Merge pull request #13640 from andrey-golubev:gapi_gcallpriv_doc

* GAPI: document GCall's recursive behavior

* Move documentation outside the class

modules/gapi/src/api/gcall_priv.hpp

@@ -19,13 +19,31 @@
 
 namespace cv {
 
+// GCall is used to capture details (arguments) passed to operation when the graph is
+// constructed. It is, in fact, just a "serialization" of a function call (to some extent). The
+// only place where new GCall objects are constructed is KernelName::on(). Note that GCall not
+// only stores its input arguments, but also yields operation's pseudo-results to return
+// "results".
+// GCall arguments are GArgs which can wrap either our special types (like GMat) or other
+// stuff user may pass according to operation's signature (opaque to us).
+// If a dynamic g-object is wrapped in GArg, it has origin - something where that object comes
+// from. It is either another function call (again, a GCall) or nothing (for graph's starting
+// points, for example). By using these links, we understand what the flow is and construct the
+// real graph. Origin is a node in a graph, represented by GNode.
+// When a GCall is created, it instantiates it's appropriate GNode since we need an origin for
+// objects we produce with this call. This is what is stored in m_node and then is used in every
+// yield() call (the framework calls yield() according to template signature which we strip then
+// - aka type erasure).
+// Here comes the recursion - GNode knows it is created for GCall, and GCall stores that node
+// object as origin for yield(). In order to break it, in GNode's object destructor this m_node
+// pointer is reset (note - GCall::Priv remains alive). Now GCall's ownership "moves" to GNode
+// and remains there until the API part is destroyed.
 class GCall::Priv
 {
 public:
     std::vector<GArg> m_args;
     const GKernel     m_k;
 
-    // FIXME: Document that there's no recursion here.
     // TODO: Rename to "constructionNode" or smt to reflect its lifetime
     GNode             m_node;