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Define custom control attributes for complex data types

Use attribute converter and UxmlObject to define UXML attributes for complex data types.
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You can use attribute converters and UxmlObject to support complex data types attributes for your custom controls.

Attribute converters

Attribute converters convert a UxmlAttribute type to and from a string. It provides a set of built-in converters for common types, such as 
int
, 
float
, 
bool
, 
string
, and 
Vector2
. For complex data types, such as lists or custom classes, you need to create custom converters.
The following C# code example creates a custom class called 
Person
and a custom control class called 
Department
. The 
Department
class contains a 
Person
field for the manager and a list of 
Person
objects for the employees.
using System;using System.Collections.Generic;using UnityEngine.UIElements;namespace AttributeConverterExample{    [Serializable]    public class Person    {        public string name;        public int age;        public string nationality;    }    [UxmlElement]    public partial class Department : VisualElement    {        [UxmlAttribute]        public Person manager;        [UxmlAttribute]        public List<Person> employees;    }}
If you edit the custom control in UI Builder, you get the following error message:
[UxmlElement] 'Department' has a [UxmlAttribute] 'manager' of an unknown type 'Person'. To fix this error, define a custom UxmlAttributeConverter<Person>.
To address this, create an attribute converter for the 
Person
class that inherits from 
UxmlAttributeConverter<Person>
and implements the 
ToString
and 
FromString
methods to convert the 
Person
object to and from a string. The following example creates a converter for the 
Person
class:
using UnityEditor.UIElements;namespace AttributeConverterExample{    public class PersonConverter : UxmlAttributeConverter<Person>    {        const char k_Separator = ':';        public override string ToString(Person value)        {            return $"{value.name}{k_Separator}{value.age}{k_Separator}{value.nationality}";        }        public override Person FromString(string value)        {            var person = new Person();            var split = value.Split(k_Separator);            if (split.Length == 3)            {                person.name = split[0];                person.age = int.Parse(split[1]);                person.nationality = split[2];            }            return person;        }    }}
The converter combines the three values into a single string format, with the pattern of 
[name]:[age]:[nationality]
. The 
FromString
method splits the string into the three values and assigns them to the 
Person
object.
The following shows an example usage of the 
Department
element in a UXML file:
<ui:UXML xmlns:ui="UnityEngine.UIElements">    <AttributeConverterExample.Department name="Dunder Mifflin" manager="Michael Scott:58:USA" employees="Dwight Schrute:53:USA,Jim Halpert:44:USA" /></ui:UXML>

UxmlObjects

When using attribute converters, as data types become more intricate or when dealing with extensive lists, the generated strings can quickly grow convoluted and unwieldy. To simplify this, you can use 
UxmlObjects
.
UxmlObjects
 are classes instantiated from UXML that contain UXML attributes.
To understand how to use 
UxmlObjects
, take the previous 
Person
and 
Department
classes example, convert them into 
UxmlObjects
as shown in the following example:
using System;using System.Collections.Generic;using UnityEngine.UIElements;namespace UxmlObjectExample{    [UxmlObject]    public partial class Person    {        [UxmlAttribute]        public string name;        [UxmlAttribute]        public int age;        [UxmlAttribute]        public string nationality;    }    [UxmlElement]    public partial class Department : VisualElement    {        [UxmlObjectReference("manager")]        public Person manager;        [UxmlObjectReference("employees")]        public List<Person> employees;    }}
The 
Person
class now resembles a custom element, and uses 
UxmlAttribute
for attribute declaration and introducing the UxmlObject attribute to signify its status as a 
UxmlObject
.
Instead of using 
UxmlAttribute
for 
UxmlObject
fields, this example uses UxmlObjectReference to assign a 
name
. The name specifies the parent element of the 
UxmlObject
.
Previously, all 
UxmlObjects
were stored as direct children of the element, which caused scalability issues when multiple 
UxmlObject
fields were present. It was difficult to distinguish which 
UxmlObjects
belonged to which field.
The 
UxmlObjectReference
attribute resolves this issue by specifying a name for the 
UxmlObject
. This name corresponds to the UXML tag name. In the previous example, the 
manager
and 
employees
fields belong to the 
Person
class. The 
UxmlObjectReference
attribute ensures that the 
Person
class is correctly assigned to the 
manager
and 
employees
fields.
The following shows an example usage in a UXML file:
<ui:UXML xmlns:ui="UnityEngine.UIElements">    <UxmlObjectExample.Department name="Department One">        <manager>            <UxmlObjectExample.Person name="Manager One" age="58" nationality="Canadian" />        </manager>        <employees>            <UxmlObjectExample.Person name="Person Two" age="53" nationality="Canadian" />            <UxmlObjectExample.Person name="Person Three" age="44" nationality="Canadian" />        </employees>    </UxmlObjectExample.Department></ui:UXML>
注意
UxmlObjectReferenceFields
also works with interfaces. However, you can only assign 
UxmlObjects
that implement the specified interface. For an example of using interfaces, refer to UxmlObjectReferenceAttribute.

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