What I currently do:
/* FIXME: Actually I just want to call that callback ... no idea how that can be done without using a signal */
/* So I just used some random unused signal for it */
gulong handler_id = g_signal_connect_swapped (window, "direction-changed", (GCallback) mycallback, window);
g_signal_emit_by_name (window,"direction-changed");
g_signal_handler_disconnect (window, handler_id);
Works well, but is an ugly hack.
I know it is somehow possible to call my callback by using g_closure_invoke() in some way, though so far I did not get that to work.
If it matters, the callback will be filled with a function which looks like that:
static void myCallbackMethod (GtkWindow *window);
A GCallback is just a function pointer of a certain type. You can call the function directly, just like any other C function call.
mycallback (window);
Related
I found conception of Delegates pretty hard for me. I really do not understand why I can't simply pass one function to another and need to wrap it to Delegate. I read in docs that there is some cases when I do not know it's name and Delegate is only way to call it.
But now I have trouble in understanding conception of callbacks. I tried to find more information, but I can't understand is it's simply call of other function or what is it.
Could you show examples of D callbacks and explain where they can be helpful?
import vibe.d;
shared static this()
{
auto settings = new HTTPServerSettings;
settings.port = 8080;
listenHTTP(settings, &handleRequest);
}
void handleRequest(HTTPServerRequest req,
HTTPServerResponse res)
{
if (req.path == "/")
res.writeBody("Hello, World!", "text/plain");
}
&handleRequest is it callback? How it's work and at what moment it's start?
So within memory a function is just a pile of bytes. Like an array, you can take a pointer to it. This is a function pointer. It has a type of RETT function(ARGST) in D. Where RETT is the return type and ARGST are the argument types. Of course attributes can be applied like any function declaration.
Now delegates are a function pointer with a context pointer. A context pointer can be anything from a single integer (argument), call frame (function inside of another) or lastly a class/struct.
A delegate is very similar to a function pointer type at RETT delegate(ARGST). They are not interchangeable, but you can turn a function pointer into a delegate pointer pretty easily.
The concept of a callback is to say, hey I know you will know about X so when that happens please tell me about X by calling this function/delegate.
To answer your question about &handleRequest, yes it is a callback.
You can pass functions to other functions to later be called.
void test(){}
void receiver(void function() fn){
// call it like a normal function with 'fn()'
// or pass it around, save it, or ignore it
}
// main
receiver(&test); // 'test' will be available as 'fn' in 'receiver'
You need to prepend the function name as argument with & to clarify you want to pass a function pointer. If you don't do that, it will instead call that function due to UFCS (calling without braces). It is not a delegate yet.
The function that receives your callable may do whatever it wants with it. A common example is in your question, a web service callback. First you tell the framework what should be done in case a request is received (by defining actions in a function and making that function available for the framework), and in your example enter a loop with listenHTTP which calls your code when it receives a request. If you want to read more on this topic: https://en.wikipedia.org/wiki/Event_(computing)#Event_handler
Delegates are function pointers with context information attached. Say you want to add handlers that act on other elements available in the current context. Like a button that turns an indicator red. Example:
class BuildGui {
Indicator indicator;
Button button;
this(){
... init
button.clickHandler({ // curly braces: implicit delegate in this case
indicator.color = "red"; // notice access of BuildGui member
});
button.clickHandler(&otherClickHandler); // methods of instances can be delegates too
}
void otherClickHandler(){
writeln("other click handler");
}
}
In this imaginary Button class all click handlers are saved to a list and called when it is clicked.
There were several questions in the OP. I am going to try to answer the following two:
Q: Could you show examples of D callbacks and explain where they can be helpful?
A: They are commonly used in all languages that support delegates (C# for an example) as event handlers. - You give a delegate to be called whenever an event is triggered. Languages that do not support delegates use either classes, or callback functions for this purpose. Example how to use callbacks in C++ using the FLTK 2.0 library: http://www.fltk.org/doc-2.0/html/group__example2.html. Delegates are perfect for this as they can directly access the context. When you use callbacks for this purpose you have to pass along all the objects you want to modify in the callback... Check the mentioned FLTK link as an example - there we have to pass a pointer to the fltk::Window object to the window_callback function in order to manipulate it. (The reason why FLTK does this is that back FLTK was born C++ did not have lambdas, otherwise they would use them instead of callbacks)
Example D use: http://dlang.org/phobos/std_signals.html
Q: Why I can't simply pass one function to another and need to wrap it to Delegate?
