch 0.3.2, the abandoned crate from which this library was forked.

Changed

• updated assorted dependencies

Fixed

• fixed an exponential performance blow-up with deep nesting
• fixed percent height values, which were using parent width
• recomputing layout no longer moves children of non-zero-positioned parent
• fixed broken Swift bindings