Binding Examples

Field Binding Example

Here's a real-world text input binding similar to the implementation in @mobx-sentinel/react:

import { FormBinding, FormField } from '@mobx-sentinel/form';
import { makeObservable, computed, action } from 'mobx';

class InputBinding implements FormBinding {
  constructor(
    private readonly field: FormField, // Accepting single field
    public config: {
      getter: () => string | null;
      setter: (value: string) => void;
      id?: string;
      onChange?: (e: React.ChangeEvent) => void;
      onFocus?: (e: React.FocusEvent) => void;
      onBlur?: (e: React.FocusEvent) => void;
    }
  ) {
    makeObservable(this);
  }

  // Not @computed: `form.bind()` replaces `config` on every call,
  // so while observed, a computed would keep an earlier getter's value
  get value() {
    return this.config.getter() ?? ''; // Read the value from the model
  }

  @action
  onChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    this.config.setter(e.currentTarget.value); // Update the value from the input
    this.field.markAsChanged('intermediate'); // While editing, delay error reporting
    this.config.onChange?.(e); // Support extending handlers via config
  };

  onBlur = (e: React.FocusEvent<HTMLInputElement>) => {
    this.field.finalizeChangeIfNeeded(); // Ensure to report errors when they left the input
    this.config.onBlur?.(e);
  };

  onFocus = (e: React.FocusEvent<HTMLInputElement>) => {
    this.field.markAsTouched();
    this.config.onFocus?.(e);
  };

  @computed
  get errorMessages() {
    if (!this.field.isErrorReported) return null; // Check whether to report errors
    return Array.from(this.field.errors).join(', ') || null;
  }

  // The outport props: the returned value is passed to the view component
  get props() {
    return {
      type: 'text',
      value: this.value,
      id: this.config.id ?? this.field.stableId,
      onChange: this.onChange,
      onFocus: this.onFocus,
      onBlur: this.onBlur,
      'aria-invalid': this.field.isErrorReported,
      'aria-errormessage': this.errorMessages ?? undefined,
    };
  }
}

Checkbox Binding Example

Checkboxes use immediate finalization since there's no intermediate state:

class CheckBoxBinding implements FormBinding {
  constructor(
    private readonly field: FormField, // Accepting single field
    public config: {
      getter: () => boolean;
      setter: (value: boolean) => void;
      id?: string;
      onChange?: (e: React.ChangeEvent) => void;
      onFocus?: (e: React.FocusEvent) => void;
    }
  ) {
    makeObservable(this);
  }

  // Not @computed, as it reads `config`
  get checked() {
    return this.config.getter();
  }

  @action
  onChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    this.config.setter(e.currentTarget.checked);
    this.field.markAsChanged(); // Defaults to 'final'; No delay is needed for checkboxes
    this.config.onChange?.(e);
  };

  onFocus = (e: React.FocusEvent<HTMLInputElement>) => {
    this.field.markAsTouched();
    this.config.onFocus?.(e);
  };

  @computed
  get errorMessages() {
    if (!this.field.isErrorReported) return null;
    return Array.from(this.field.errors).join(', ') || null;
  }

  get props() {
    return {
      type: 'checkbox',
      id: this.config.id ?? this.field.stableId,
      checked: this.checked,
      onChange: this.onChange,
      onFocus: this.onFocus,
      'aria-invalid': this.field.isErrorReported,
      'aria-errormessage': this.errorMessages ?? undefined,
    };
  }
}

Multi-Field Binding Example

Bindings can work with multiple fields, useful for components like labels that need to aggregate state:

class LabelBinding implements FormBinding {
  constructor(
    private readonly fields: FormField[], // Accepting multiple fields
    public config: {
      htmlFor?: string;
    }
  ) {
    makeObservable(this);
  }

  // Not @computed: `stableId` is composed from a plain field on the form,
  // so while observed, a computed would keep the first id it saw
  get firstFieldStableId() {
    return this.fields.at(0)?.stableId;
  }

  @computed
  get firstErrorMessage() {
    for (const field of this.fields) {
      if (!field.isErrorReported) continue;
      for (const error of field.errors) {
        return error;
      }
    }
    return null;
  }

  get props() {
    return {
      htmlFor: this.config.htmlFor ?? this.firstFieldStableId,
      'aria-invalid': this.fields.some((field) => field.isErrorReported),
      'aria-errormessage': this.firstErrorMessage ?? undefined,
    };
  }
}

This binding aggregates error states from multiple fields, showing the first error message of the fields whose errors are reported.

Form Binding Example

Bindings can also operate on the entire form, useful for submit buttons:

class SubmitButtonBinding implements FormBinding {
  constructor(
    private readonly form: Form<unknown>, // Accepting the form
    public config: {
      onClick?: (e: React.MouseEvent) => void;
      onMouseOver?: (e: React.MouseEvent) => void;
    }
  ) {
    makeObservable(this);
  }

  @computed
  get busy() {
    return this.form.isSubmitting || this.form.isValidating;
  }

  onClick = (e: React.MouseEvent<HTMLButtonElement>) => {
    this.form.submit().catch((e) => void e);
    this.config.onClick?.(e);
  };

  onMouseOver = (e: React.MouseEvent<HTMLButtonElement>) => {
    this.form.reportError(); // Show errors on hover (like attempting to click)
    this.config.onMouseOver?.(e);
  };

  get props() {
    return {
      onClick: this.onClick,
      onMouseOver: this.onMouseOver,
      disabled: !this.form.canSubmit,
      'aria-busy': this.busy,
      'aria-invalid': !this.form.isValid,
    };
  }
}

Best Practices

Follow these patterns when creating bindings:

1. Reading configuration
   • Read this.config in plain getters, not @computed ones, since form.bind() replaces it on every call (see Why Not @computed)
   • Use @computed only for members that read nothing but observables, like field.errors
2. Extending event handlers
   • Accept configuration handlers as optional (onChange?: ...)
   • Call configuration handlers AFTER internal logic (this.config.onChange?.(e))
   • This allows users to extend behavior without overriding the binding's core functionality
3. Overriding/extending props
   • Accept props as optional (id?: string)
   • Use user-specified values in conjunction with default behavior:
     • Override with fallback: this.config.id ?? this.field.stableId (stableId, not id -- see Element IDs)
     • Extend with combination: this.config.disabled || this.disabled
4. Normalizing problematic values
   • Provide sensible defaults for null/undefined values (e.g., ?? "" for strings)
   • Handle special cases where valueAsNumber returns NaN or valueAsDate returns null
5. Field state management
   • Mark as touched on focus: field.markAsTouched()
   • Mark as changed with appropriate finalization (see Intermediate vs Final Changes)
   • Finalize on blur for text inputs: field.finalizeChangeIfNeeded()
   • Check field.isErrorReported before showing errors
6. Accessibility
   • Generate and manage unique element IDs for connecting labels and inputs (using id on inputs and htmlFor on labels)
   • Include aria-invalid based on field.isErrorReported
   • Include aria-errormessage with error text, or undefined if no errors
   • For form-level bindings (submit buttons), use aria-busy to indicate loading states