databinding-patterns

# Data Binding Patterns Agent defines predictable data flow between XAML views and observable .NET state so bindings stay correct, diagnosable, and performant. ## When to Use | Condition | Use | |---|---| | Work wires view state to a view-model property, collection, or command parameter | Use this skill | | Work selects a binding mode or update trigger for editable UI | Use this skill | | Work adds converters, validation, `StringFormat`, or `TargetNullValue` | Use this skill | | Work tunes WinUI 3 or UWP binding performance with compiled bindings | Use this skill | ## When Not to Use | Condition | Route | |---|---| | Work restructures overall MVVM boundaries | Use `mvvm-patterns-dotnet` | | Work focuses on command implementations and behavior mapping | Use `commands-behaviors-patterns` | | Work edits domain services with no UI binding impact | Use the matching service skill | | Work runs in a UI stack without XAML-style binding | Follow the platform-native pattern | ## Required Inputs | Input | Required | Notes | |---|---|---| | Bound properties and collections | Yes | Identify sources, targets, and null behavior. | | Edit flow | Yes | Identify when source updates occur. | | Target platforms | Yes | Identify WPF, MAUI, WinUI 3, or UWP rules. | | Validation rules | No | Identify user-facing errors and blocking conditions. | | Performance sensitivity | No | Identify high-frequency lists, forms, or startup paths. | ## Pattern Matrix | Concern | Preferred Pattern | Guardrail | Pass Target | |---|---|---|---| | Read-only display | `OneWay` or `OneTime` as appropriate | Avoid `TwoWay` on labels and passive text | Target reflects the intended source timing | | Editable form field | `TwoWay` plus deliberate update trigger | Match trigger cost to the user experience | Source updates at the intended time | | View formatting | `StringFormat`, `TargetNullValue`, or small converters | Keep domain rules out of converters | Presentation transforms stay view-focused | | High-traffic WinUI 3 or UWP path | `x:Bind` or compiled binding where appropriate | Keep update direction explicit | Binding compiles and stays responsive | | Nested templates and extracted controls | Explicit data-context forwarding | Avoid accidental context loss | Child content resolves the intended source | ## Workflow | Step | Agent Action | Test | Pass | |---|---|---|---| | 1 | Agent maps each binding source, target, mode, and null path. | Review the binding map. | Every bound control has an explicit data-flow choice. | | 2 | Agent selects binding mode and update trigger based on edit timing and cost. | Exercise one edit path per mode in scope. | Update timing matches the intended user experience. | | 3 | Agent adds converters, formatting, and null handling only where presentation logic belongs in the view. | Search XAML and view-model code for duplicated formatting logic. | Transform logic lives in one place. | | 4 | Agent wires validation and error visuals through platform-standard mechanisms. | Trigger one invalid path and one recovery path. | Error state appears and clears correctly. | | 5 | Agent applies compiled binding or path simplification on performance-sensitive screens. | Build and exercise one list or form path. | Binding stays responsive. | | 6 | Agent verifies data-context propagation and local overrides. | Open nested controls or templates in scope. | Child views receive the intended context. | ## Rules | Topic | Rule | |---|---| | Mode selection | Match `OneWay`, `TwoWay`, `OneTime`, or `OneWayToSource` to the control intent. | | Update timing | Use `PropertyChanged`, `LostFocus`, explicit update, or compiled-binding defaults deliberately. | | Converters | Keep converters small and presentation-focused. | | Null display | Add `TargetNullValue` or view-model defaults intentionally. | | Diagnostics | Build and exercise touched views after binding-path changes. | ## Verification Checklist | Check | Test | Pass | |---|---|---| | Binding mode | Review editable and read-only controls. | No control uses an incorrect mode. | | Update trigger | Type through one form path. | Source updates occur at the intended time. | | Formatting ownership | Inspect XAML and view-model responsibilities. | Presentation transforms stay outside business logic. | | Validation visuals | Trigger invalid and corrected inputs. | Error feedback follows state changes. | | Data-context propagation | Render nested bound content. | Child content resolves the intended source. | ## Outputs - Binding map with modes, update triggers, and null behavior - Converter and formatting plan - Validation and error-visual guidance - Performance notes for compiled bindings and context propagation - Verification steps for binding correctness and responsiveness