The mapping of Slice interfaces revolves around the idea that, to invoke a remote operation, you call a member function on a local class instance that represents the remote object. This makes the mapping easy and intuitive to use because, for all intents and purposes (apart from error semantics), making a remote procedure call is no different from making a local procedure call.
On the client side, interfaces map to a protocol with member functions that correspond to the operations on those interfaces. Consider the following simple interface:
@interface EXSimplePrx : ICEObjectPrx
// Mapping‑internal methods here...
@end
@protocol EXSimplePrx <ICEObjectPrx>
‑(void) op;
‑(void) op:(ICEContext *)context;
@end;
As you can see, the compiler generates a proxy protocol EXSimplePrx and a proxy class
EXSimplePrx. In general, the generated name for both protocol and class is
<module-prefix><interface-name>Prx .
In the client’s address space, an instance of EXSimplePrx is the local ambassador for a remote instance of the
Simple interface in a server and is known as a
proxy class instance. All the details about the server-side object, such as its address, what protocol to use, and its object identity are encapsulated in that instance.
Note that EXSimplePrx derives from
ICEObjectPrxx, and that
EXSimplePrx adopts the
ICEObjectPrx protocol. This reflects the fact that all Slice interfaces implicitly derive from
Ice::Object. For each operation in the interface, the proxy protocol has two methods whose name is derived from the operation. For the preceding example, we find that the operation
op is mapped to two methods,
op and
op:.
The second method has a trailing parameter of type ICEContext. This parameter is for use by the Ice run time to store information about how to deliver a request; normally, you do not need to supply a value here and can pretend that the trailing parameter does not exist. (We examine the
ICEContext parameter in detail in
Chapter 32. The parameter is also used by IceStorm—see
Chapter 45.)
Client-side application code never manipulates proxy class instances directly. In fact, you are not allowed to instantiate a proxy class directly. Instead, proxy instances are always instantiated on behalf of the client by the Ice run time, so client code never has any need to instantiate a proxy directly.
Proxies are immutable: once the run time has instantiated a proxy, that proxy continues to denote the same remote object and cannot be changed. This means that, if you want to keep a copy of a proxy, it is sufficient to call
retain on the proxy. (You can also call
copy on a proxy because
ICEObjectPrx implements
NSCopying. However, calling
copy has the same effect as calling
retain.)
Proxies are always passed and returned as type id<<module‑prefix><interface-name>Prx>. For example, for the preceding
Simple interface, the proxy type is
id<EXSimplePrx>.
The ICEObjectPrx base class provides two class methods that allow you cast a proxy from one type to another.
A checkedCast tests whether the object denoted by a proxy implements the specified interface. If so, the cast returns a proxy to the specified interface; otherwise, if the object denoted by the proxy does not implement the specified interface, the cast returns
nil. Checked casts are typically used to safely down-cast a proxy to a more derived interface. For example, assuming we have Slice interfaces
Base and
Derived, you can write the following:
id<EXBasePrx> base = ...; // Initialize base proxy
id<EXDerivedPrx> derived = [EXDerivedPrx checkedCast:base];
if(derived != nil)
{
// base implements run‑time type Derived
// use derived...
} else {
// Base has some other, unrelated type
}
The expression [EXDerivedPrx checkedCast:base] tests whether
base points at an object of type
Derived (or an object with a type that is derived from
Derived). If so, the cast succeeds and
derived is set to point at the same object as
base. Otherwise, the cast fails and
derived is set to
nil. (Proxies that "point nowhere" are represented by
nil.)
A checkedCast typically results in a remote message to the server.
1 The message effectively asks the server "is the object denoted by this proxy of type
Derived?" The reply from the server is communicated to the application code in form of a successful (non-
nil) or unsuccessful (
nil) result. Sending a remote message is necessary because, as a rule, there is no way for the client to find out what the actual run-time type of a proxy is without confirmation from the server. (For example, the server may replace the implementation of the object for an existing proxy with a more derived one.) This means that you have to be prepared for a
checkedCast to fail. For example, if the server is not running, you will receive an
ICEConnectionRefusedException; if the server is running, but the object denoted by the proxy no longer exists, you will receive an
ICEObjectNotExistException.
id<EXBasePrx> base;
base = ...; // Initialize base to point at a Derived
id<EXDerivedPrx> derived = [EXDerivedPrx uncheckedCast:base];
// Use derived...
