Binding Modes

Aspid.MVVM supports four data binding modes between the ViewModel and the View.

Contents

• Overview
• OneWay
• TwoWay
• OneTime
• OneWayToSource
• Automatic mode detection
• Explicit mode
• Restricting modes in the Inspector

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Overview

public enum BindMode
{
    None = 0,
    OneWay = 1,
    TwoWay = 2,
    OneTime = 3,
    OneWayToSource = 4
}

Mode	Direction	When to use
OneWay	ViewModel → View	Displaying data (text, progress bar, icon)
TwoWay	ViewModel ↔ View	Input fields, sliders, toggles
OneTime	ViewModel → View (once)	Static data, commands
OneWayToSource	View → ViewModel	Component references, UI events

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OneWay

Data flows from the ViewModel to the View only. When the property changes, the binder updates the UI. UI changes never reach the ViewModel.

[ViewModel]
public partial class StatsViewModel
{
    [OneWayBind] private int _health;
    [OneWayBind] private string _playerName;
}

Implementation: OneWayBindableMember<T> stores the value and a Changed event. On Add() the binder immediately receives the current value and subscribes to Changed.

Typical binders: TextBinder, ImageSpriteBinder, ImageFillBinder, GraphicColorBinder.

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TwoWay

Two-way synchronization. A ViewModel change updates the View, and a View change updates the ViewModel.

[ViewModel]
public partial class FormViewModel
{
    [TwoWayBind] private string _inputText;
    [TwoWayBind] private bool _isEnabled;
    [TwoWayBind] private float _volume;
}

Implementation: TwoWayBindableMember<T> supports all four modes. For TwoWay/OneWayToSource binders Add() also subscribes to IReverseBinder<T>.ValueChanged.

Loop protection: binders such as InputFieldBinder keep an _isNotifyValueChanged flag to prevent infinite recursion.

Typical binders: InputFieldBinder, SliderValueBinder, ToggleIsOnBinder.

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OneTime

The value is delivered once, when binding. Later changes are not tracked.

[ViewModel]
public partial class PlayerViewModel
{
    // OneTime automatically for const
    [Bind] private const string Title = "Player Stats";

    // OneTime automatically for readonly
    [Bind] private readonly int _maxHealth;

    // Explicit OneTime
    [OneTimeBind] private IRelayCommand _saveCommand;
}

Implementation: OneTimeBindableMember<T> is a singleton per T. Add() calls SetValue once and returns null (no IBinderRemover is needed).

When to use: commands (IRelayCommand), configuration data, static labels.

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OneWayToSource

Data flows from the View to the ViewModel only. The ViewModel cannot push values to the View.

[ViewModel]
public partial class FormViewModel
{
    [OneWayToSourceBind] private string _userInput;
}

Implementation: OneWayToSourceBindableMember<T> stores no value. It subscribes to IReverseBinder<T>.ValueChanged and forwards changes to the ViewModel.

When to use: user input without an initial value, component references through ComponentToSourceMonoBinder.

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Automatic mode detection

[Bind] without arguments picks the mode itself:

Field	Detected mode
const	OneTime
readonly	OneTime
Mutable field	TwoWay

[ViewModel]
public partial class ExampleViewModel
{
    [Bind] private const string Title = "Hello";     // → OneTime
    [Bind] private readonly int _id;                  // → OneTime
    [Bind] private string _name;                      // → TwoWay
    [Bind] private float _value;                      // → TwoWay
}

tip

Prefer the explicit attributes ([OneWayBind], [TwoWayBind], …) for readability.

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Explicit mode

Through the [Bind] argument

[Bind(BindMode.OneWay)] private string _text;
[Bind(BindMode.TwoWay)] private float _slider;
[Bind(BindMode.OneTime)] private IRelayCommand _command;
[Bind(BindMode.OneWayToSource)] private string _userInput;

Through the shorthand attributes

[OneWayBind] private string _text;
[TwoWayBind] private float _slider;
[OneTimeBind] private IRelayCommand _command;
[OneWayToSourceBind] private string _userInput;

Both forms are equivalent. The shorthand attributes are just shorter.

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Restricting modes in the Inspector

On the binder side the allowed modes can be limited with [BindModeOverride]:

// OneWay and OneTime only
[BindModeOverride(BindMode.OneWay, BindMode.OneTime)]
public class TransformPositionBinder : TargetBinder<Transform, Vector3>, IVector3Binder
{
    // TwoWay and OneWayToSource are not offered in the Inspector
}

Some binders support every mode:

[BindModeOverride(IsAll = true)]
public class DebugLogBinder : MonoBinder
{
    // All modes available
}

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Summary

Mode	ViewModel → View	View → ViewModel	Updates	Good for
OneWay	✅	❌	On every change	Displaying data
TwoWay	✅	✅	On every change (both sides)	Interactive elements
OneTime	✅	❌	Only when binding	Static data, commands
OneWayToSource	❌	✅	On View change	Input, component references

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See also

• Architecture, how the binding pipeline works
• ViewModels, declaring bindable members
• Binders, writing binders with specific modes