Dynamic Binding — Data Binding

Dynamic Path Expression

Prefix the path with expr: to signal that it is an expression, not a literal path. The expression is evaluated using the form's current value map:

// Static path (always binds to items[0].price):
"binding": { "source": "$json", "path": "items[0].price" }

// Dynamic path (binds to items[n].price where n comes from another field):
"binding": {
  "source": "$json",
  "path":   "expr:items[values['selected-index']].price"
}

Example — Item Detail Form

A user selects a line item from a grid, then edits its details in a separate panel. The detail fields dynamically bind to the selected item's index:

// Form schema (abridged)
{
  "controls": [
    // Hidden field — populated programmatically when user clicks a row
    {
      "id":   "selected-index",
      "type": "hidden",
      "binding": { "source": "$json", "path": "selectedIndex" }
    },

    // Detail fields — paths depend on selected-index
    {
      "id":    "item-name",
      "type":  "text",
      "label": "Item Name",
      "binding": {
        "source": "$json",
        "path":   "expr:items[values['selected-index']].name"
      }
    },
    {
      "id":    "item-price",
      "type":  "number",
      "label": "Unit Price",
      "binding": {
        "source": "$json",
        "path":   "expr:items[values['selected-index']].price"
      }
    }
  ]
}

// When the user selects row 2:
engine.setValue('selected-index', 2);
// item-name now reads from items[2].name
// item-price now reads from items[2].price

Expression Evaluation Rules

Expressions are evaluated using the same expression engine as visibilityRule
The expression has access to the values object (current form value map)
The expression must evaluate to a valid path string
Expressions are re-evaluated whenever the value map changes — the binding path updates reactively
If the expression throws or returns a non-string, the path is treated as undefined and the control value is undefined

When Not to Use Dynamic Binding

Scenario	Better Approach
The path is always static	Use a plain `path` string — simpler and faster
You need to show an array of items	Use a `repeater` control — designed for this use case
The path depends on a server-side value	Compute the path server-side and pass as a static schema string at load time

Dynamic Binding Has Performance Overhead Every change to the value map triggers re-evaluation of all dynamic path expressions. For forms with many dynamic bindings, this can cause extra re-render cycles. Prefer static paths where possible, and use dynamic binding only when the path genuinely cannot be known at schema authoring time.

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