Discover the HIDDEN POWER of Unity Property Bags and Visitors

Summary

This video delves into Unity's advanced runtime properties system, focusing on how property bags, paths, and visitors can be used to traverse and manipulate object data efficiently. It explains how to expose object properties generically, build decoupled systems, and implement custom visitor logic for advanced data handling. The video also touches on using attributes like 'bindable' and explores performance optimizations through code generation. Real-world examples, including inventory systems and character data, showcase the power and flexibility of Unity’s properties API, making it a valuable tool for serialization, UI generation, and data introspection.

Takeaways

• 😀 Unity Properties API allows for powerful runtime data traversal and access to the structure of any object inside Unity.
• 😀 The API uses property bags, paths, and visitors to expose objects generically and efficiently, without relying on reflection for most cases.
• 😀 To traverse object properties, developers need to use the 'PropertyBag' class, which can be generated using the 'getPropertyBag' method for a given type.
• 😀 The 'CreateProperty' attribute is essential for exposing private fields and internal properties for traversal in the property bag.
• 😀 The visitor programming pattern in Unity properties allows for writing generic logic that works with any object type, without needing to modify the object itself.
• 😀 Custom visitor logic can be defined for different property types (e.g., strings, integers) to handle them in specific ways.
• 😀 The system supports advanced features like creating custom property paths and navigating nested objects, which can be useful for complex data structures.
• 😀 Unity's low-level APIs allow for implementing your own visitors by using interfaces like 'IPropertyBagVisitor' and 'IPropertyVisitor', giving you full control over data traversal.
• 😀 The properties API can be used for tasks like generic runtime introspection, custom serialization, UI generation, live data binding, and state validation.
• 😀 By opting into code generation, you can avoid runtime performance overhead and make property bags available immediately at domain load time.
• 😀 The Unity properties API is particularly useful in systems like behavior trees or runtime authoring systems, where arbitrary object graphs need to be traversed and deserialized in a highly extensible way.

Q & A

What is the Unity properties API and why is it useful?

The Unity properties API is a runtime data traversal system that allows developers to access the structure of any object inside Unity. It is useful because it provides powerful and generic access to objects, making it easier to build data-driven systems that are decoupled from specific types, with or without reflection.

What is a property bag in Unity, and how does it work?

A property bag is a structure that describes the properties of a class (like score and name in the example) so that they can be accessed, traversed, and modified at runtime. It uses a static method, `getPropertyBag`, to generate a bag for a given type, and it can iterate over the properties using the `getProperties` method.

How does Unity handle serialization for properties that are not explicitly public?

Unity will expose properties that are marked with the `CreateProperty` attribute, even if they are not public. Private and internal fields can also be exposed if they are tagged with `SerializedField`, `SerializedReference`, or `CreateProperty`. However, fields not tagged with these attributes will not be exposed by default.

What is the difference between a traditional visitor pattern and Unity's visitor pattern?

In Unity's visitor pattern, types do not need to know about the visitor. They do not implement an `Accept` method or need to be part of any inheritance hierarchy. Instead, Unity generates metadata for the type (the property bag), and the visitor traverses the structure from the outside, making it more generic and flexible.

What is a bindable property in Unity, and how is it used in a visitor pattern?

A bindable property is a property marked with a custom attribute (like `Bindable`) that allows it to be visited during traversal. The visitor can collect all bindable properties, track them, and perform actions, such as printing out their paths or modifying their values, based on the visitor logic.

How can Unity's properties API be used for custom serialization?

Unity's properties API allows you to use visitor patterns for custom serialization by traversing object graphs and assigning values directly to properties, even in deeply nested structures. This is useful for highly generic, extensible, and type-safe serialization, as seen with behavior trees and runtime authoring systems.

What role do property adapters play in Unity's visitor pattern?

Property adapters allow you to define custom logic for visiting properties of specific types. By implementing an adapter for types like `int`, `float`, or `string`, you can override the default behavior of visiting a property and execute type-specific logic during traversal.

How does Unity's lazy reflection work in the context of property bags?

Unity's property bags use a form of lazy reflection. The first time a property bag is used, it performs reflection to find all available properties. However, this reflection is optimized to occur only once, and after that, no additional runtime performance overhead is introduced unless code generation is enabled.

How can you improve performance when using property bags in Unity?

To improve performance, you can opt into code generation, which eliminates the need for runtime reflection. By tagging the assembly with the `GeneratePropertyBagsForAssembly` attribute and marking the data structure with `GeneratePropertyBag`, Unity will generate property bags at compile-time, reducing runtime overhead.

How does the Unity properties API support different use cases like UI generation and state validation?

The Unity properties API can be used for a wide range of use cases, such as UI generation, live data binding, state validation, and custom serialization. It allows you to traverse complex object graphs at runtime, making it ideal for tasks like building UI systems, validating object states, or serializing structured data.