Crate taffy

Taffy

Taffy is a flexible, high-performance library for UI layout. It currently implements the Block, Flexbox and Grid layout algorithms from the CSS specification. Support for other paradigms is planned. For more information on this and other future development plans see the roadmap issue.

Architecture

Taffy is based on a tree of “UI nodes” similar to the tree of DOM nodes that one finds in web-based UI. Each node has:

• A Style struct which holds a set of CSS styles which function as the primary input to the layout computations.
• A Layout struct containing a position (x/y) and a size (width/height) which function as the output of the layout computations.
• Optionally:
  • A Vec set of child nodes
  • “Context”: arbitary user-defined data (which you can access when using a “measure function” to integrate Taffy with other kinds of layout such as text layout)

Usage of Taffy consists of constructing a tree of UI nodes (with associated styles, children and context), then calling function(s) from Taffy to translate those styles, parent-child relationships and measure functions into a size and position in 2d space for each node in the tree.

High-level API vs. Low-level API

Taffy has two APIs: a high-level API that is simpler and easier to get started with, and a low-level API that is more flexible gives greater control.

We would generally recommend the high-level API for users using Taffy standalone and the low-level API for users wanting to embed Taffy as part of a wider layout system or as part of a UI framework that already has it’s own node/widget tree representation.

High-level API

The high-level API** consists of the TaffyTree struct which contains a tree implementation and provides methods that allow you to construct a tree of UI nodes. Once constructed, you can call the compute_layout_with_measure method to compute the layout (passing in a “measure function” closure which is used to compute the size of leaf nodes), and then access the layout of each node using the layout method.

When using the high-level API, Taffy will take care of node storage, caching and dispatching to the correct layout algorithm for a given node for you. See the TaffyTree struct for more details on this API.

Examples which show usage of the high-level API include:

• basic
• flexbox_gap
• grid_holy_grail
• measure

In particular, the “measure” example shows how to integrate Taffy layout with other layout modalities such as text or image layout when using the high level API.

Low-level API

The low-level API consists of a set of traits (notably the PartialLayoutTree trait) which define an interface behind which you must implement your own tree implementation, and a set of functions such as compute_flexbox_layout and compute_grid_layout which implement the layout algorithms (for a single node at a time), and are designed to be flexible and easy to integrate into a wider layout or UI system.

When using this API, you must handle node storage, caching, and dispatching to the correct layout algorithm for a given node yourself. See the PartialLayoutTree trait for more details on this API.

Examples which show usage of the high-level API are:

• custom_layout_tree_vec which implements a custom Taffy tree using a Vec as an arena with NodeId’s being index’s into the Vec.
• custom_layout_tree_owned which implements a custom Taffy tree using directly owned children with NodeId’s being pointers.

Re-exports

• pub use crate::geometry::*;
• pub use crate::style::*;
• pub use crate::tree::*;
• pub use crate::util::*;

Modules

• compute
  The layout algorithms themselves
• geometry
  Geometric primitives useful for layout
• prelude
  Commonly used types
• style
  A typed representation of CSS style properties in Rust. Used as input to layout computation.
• style_helpers
  Helper functions which it make it easier to create instances of types in the style and geometry modules.
• tree
  Contains both a high-level interface to Taffy using a ready-made node tree, and a trait for defining a custom node trees / utility types to help with that.
• util
  Helpful misc. utilities such as a function to debug print a tree

Structs

• Style
  A typed representation of the CSS style information for a single node.
• TaffyTree
  An entire tree of UI nodes. The entry point to Taffy’s high-level API.

Traits

• LayoutTree
  Extends PartialLayoutTree with an additional guarantee: that the child/children methods can be used to recurse infinitely down the tree. Enables Taffy’s rounding and debug printing methods to be used.
• PartialLayoutTree
  This if the core abstraction in Taffy. Any type that correctly implements PartialLayoutTree can be laid out using Taffy’s algorithms.

Functions

• compute_block_layout
  Computes the layout of PartialLayoutTree according to the block layout algorithm
• compute_cached_layout
  Updates the stored layout of the provided node and its children
• compute_flexbox_layout
  Computes the layout of PartialLayoutTree according to the flexbox algorithm
• compute_grid_layout
  Grid layout algorithm This consists of a few phases:
• compute_hidden_layout
  Creates a layout for this node and its children, recursively. Each hidden node has zero size and is placed at the origin
• compute_layout
  Updates the stored layout of the provided node and its children
• compute_leaf_layout
  Compute the size of a leaf node (node with no children)
• print_tree
  Prints a debug representation of the computed layout for a tree of nodes, starting with the passed root node.
• round_layout
  Rounds the calculated Layout to exact pixel values