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SPM library providing typed access to bit-level information packed into Swift Data bytes.
.package(url: "https://github.com/didactek/deft-layout.git", from: "0.5.0")

Deft Layout

A library providing typed access to bit-level information packed into Swift Data bytes.

Part of the DEvices from swiFT (Deft) family.


  • maximize call-site readability
  • encourage discovery
    • minimize use of external libraries
  • exemplify unit-testable design


Swift Package Manager


DeftLayout is a framework for mapping values of Swift data types to the bit-level positions used in device protocols. It performs the role of functions like pack/unpack in Ruby or Python, or of bitwise boolean operations, bitfields, and unions in C.

The mapping is defined by deriving a class from a base class (ByteDescription, ByteArrayDescription, etc.) that manages the storage, and then adding properties to the derived class that define what should be stored. The properties have Swift data types (notably supporting enums with a storage class), but are annotated with an @Position property wrapper that maps the property's storage back into the packed/encoded base storage. See the documentation of these classes for details. (@Position wrappers adopt the CoderAdapter protocol, which requires they provide and set up a ByteCoder that it wires to the AssembledMessage storage.)

This suggested pattern tries to minimize the need for raw bit operations, hardcoded masks, and other "magic" constants. The @Position wrappers aspire to adopt positional notation used in device datasheets.

  • Note: DeftLayout is optimized for readability and not efficiency. In most cases, the message it is assembling or decoding is small, and the communications with the device will be much more work than the encoding here.


Describe messages

The underlying encoded bytes of the message are represented by an AssembledMessage.

The DeftLayout module provides support for mapping particular bits in AssembledMessage Data to properties in a message object.

Typical hierarchy for a mapping class:

BitStorageCore // manages the AssembledMessage

[ByteArray]Description // provides @Position wrappers that "make sense" for the representation

[UserClass]Layout // uses @Position wrappers to add properties of the message

Example: a packed, 5-byte message

The TEA5767 radio tuner has only one command, consisting of a write of 5 bytes. Its datasheet describes the bytes by byte index and bits within the 8-bit byte. The ByteArrayDescription best supports this longer array with byte-oriented descriptions.

class TEA5767_WriteLayout: ByteArrayDescription {
    enum SearchStopLevel: UInt8, BitEmbeddable {
        case low = 0b01
        case medium = 0b10
        case high = 0b11
    @Position(ofByte: 3, msb: 6, lsb: 5)
    var searchStopLevel: SearchStopLevel = .high
    // ...

Example: a 2-byte message as a big-endian word

The MCP9808 defines a number of different 1- or 2-byte messages. The 2-byte messages are documented in the datasheet as big-endian words. The WordDescription @Position wrappers idiomatically handle positioning bits between 0 and 15 and encode them to bytes with the expected endian-ness:

class MCP9808_AmbientTemperatureRegister: WordDescription {
    enum LimitFlag: UInt8, BitEmbeddable {
        case withinLimit = 0
        case outsideLimit = 1
    @Position(bit: 13)
    var AmbientVsLower: LimitFlag = .withinLimit

    @Position(msb: 12, lsb: 0, .extendNegativeBit)
    var temperatureSixteenthCelsius: Int = 0

Because WordDescription describes precisely two bytes (or one word), its @Position structs do not offer byte or word index/offset.

Using Message Layouts

Obtain the assembled bytes via the storage property via the base class.

To decode data, populate the underlying AssembledMessage storage with bytes, then read the properties via the layout class.


@Position wrappers adopt the CoderAdapter protocol, which requires they provide and set up a ByteCoder that it wires to the AssembledMessage storage.


The Swift 5.1 compiler will crash if more than one property wrapper structure is defined in the application with the same name, even when those structures are defined within the scope of different classes.

This appears fixed in 5.2, but Swift binaries are not yet available for the RPi.


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