Dynamic Creation and Use of Components

Article number: KB000028

General Information

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Description

Application form development means that the forms are populated with components, then component properties and events are set up. A large number of components added in form design mode, make the form loading more time-consuming and use more memory. If some of the components are not displayed on the form all the time but are shown conditionally, they can be created dynamically at the runtime instead of being added to the form. The code should have declared a variable with the corresponding type:

Class TESTForm: Form
    btnRun: Button;
    t: Timer;
    //...

To create a component, initialize an instance of the corresponding class using the New keyword and assign value to the declared variable. If a visual component is created (a component inherited from the IControl interface), use the Parent property to indicate the parent component, in the area of which the created component is drawn. If the form itself is the parent component, the user can indicate the reserved Self ID as the value of the Parent property. Indicating parent component enables the user to remove created components correctly after the form finishes working:

    //...
    btnRun := New Button.Create;
    btnRun.Parent := Self;
    //...

It is not necessary to indicate parent component for non-visual components (inherited form the IComponent interface). Non-visual components can be used right after initializing the variable of the respective class.

    //...
    t := New Timer.Create;
    t.Enabled := False;
    //...

Procedures with corresponding signature should be created to handle components' events. To subscribe a component to an event, indicate procedure name as the value of the corresponding event:

    //...
    //Event handler for OnClick
    Sub btnOnClick(Sender: Object; Args: IMouseEventArgs);
    Begin
        t.Enabled := True;
    End Sub btnOnClick;
    //Timer event handler
    Sub OnTimer(Sender: Object; Args: IEventArgs);
    Begin

    End Sub OnTimer;
    //...

    //...
    //Components subscription for events
    btnRun.OnClick := btnOnClick;
    t.OnTimer := OnTimer;
    //...

When a component gets subscribed to an event, additional links to this event are created. If the component is a non-visual one, or the parent component is undefined, these links do not let the garbage collector clean the memory allocated to the component after the form runtime is completed. This results in keeping the objects in memory. To avoid memory leakage, use the Dispose statement to delete components in such cases:

    Sub TESTFormOnClose(Sender: Object; Args: IEventArgs);
    Begin
        Dispose t;
    End Sub TESTFormOnClose;

If it is required to delete a visual component at the form runtime, use the FreeComponent method together with the Dispose statement. FreeComponent removes visual part of the component, while Dispose deletes the component instance and frees the memory:

    //...
    FreeComponent(btnRun);
    Dispose btnRun;
    //...