Add UIRef

This commit adds an opaque `UIRef props state` type constructor which represents
a reference to UI component. The following functions available for working with
UI refs:

  * getSelf returns a refernce to a current React component

  * runUI is a handler which gets an UI reference and executes React effects
    with it (it also checks if current UI reference is still mounted and so
    valid)

The motivation for this change is to add an ability to defer React for a some
period of time (XHR, timer).

Author: andreypopp

src/React.purs

@@ -7,6 +7,7 @@ module React where
   foreign import data ReadStateEff :: * -> !
   foreign import data WriteStateEff :: * -> !
   foreign import data UI :: *
+  foreign import data UIRef :: * -> * -> *
   foreign import data EventHandler :: * -> *
 
   foreign import noop0
@@ -99,6 +100,33 @@ module React where
     \ }"
     :: forall state eff. Eff (r :: ReadStateEff state | eff) state
 
+  foreign import getSelf
+    " function getSelf() { \
+    \   return __current;  \
+    \ }"
+    :: forall eff props state.
+    Eff (p :: ReadPropsEff props, r :: ReadStateEff state | eff) (UIRef props state)
+
+  foreign import runUI
+    " function runUI(ref) {         \
+    \   return function(action) {   \
+    \     return function() {       \
+    \       if (ref.isMounted()) {  \
+    \         __current = ref;      \
+    \         try {                 \
+    \           return action();    \
+    \         } finally {           \
+    \           __current = null;   \
+    \         }                     \
+    \       }                       \
+    \     }                         \
+    \   }                           \
+    \ }"
+    :: forall eff props state result.
+    UIRef props state
+    -> Eff (p :: ReadPropsEff props, r :: ReadStateEff state, w :: WriteStateEff state | eff) result
+    -> Eff (eff) result
+
   foreign import mkUI
     " var __current;                                    \
     \ function mkUI(ss) {                               \