A control is an instance of class TControl or its subclass. A control is a component defined in addition with user interface. The base class TControl defines the parent-child relationship among controls which reflects the containment relationship among user interface elements.

Control Tree

Controls are related to each other via parent-child relationship. Each parent control can have one or several child controls. A parent control is in charge of the state transition of its child controls. The rendering result of the child controls are usually used to compose the parent control's presentation. The parent-child relationship brings together controls into a control tree. A page is at the root of the tree, whose presentation is returned to the end-users.

The parent-child relationship is usually established by the framework via templates. In code, you may explicitly specify a control as a child of another using one of the following methods,

where the property Controls refers to the child control collection of the parent.

Control Identification

Each control has an ID property that can be uniquely identify itself among its sibling controls. In addition, each control has a UniqueID and a ClientID which can be used to globally identify the control in the tree that the control resides in. UniqueID and ClientID are very similar. The former is used by the framework to determine the location of the corresponding control in the tree, while the latter is mainly used on the client side as HTML tag IDs. In general, you should not rely on the explicit format of UniqueID or ClientID.

Naming Containers

Each control has a naming container which is a control creating a unique namespace for differentiating between controls with the same ID. For example, a TRepeater control creates multiple items each having child controls with the same IDs. To differentiate these child controls, each item serves as a naming container. Therefore, a child control may be uniquely identified using its naming container's ID together with its own ID. As you may already have understood, UniqueID and ClientID rely on the naming containers.

A control can serve as a naming container if it implements the INamingContainer interface.

ViewState and ControlState

HTTP is a stateless protocol, meaning it does not provide functionality to support continuing interaction between a user and a server. Each request is considered as discrete and independent of each other. A Web application, however, often needs to know what a user has done in previous requests. People thus introduce sessions to help remember such state information.

PRADO borrows the viewstate and controlstate concept from Microsoft ASP.NET to provides additional stateful programming mechanism. A value storing in viewstate or controlstate may be available to the next requests if the new requests are form submissions (called postback) to the same page by the same user. The difference between viewstate and controlstate is that the former can be disabled while the latter cannot.

Viewstate and controlstate are implemented in TControl. They are commonly used to define various properties of controls. To save and retrieve values from viewstate or controlstate, use following methods,

where $this refers to the control instance, Name refers to a key identifying the persistent value, $defaultValue is optional. When retrieving values from viewstate or controlstate, if the corresponding key does not exist, the default value will be returned.