////WIP

NOTE: Layouts are an advanced, mostly internal feature of the input system. It is not necessary to understand this feature to use the input system. Knowledge of the layout system is mostly useful when wanting to support custom devices or when wanting to modify the behavior of existing devices.

Layouts

A layout describes a memory format and the controls to build in order to read and write data to/from memory in the given format.

Layouts are the central mechanism by which the input system learns about types of input devices and input controls. Each layout represents a specific composition of input controls. By matching the description of a device to a layout, the input system is able to create the correct type of device and interpret the incoming input data correctly.

The set of currently understood layouts can be browsed from the input debugger.

A layout has two primary functions:

• Describe a certain memory layout containing input data.
• Assign names, structure, and meaning to the controls operating on the data.

A layout can either be for a control on a device (e.g. Stick) or for a device itself (i.e. something based on InputDevice).

Layouts are only loaded when needed -- usually when creating a new device. A layout can be manually loaded through InputSystem.LoadLayout. This returns an InputControlLayout instance containing the fully merged (i.e. containing any information inherited from based layouts and/or affected by layout overrides), final structure of the layout.

New layouts can be registered through InputSystem.RegisterLayout.

Layout Formats

New layouts can be added in one of three ways.

1. Represented by C# structs and classes.
2. In JSON format.
3. Built-on the fly at runtime using what's called "layout builders".

Layout from Type

In its most basic form, a layout can simply be a C# class (derived from InputControl for a control layout or derived from InputDevice for a device layout).

// The InputControlLayout attribute is not strictly necessary here.
// However, it can be used to set additional properties (such as
// a custom display name for the layout).
[InputControlLayout]
public class MyDevice : InputDevice
{
    public AxisControl axis { get; private set; }
    public ButtonControl button { get; private set; }

    protected override void FinishSetup(InputDeviceBuilder builder)
    {
        base.FinishSetup(builder);

        axis = builder.GetControl<AxisControl>("axis");
        button = builder.GetControl<ButtonControl>("button");
    }
}

The layout can be registered with InputSystem.RegisterLayout. This works the same for control and for device layouts.

// NOTE: This should generally be done from InitializeOnLoad/
// RuntimeInitializeOnLoad code.
InputSystem.RegisterLayout<MyDevice>();

When the layout is instantiated, the system will look at every field and property defined directly in the type to potentially turn it into one or more control items.

1. If the field or property is annotated with InputControlAttribute, the attribute's properties will be applied to the control item. Some special defaults apply in this case:
   • If no offset is set and the attribute is applied to a field, offset defaults to the offset of the field.
   • If no name is set, it defaults to the name of the property/field.
   • If no layout is set, it is inferred from the type of the field/property.
2. If the field or property has a struct type which implements IInputStateTypeInfo, the field is considered to be an embedded state struct and the field/property will be recursed into to gather controls from it.
3. Otherwise, if the type of the field or property is based on InputControl, a control item will be added similar to case 1) when the member is annotated with InputControlAttribute.

Using a State Structure

When implementing support for a new input device, there usually is an existing data format in which input for the device is received. The easiest way to add support for the given data format is to describe it with a C# struct annotated with InputControlAttribute.

// It can be useful to use LayoutKind.Explicit to force explicit field offsets
// so as to not get caught by alignment/packing difference introduced by the VM.
public struct MyDeviceState : IInputStateTypeInfo
{
    public static FourCC Format => new FourCC('M', 'D', 'E', 'V');

    [InputControl(name = "button1", layout = "Button", bit = 0)]
    [InputControl(name = "button2", layout = "Button", bit = 1)]
    [InputControl(name = "dpad", layout = "Dpad", bit = 2, sizeInBits = 4)]
    [InputControl(name = "dpad/up", bit = 2)]
    [InputControl(name = "dpad/down", bit = 3)]
    [InputControl(name = "dpad/left", bit = 4)]
    [InputControl(name = "dpad/right", bit = 5)]
    public int buttons;

    [InputControl(layout = "Stick")]
    public Vector2 stick;

    [InputControl(layout = "Axis")] // Automatically converts from byte to float.
    public byte trigger;

    public FourCC GetFormat()
    {
        return Format;
    }
}

// The device must be directed to the state struct we have created.
[InputControlLayout(stateType = typeof(MyDeviceState)]
public class MyDevice : InputDevice
{
}

Layout from JSON

TODO

Layout Builders

Finally, layouts can also be built on the fly in code. This is useful for device interfaces such as HID that supply descriptive information for each device.

TODO

Layout Inheritance

A layout can be derived from an existing layout. This process is based on a simple process of merging the information from a derived layout on top of the information present in the derived layout.

• For layouts defined as types, the base layout TODO
• For layouts defined in JSON, TODO
• For layouts ...

Control Items

Each layout is comprised of zero or more control items. Each item either describes a new control or modifies the properties of an existing control. The latter can also reach down into the hierarchy and modify properties of a control added implicitly as a child by another item.

    // Add a dpad control.
    [InputControl(layout = "Dpad")]
    // And now modify the properties of the "up" control that was added by the
    // "Dpad" layout above.
    [InputControl(name = "dpad/up", displayName = "DPADUP")]
    public int buttons;

The following table details the properties that a control item can have. These can be set either as properties on InputControlAttribute, as properties on the control in JSON, or through methods on InputControlLayout.Builder.ControlBuilder.

Property	Description
name	Defaults to the name of the field/property that InputControlAttribute is applied to.
displayName
shortDisplayName
layout
variants
aliases
usages
offset	The byte offset at which the state for the control is found.
bit
sizeInBits	The total size of the control's state, in bits.
arraySize
parameters
processors
noisy	Whether the control is to be considered noisy.
synthetic	Whether the control is to be considered synthetic.
defaultState	What value to initialize the state memory for the control to by default. This is not the same as a default value but instead TODO.
useStateFrom
minValue
maxValue

Layout Overrides

It is possible to non-destructively alter aspects of an existing layout using what's called "layout overrides".

Built-In Layouts

Controls

Layout	Description
Stick	A thumbstick-like controls. Based on Vector2. Has an X and a Y axis as well as up, down, left, and right buttons corresponding to the cardinal directions.

Devices

Layout	Description