Signals and Slots in D

[J Duncan]

Lutger wrote:
> Walter Bright wrote:
> 
>> Ok, I admit I don't understand S&S. But let's try starting with Qt's 
>> canonical example from http://doc.trolltech.com/3.3/signalsandslots.html:
>>
>>     class Foo : public QObject
>>     {
>>         Q_OBJECT
>>     public:
>>         Foo();
>>         int value() const { return val; }
>>     public slots:
>>         void setValue( int );
>>         {
>>           if ( v != val ) {
>>             val = v;
>>             emit valueChanged(v);
>>           }
>>         }
>>     signals:
>>         void valueChanged( int );
>>     private:
>>         int val;
>>     };
>>
>>     Foo a, b;
>>     connect(&a, SIGNAL(valueChanged(int)), &b, SLOT(setValue(int)));
>>     b.setValue( 11 ); // a == undefined  b == 11
>>     a.setValue( 79 ); // a == 79         b == 79
>>     b.value();        // returns
>>
>> It seems that I can do this in D with:
>>
>>     class Foo
>>     {
>>         this();
>>         int value() { return val; }
>>
>>         void setValue( int );
>>         {
>>           if ( v != val ) {
>>             val = v;
>>             valueChanged(v);
>>           }
>>         }
>>
>>         void valueChanged( int i )
>>     {
>>         foreach (dg; slots)
>>         dg(i);
>>     }
>>
>>     void connect( void delegate(int i) dg)
>>     {
>>         slots ~= dg;
>>     }
>>
>>     private:
>>     void delegate(int i)[] slots;
>>
>>         int val;
>>     };
>>
>>     Foo a = new Foo;
>>     Foo b = new Foo;
>>     a.connect(&b.setValue);
>>     b.setValue( 11 ); // a == undefined  b == 11
>>     a.setValue( 79 ); // a == 79         b == 79
>>     b.value();        // returns 79
>>
>> There's no casting, it's statically typesafe. Some of the boilerplate 
>> can be eliminated with a mixin. Is that all there is to it, or have I 
>> completely missed the boat?
> 
> 
> Almost, it's very simple, like the plain old function pointer as 
> callback in C. The main difference is:
> - when an object which has member functions connected gets detroyed, 
> there is no problem, no segfaults.
> - signals and slots are not as tightly coupled to a specific class as in 
> your example. Be it through introspection or ifti, class B needs to know 
> nothing about A except where it can connect, meaning more less coupling 
> between classes. I think thats all there is to it in essence.


Yeah thats pretty much it, everything can be done with templates etc. 
What we dont really have yet is a standardized introspection. One of the 
nice things about qt is it provides a typesafe callback which c++ doesnt 
have. D has the Wonderful delegate, so at least we get builtin typesafe 
callbacks. But on my D wishlist is introspection - or a builtin 
messaging system, or at least a standardized implementation in phobos. 
But its not a big deal by any means, I just think people who have used 
qt see how s&s would be Very Cool in D.