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96 lines
3.1 KiB
Plaintext
96 lines
3.1 KiB
Plaintext
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1.0 Marshallers
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===============
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Marshallers offer the opportunity to control the emittion process, and
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collate return values. By default signals have a marshaller which does
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nothing, and throws away all but the last return value.
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A marshaller can be any class with:
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struct SomeMarshal
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{
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// both typedefs must be defined.
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typedef Type1 InType;
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typedef Type2 OutType;
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// Return final return code.
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OutType value();
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// Return value if marshaller does not get built
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static OutType default_value();
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// Captures return codes and returns TRUE to stop emittion.
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bool marshal(const InType&);
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SomeMarshal();
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};
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The function marshal() will be called with the return value of each slot
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connected to the signal as they are called. The emittion process can be
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stopped by marshal() returning true.
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Once all the slots have been called, value() is called to determine what
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to return to the emitter. If the signal gets called and there is
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nothing attached the default_value() is used.
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2.0 Possible uses
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=================
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A marshaller could stop emittion when the signal had been handled:
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struct StopOnTrue
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{
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typedef bool InType;
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typedef bool OutType;
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OutType return_value_;
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OutType value() { return return_value_; }
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static OutType default_value() { return false; }
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bool marshal(const InType& val) { return_value_ = val; return val; }
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StopOnTrue() : return_value_(false) {}
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};
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marshal() here returns true as soon as one of the things connected to the
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signal returns true. It also keeps track of whether anything stopped the
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emittion in the return_value_ member. This way when something emits the
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signal, it can tell whether the signal was dealt with (signal.emit()
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returns true) or not (signal.emit() returns false).
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Now, if OutType was a list or a vector, marshal() could push_back() all
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the values, returning to the emitter all the return values rather than just
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one.
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3.0 Standard Marshallers
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========================
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The following marshallers are provided by default.
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Marshal<void>
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Marshal<T>
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Marshal<R> (untested, may not be portable)
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FixedMarshal<T,V>
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FastMarshal<T>
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where
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- T can be a type, class, or pointer
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- R can be a reference
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- V is the initial value of the marshaller
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which is returned if nothing is connected.
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All of the standard defined marshallers, with the exception of
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FastMarshal, check for a possiblity of a skipped return code via
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RetCode::check_ignore(). You can cause a return code to be
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ignored by the marshaller by setting RetCode::ignore() prior to returning
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to a function. Please note that you must be sure that you are returning to
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signal call to use ignore(). Calling at other times may result in other
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functions incorrectly skipping.
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The check_ignore function is thread safe which may induce
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unnecessary delays in slot calling. To avoid this overhead
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either define the signal to have a void return or
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use a FastMarshal.
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(This will all be replaced by an exeption mechanism at some point;
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however, current compiler technology is too slow for this to
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work.)
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