The -object provider (MOP) mechanism

Purpose

The object provider mechanism -exists to enable control types to be established at run time. It's a form -of Run Time Type Identification (RTTI). More specifically it allows a caller -to find a control in the run-time hierarchy that implements a particular interface, -or mixin. From the caller's point of view it's pretty simple: all that's required -is to ask a control, any control, to get an object that supports the specified -interface, cast the pointer returned to the correct interface and then call -the desired function.

History

The Object Provider interface pre-dates -the parent pointer in CCoeControl. Instead it uses a MopParent pointer -which is normally the parent control but may be the AppUi or even the CoeEnv object. -It was designed to meet Series 60 requirements for navigation of the run-time -hierarchy. Internally it is now implemented by the framework using the parent -pointer where possible which simplifies the control development process.

Description

Interfaces are only identifiable by -the Object Provider framework if they have an associated Interface UID. The -Interface UID must be declared in the class definition. Fortunately the UID -is invisible to the caller which simply uses the interface (mixin) name.

A -class which implements an identifiable mixin, i.e. one with an Interface UID, -can 'supply' a pointer to itself when the Object Provider framework requests -it. If a control is not able to supply the mixin requested it passes the request -on to the next control in the hierarchy.

The sequence diagram below -illustrates the Object Provider process for supplying an interface pointer -from the run-time hierarchy.

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Object1 wishes to call a function on MInterface. -It calls MopGetObject() on the nearest control in the run-time -hierarchy. The Object Provider Framework identifies the Uid for Minterface and -then passes the request up the hierarchy until an object is found (Object4) -that implements Minterface.

Calling MopGetGetObjectNoChaining() will -call MopSupplyObject() but not MopNext(). -It returns Null if the class does not implement the specified mixin.