A: You do not have to wrap to delegates - it depends what you want to accomplish... Sometimes passing callbacks will just work for you. You can't access context in which you may want to call the callback, but delegates can. You can, however pass the context along (and that is what some C/C++ libraries do).
I think what you are asking is explained in the D language reference
Quote 1:
A function pointer can point to a static nested function
Quote 2:
A delegate can be set to a non-static nested function
Take a look at the last example in that section and notice how a delegate can be a method:
struct Foo
{
int a = 7;
int bar() { return a; }
}
int foo(int delegate() dg)
{
return dg() + 1;
}
void test()
{
int x = 27;
int abc() { return x; }
Foo f;
int i;
i = foo(&abc); // i is set to 28
i = foo(&f.bar); // i is set to 8
}
There are already excellent answers. I just want to try to make simple summary.
Simply: delegate allows you to use methods as callbacks.
In C, you do the same by explicitly passing the object (many times named context) as void* and cast it to (hopefully) right type:
void callback(void *context, ...) {
/* Do operations with context, which is usually a struct */
doSomething((struct DATA*)context, ...);
doSomethingElse((struct DATA*)context, ...);
}
In C++, you do the same when wanting to use method as callback. You make a function taking the object pointer explicitly as void*, cast it to (hopefully) right type, and call method:
void callback(void* object, ...) {
((MyObject*)object)->method(...);
}
Delegate makes this all implicitly.
In a custom converter, I am checking whether a sequence item is some type. So far I've had this code (simplified)
namespace bp=boost::python;
/* ... */
static void* convertible(PyObject* seq_ptr){
if(!PySequence_Check(seq_ptr)) return 0;
for(int i=0; i<PySequence_Size(seq_ptr); i++)
if(!bp::extract<double>(PySequence_GetItem(seq_ptr,i)).check()) return 0;
/* ... */
}
/* ... */
but this is leaking memory, since PySequence_GetItem is returning a new reference. So either I can do something like this in the loop:
PyObject* it=PySequence_GetItem(seq_ptr,i);
bool ok(bp::extract<double>(it).check();
Py_DECREF(it); // will delete the object which had been newly created
if(!ok) return 0;
but that is quite clumsy; I could make a stand-alone function doing that, but that is where I recalled bp::handle implementing the ref-counting machinery; so something like this might do:
if(!bp::extract<double>(bp::handle<>(PySequence_GetItem(seq_ptr,i))).check()) return 0;
but this page mentions using handles as temporaries is discouraged. Why? Can the object be destroyed before .check() is actually called? Is there some other elegant way to write this?
The object will not be destroyed before the .check() is called and is safe in the posted context.
The recommendation to not use temporaries is due to the unspecified order of evaluation of the arguments and exception safety. If there is only one order in which arguments can be evaluated, such as in your example, then it is safe. For instance, consider function bad() which always throws an exception:
f(boost::python::handle<>(PySequence_GetItem(...)), bad());
If bad() gets evaluated between PySequence_GetItem(...) and boost::python::handle<>(...), then the new reference will be leaked as the stack will begin to unwind before the construction of boost::python::handle<>. On the other hand, when a non-temporary is used, there is no chance for something to throw between PySequence_GetItem() and boost::python::handle<>(), so the following is safe in the presence of exceptions:
boost::python::handle<> item_handle(PySequence_GetItem(...));
f(item_handle, bad());
Consider reading Herb Sutter's GotW #56: Exception-Safe Function Calls for more details.
I have a function className.doSomething() which returns an observable.I want to know the time when this function was called or subscribed.
className.doSomething()
:callAtSubscribe( "print current time")
:tap()
:subscribe()
Need something appropriate in place of callAtSubscribe.
You can wrap the method in an Observable.Create. This will allow you to track when the subscription is made. If you then wrapped that in a method, you could track when the method was called. This would allow you to see when the method was called (but potentially not subscribed to the result).
/**
* Module dependencies.