An uncheckedCast provides a down-cast
without consulting the server as to the actual run-time type of the object. You should use an
uncheckedCast only if you are certain that the proxy indeed supports the more derived type: an
uncheckedCast, as the name implies, is not checked in any way; it does not contact the object in the server and, if the proxy does not support the specified interface, the cast does not return null. If you use the proxy resulting from an incorrect
uncheckedCast to invoke an operation, the behavior is undefined. Most likely, you will receive an
ICEOperationNotExistException, but, depending on the circumstances, the Ice run time may also report an exception indicating that unmarshaling has failed, or even silently return garbage results.
Despite its dangers, uncheckedCast is still useful because it avoids the cost of sending a message to the server. And, particularly during initialization (see
page 587), it is common to receive a proxy of type
id<ICEObjectPrx>, but with a known run-time type. In such cases, an
uncheckedCast saves the overhead of sending a remote message.
Note that an uncheckedCast is
not the same as an ordinary cast. The following is incorrect and has undefined behavior:
Both checkedCast and
uncheckedCast call
autorelease on the proxy they return so, if you want to prevent the proxy from being deallocated once the enclosing autorelease pool is drained, you must call
retain on the returned proxy.
The ICEObjectPrx provides a variety of methods for customizing a proxy (see
Section 32.11). Since proxies are immutable, each of these "factory methods" returns a copy of the original proxy that contains the desired modification. For example, you can obtain a proxy configured with a ten-second timeout as shown below:
id<ICEObjectPrx> proxy = [communicator stringToProxy:...];
proxy = [proxy ice_timeout:10000];
A factory method returns a new (autoreleased) proxy object if the requested modification differs from the current proxy, otherwise it returns the original proxy. The returned proxy is always of the same type as the source proxy.
Apart from the methods discussed in Section 18.11.2, proxy handles also support comparison with
isEqual. Note that
isEqual uses
all of the information in a proxy for the comparison. This means that not only the object identity must match for a comparison to succeed, but other details inside the proxy, such as the protocol and endpoint information, must be the same as well. In other words, comparison with
isEqual tests for
proxy identity,
not object identity. A common mistake is to write code along the following lines:
id<ICEObjectPrx> p1 = ...; // Get a proxy...
id<ICEObjectPrx> p2 = ...; // Get another proxy...
if (![p1 isEqual:p2]) {
// p1 and p2 denote different objects // WRONG!
} else {
// p1 and p2 denote the same object // Correct
}
Even though p1 and
p2 differ, they may denote the same Ice object. This can happen if, for example,
p1 and
p2 embed the same object identity, but use a different protocol to contact the target object. Similarly, the protocols might be the same, but could denote different endpoints (because a single Ice object can be contacted via several different transport endpoints). In other words, if two proxies compare equal with
isEqual, we know that the two proxies denote the same object (because they are identical in all respects); however, if two proxies compare unequal with
isEqual, we know absolutely nothing: the proxies may or may not denote the same object.
@protocol ICEObjectPrx <NSObject, NSCopying>
// ...
‑(NSComparisonResult) compareIdentity:(id<ICEObjectPrx>)p;
‑(NSComparisonResult) compareIdentityAndFacet:(id<ICEObjectPrx>)p;
@end
The compareIdentity method compares the object identities embedded in two proxies while ignoring other information, such as facet and transport information. To include the facet name (see
Chapter 34) in the comparison, use
compareIdentityAndFacet instead.
compareIdentity and
compareIdentityAndFacet allow you to correctly compare proxies for object identity. The example below demonstrates how to use
compareIdentity:
id<ICEObjectPrx> p1 = ...; // Get a proxy...
id<ICEObjectPrx> p2 = ...; // Get another proxy...
if ([p1 compareIdentity:p2] != NSOrderedSame) {
// p1 and p2 denote different objects // Correct
} else {
// p1 and p2 denote the same object // Correct
}
