Ruby Mapping for Operations
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Basic Ruby Mapping for Operations
As we saw in the Ruby mapping for interfaces, for each operation on an interface, the proxy class contains a corresponding method with the same name. To invoke an operation, you call it via the proxy. For example, here is part of the definitions for our file system:
module Filesystem
{
interface Node
{
idempotent string name();
}
// ...
}
The name operation returns a value of type string. Given a proxy to an object of type Node, the client can invoke the operation as follows:
node = ... # Initialize proxy name = node.name() # Get name via RPC
Normal and idempotent Operations in Ruby
You can add an idempotent qualifier to a Slice operation. As far as the signature for the corresponding proxy method is concerned, idempotent has no effect. For example, consider the following interface:
interface Example
{
string op1();
idempotent string op2();
}
The proxy class for this is:
class ExamplePrx < Ice::ObjectPrx
def op1(context=nil)
def op2(context=nil)
end
Because idempotent affects an aspect of call dispatch, not interface, it makes sense for the two methods to look the same.
Passing Parameters in Ruby
In-Parameters in Ruby
All parameters are passed by reference in the Ruby mapping; it is guaranteed that the value of a parameter will not be changed by the invocation.
Here is an interface with operations that pass parameters of various types from client to server:
struct NumberAndString
{
int x;
string str;
}
sequence<string> StringSeq;
dictionary<long, StringSeq> StringTable;
interface ClientToServer
{
void op1(int i, float f, bool b, string s);
void op2(NumberAndString ns, StringSeq ss, StringTable st);
void op3(ClientToServer* proxy);
}
The Slice compiler generates the following proxy for this definition:
class ClientToServerPrx < Ice::ObjectPrx
def op1(i, f, b, s, context=nil)
def op2(ns, ss, st, context=nil)
def op3(proxy, context=nil)
end
Given a proxy to a ClientToServer interface, the client code can pass parameters as in the following example:
p = ... # Get proxy...
p.op1(42, 3.14, true, "Hello world!") # Pass simple literals
i = 42
f = 3.14
b = true
s = "Hello world!"
p.op1(i, f, b, s) # Pass simple variables
ns = NumberAndString.new()
ns.x = 42
ns.str = "The Answer"
ss = [ "Hello world!" ]
st = {}
st[0] = ns
p.op2(ns, ss, st) # Pass complex variables
p.op3(p) # Pass proxy
Out-Parameters in Ruby
As in Java, Ruby functions do not support reference arguments. That is, it is not possible to pass an uninitialized variable to a Ruby function in order to have its value initialized by the function. The Java mapping overcomes this limitation with the use of holder classes that represent each out parameter. The Ruby mapping takes a different approach, one that is more natural for Ruby users.
The semantics of out parameters in the Ruby mapping depend on whether the operation returns one value or multiple values. An operation returns multiple values when it has declared multiple out parameters, or when it has declared a non-void return type and at least one out parameter.
If an operation returns multiple values, the client receives them in the form of a result array. A non-void return value, if any, is always the first element in the result array, followed by the out parameters in the order of declaration.
If an operation returns only one value, the client receives the value itself.
Here again are the same Slice definitions we saw earlier, but this time with all parameters being passed in the out direction:
struct NumberAndString
{
int x;
string str;
}
sequence<string> StringSeq;
dictionary<long, StringSeq> StringTable;
interface ServerToClient
{
int op1(out float f, out bool b, out string s);
void op2(out NumberAndString ns,
out StringSeq ss,
out StringTable st);
void op3(out ServerToClient* proxy);
}
The Ruby mapping generates the following code for this definition:
class ClientToServerPrx < Ice::ObjectPrx
def op1(context=nil)
def op2(context=nil)
def op3(context=nil)
end
Given a proxy to a ServerToClient interface, the client code can receive the results as in the following example:
p = ... # Get proxy... i, f, b, s = p.op1() ns, ss, st = p.op2() stcp = p.op3()
The operations have no in parameters, therefore no arguments are passed to the proxy methods. Since op1 and op2 return multiple values, their result arrays are unpacked into separate values, whereas the return value of op3 requires no unpacking.
Parameter Type Mismatches in Ruby
Although the Ruby compiler cannot check the types of arguments passed to a method, the Ice run time does perform validation on the arguments to a proxy invocation and reports any type mismatches as a TypeError exception.
Null Parameters in Ruby
Some Slice types naturally have "empty" or "not there" semantics. Specifically, sequences, dictionaries, and strings all can be nil, but the corresponding Slice types do not have the concept of a null value. To make life with these types easier, whenever you pass nil as a parameter or return value of type sequence, dictionary, or string, the Ice run time automatically sends an empty sequence, dictionary, or string to the receiver.
This behavior is useful as a convenience feature: especially for deeply-nested data types, members that are sequences, dictionaries, or strings automatically arrive as an empty value at the receiving end. This saves you having to explicitly initialize, for example, every string element in a large sequence before sending the sequence in order to avoid a run-time error. Note that using null parameters in this way does not create null semantics for Slice sequences, dictionaries, or strings. As far as the object model is concerned, these do not exist (only empty sequences, dictionaries, and strings do). For example, it makes no difference to the receiver whether you send a string as nil or as an empty string: either way, the receiver sees an empty string.
Optional Parameters in Ruby
Optional parameters use the same mapping as required parameters. The only difference is that Ice::Unset can be passed as the value of an optional parameter or return value. Consider the following operation:
optional(1) int execute(optional(2) string params, out optional(3) float value);
A client can invoke this operation as shown below:
i, v = proxy.execute("--file log.txt")
i, v = proxy.execute(Ice::Unset)
if v != Ice::Unset
puts "value = " + v.to_s
end
A well-behaved program must always compare an optional parameter to Ice::Unset prior to using its value. Keep in mind that the Ice::Unset marker value has different semantics than nil. Since nil is a legal value for certain Slice types, the Ice run time requires a separate marker value so that it can determine whether an optional parameter is set. An optional parameter set to nil is considered to be set. If you need to distinguish between an unset parameter and a parameter set to nil, you can do so as follows:
if optionalParam == Ice::Unset
puts "optionalParam is unset"
elsif optionalParam == nil
puts "optionalParam is None"
else
puts "optionalParam = " + optionalParam
end
Exception Handling in Ruby
Any operation invocation may throw a run-time exception and, if the operation has an exception specification, may also throw user exceptions. Suppose we have the following simple interface:
exception Tantrum
{
string reason;
}
interface Child
{
void askToCleanUp() throws Tantrum;
}
Slice exceptions are thrown as Ruby exceptions, so you can simply enclose one or more operation invocations in a begin-rescue block:
child = ... # Get child proxy...
begin
child.askToCleanUp()
rescue Tantrum => t
puts "The child says: #{t.reason}"
end
Typically, you will catch only a few exceptions of specific interest around an operation invocation; other exceptions, such as unexpected run-time errors, will usually be handled by exception handlers higher in the hierarchy. For example:
begin
child = ... # Get child proxy...
begin
child.askToCleanUp()
child.praise() # Give positive feedback...
rescue Tantrum => t
puts "The child says: #{t.reason}"
child.scold() # Recover from error...
end
rescue Ice::Exception => ex
print ex.backtrace.join("\n")
end