*/
var Rx = require('rx');
Rx.Observable.prototype.log = function (logger, sourceName) {
var source = this;
logger.debug({source : sourceName, kind : 'Created'});
return Rx.Observable.create(function(observer) {
logger.debug({source : sourceName, kind : 'Subscribe'});
return source.subscribe(observer);
});
};
This is a very cut down version of a log method I use. It can be heavily modified to include timings, wiretap of values, error, completions and terminations.
I am wondering how do I hook to a function that is in the dylib, i.e. a C function.
My target is to hook to a function CTRegistrationSetCellularDataIsEnabled that is in CoreTelephony.
Thanks!
You will need access to MobileSubtrate if you want any hope of hooking a dylib function, which is done like so (hooking a function called CFShow(), from here):
static void (*original_CFShow)(CFTypeRef obj); // a function pointer to store the original CFShow().
void replaced_CFShow(CFTypeRef obj) {
// our replacement of CFShow().
printf("Calling original CFShow(%p)...", obj);
original_CFShow(obj); // calls the original CFShow.
printf(" done.\n");
}
// hook CFShow to our own implementation.
MSHookFunction(CFShow, replaced_CFShow, &original_CFShow);
// From now on any call to CFShow will pass through replaced_CFShow first.
CFShow(CFSTR("test"));
You will need to use class dump to get the headers from the classes you need to use, and then call this function
I'm working on finishing up my server for my first iPhone application, and I want to implement a simple little feature.
I would like to run a function (perhaps method as well), if another function returns a certain value after a certain waiting period. Fairly simple concept.... right?
Here's my basic foundation.
template <typename T,class TYP>
struct funcpar{
T (*function)(TYP);
TYP parameter;
funcpar(T (*func)(TYP),TYP param);
funcpar& operator=(const funcpar& fp);
};
The goal here is to be able to call funcpar::function(funcpar::parameter) to run the stored function and parameter, and not have to worry about anything else...
When I attempted to use a void* parameter instead of the template, I couldn't copy the memory as an object (because I didn't know what the end object was going to be, or the beginning for that matter) and when I tried multiple timers, every single object's parameter would change to the new parameter passed to the new timer... With the previous struct I have a
question:
Is it possible to make an all-inclusive pointer to this type of object inside a method of a class? Can I templatize a method, and not the whole class? Would it work exactly like a function template?
I have a managing class that holds a vector of these "jobs" and takes care of everything fairly well. I just don't know how to use a templatized function with the struct, or how to utilize templates on a single method in a class..
I'm also utilizing this in my custom simple threadpool, and that's working fairly well, and has the same problems...
I have another question:
Can I possibly store a function with a parameter before it's run? Something like toRun = dontrunmeyet(withThisParameter);? Is my struct even necessary?
Am I going about this whole thing incorrectly?
If this is overly ambiguous, I can set you up with my whole code for context
In order to create a class method that takes a template parameter, yes, it would work almost exactly like a function template. For example:
class A
{
public:
template<typename T>
void my_function(const T& value) { }
};
int main()
{
A test;
test.my_function(5);
return 0;
}
Secondly, for your structure, you can actually turn that into a functor-object that by overloading operator(), lets you call the structure as-if it were a function rather than having to actually call the specific function pointer members inside the structure. For instance, your structure could be re-written to look like this:
#include <iostream>
template <class ReturnType, class ParameterType>
class funcpar
{
private:
ReturnType (*function)(ParameterType);
ParameterType parameter;
public:
funcpar(ReturnType (*func)(ParameterType),ParameterType param):
function(func), parameter(param) {}
funcpar& operator=(const funcpar& fp);
//operator() overloaded to be a function that takes no arguments
//and returns type ReturnType
ReturnType operator() ()
{
return function(parameter);
}
};
int sample_func(int value)
{
return value + 1;
}
int main()
{
funcpar<int, int> test_functor(sample_func, 5);
//you can call any instance of funcpar just like a normal function
std::cout << test_functor() << std::endl;
return 0;
}
BTW, you do need the functor object (or your structure, etc.) in order to bind a dynamic parameter to a function before the function is called in C/C++ ... you can't "store" a parameter with an actual function. Binding a parameter to a function is actually called a closure, and in C/C++, creating a closure requires a structure/class or some type of associated data-structure you can use to bind a function with a specific parameter stored in memory that is used only for a specific instance of that function call.