Documented shared Observed.

Author: 5cript

docs/tutorials/observed_rendering.md

@@ -12,11 +12,36 @@ Nui::Observed<int> number{0};
 
 const auto ui = div{}(
     observe(str, number),
-    // This function is recalled and regenerates its respective elements, 
+    // This function is recalled and regenerates its respective elements,
     // when 'str' or 'number' changes.
-    [&str, &number](){
+    [&str, &number]() -> Nui::ElementRenderer {
         const auto result = *str + std::to_string(*number);
         return span{}(result);
     }
 )
+```
+
+## shared_ptr of Observed
+
+Sometimes Observed may expire while the element affected by it is still part of the DOM.
+Nui can use weak_ptrs to avoid dangling pointers to expired Observed.
+```cpp
+#include <nui/frontend.hpp>
+
+/*...*/
+
+std::shared_ptr<Nui::Observed<std::string>> str = std::make_shared<Nui::Observed<std::string>>("test");
+
+const auto ui = div{}(
+    observe(str),
+    // This function is recalled and regenerates its respective elements,
+    // when 'str' changes. Unless str is expired.
+    [weak = std::weak_ptr{str}]() -> Nui::ElementRenderer {
+        if (auto str = weak.lock(); str)
+        {
+            return span{}(str->value());
+        }
+        return Nui::nil();
+    }
+)
 ```