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TBoldExpressionHandle

TBoldExpressionHandle evaluates an OCL expression and provides access to a single value result. It's the foundation for data binding and derived values.

Class Definition

TBoldExpressionHandle = class(TBoldRootedHandle)
public
  // Core access
  property Value: TBoldElement;

  // Configuration
  property Expression: TBoldExpression;
  property RootHandle: TBoldElementHandle;
  property Variables: TBoldOclVariables;

  // Performance
  property EvaluateInPS: Boolean;
  property UsePrefetch: Boolean;

  // Subscription
  property Subscribe: Boolean;
end;

How It Works

flowchart LR
    subgraph Input[" Input "]
        Root[RootHandle<br/>Context Element]
        Expr[Expression<br/>OCL Query]
        Vars[Variables<br/>Parameters]
    end

    subgraph Handle[" TBoldExpressionHandle "]
        Eval[OCL Evaluator]
    end

    subgraph Output[" Output "]
        Value[Value<br/>Result Element]
    end

    Root --> Eval
    Expr --> Eval
    Vars --> Eval
    Eval --> Value

Properties

Expression

The OCL expression to evaluate:

// Simple attribute access
BoldExpressionHandle1.Expression := 'name';

// Calculated value
BoldExpressionHandle1.Expression := 'orders->size';

// Complex expression
BoldExpressionHandle1.Expression := 'orders->select(status = ''Pending'')->collect(total)->sum';

RootHandle

The context element for expression evaluation:

// Evaluate expression relative to selected customer
BoldExpressionHandle1.RootHandle := CustomerListHandle;
BoldExpressionHandle1.Expression := 'orders->size';

Value

The result of the expression:

var
  OrderCount: Integer;
begin
  OrderCount := (BoldExpressionHandle1.Value as TBAInteger).AsInteger;
  ShowMessage(Format('Customer has %d orders', [OrderCount]));
end;

Variables

OCL variables for parameterized expressions:

// Expression with parameter
BoldExpressionHandle1.Expression := 'orders->select(total > minAmount)->size';
BoldOclVariables1.SetVariable('minAmount', '1000');
BoldExpressionHandle1.Variables := BoldOclVariables1;

Subscribe

When True, automatically updates when source data changes:

// Enable auto-refresh
BoldExpressionHandle1.Subscribe := True;

EvaluateInPS

When True, the OCL expression is evaluated in the database (Persistence Storage) instead of in memory. This is essential for aggregates on large datasets:

// Count active customers in database - fast even with millions of records
BoldExpressionHandle1.EvaluateInPS := True;
BoldExpressionHandle1.Expression := 'Customer.allInstances->select(active)->size';

Common Use Cases

Display Calculated Values

// Show order total
BoldExpressionHandle1.RootHandle := OrderHandle;
BoldExpressionHandle1.Expression := 'items->collect(quantity * unitPrice)->sum';
// Bind to TBoldEdit or TBoldLabel

Conditional Logic

// Check if customer has unpaid orders
BoldExpressionHandle1.Expression := 'orders->exists(paid = false)';

procedure CheckPaymentStatus;
var
  HasUnpaid: Boolean;
begin
  HasUnpaid := (BoldExpressionHandle1.Value as TBABoolean).AsBoolean;
  if HasUnpaid then
    ShowMessage('Customer has unpaid orders');
end;

// Get related object
BoldExpressionHandle1.RootHandle := OrderHandle;
BoldExpressionHandle1.Expression := 'customer';

var
  Customer: TCustomer;
begin
  Customer := BoldExpressionHandle1.Value as TCustomer;
end;

Aggregate Calculations

// Sum of all order totals
BoldExpressionHandle1.Expression := 'Order.allInstances->collect(total)->sum';

// Average order value
BoldExpressionHandle1.Expression := 'Order.allInstances->collect(total)->average';

// Count active customers
BoldExpressionHandle1.Expression := 'Customer.allInstances->select(active)->size';

Binding to UI Components

With TBoldEdit

// Design time: Set BoldHandle property
BoldEdit1.BoldHandle := BoldExpressionHandle1;

With TBoldLabel

// Display calculated value
BoldLabel1.BoldHandle := BoldExpressionHandle1;

With TBoldCheckBox

// Boolean expression
BoldExpressionHandle1.Expression := 'active';
BoldCheckBox1.BoldHandle := BoldExpressionHandle1;

Chaining Handles

// System -> Customer List -> Current Customer -> Order Count
BoldSystemHandle1                           // System
  -> BoldListHandle1 (Customer.allInstances) // All customers
    -> BoldExpressionHandle1 (orders->size)  // Order count for current

Read-Only vs Editable

Expression handles are typically read-only for complex expressions. For editable values, use simple attribute expressions:

// Editable (simple attribute)
BoldExpressionHandle1.Expression := 'name';

// Read-only (calculated)
BoldExpressionHandle2.Expression := 'firstName + '' '' + lastName';

Performance Tips

  1. Set Subscribe := False for one-time evaluations
  2. Use EvaluateInPS := True for aggregates on large datasets
  3. Cache results when expression is expensive and data rarely changes

See Also