Add docs for ExampleIterator

src/spectator/example_iterator.cr

@@ -1,40 +1,69 @@
 module Spectator
+  # Iterates through all examples in a group and its nested groups.
   class ExampleIterator
     include Iterator(Example)
 
+    # Stack that contains the iterators for each group.
+    # A stack is used to track where in the tree this iterator is.
     @stack : Array(Iterator(ExampleComponent))
 
+    # Creates a new iterator.
+    # The `group` is the example group to iterate through.
     def initialize(@group : Iterable(ExampleComponent))
       iter = @group.each.as(Iterator(ExampleComponent))
       @stack = [iter]
     end
 
+    # Retrieves the next `Example`.
+    # If there are none left, then `Iterator::Stop` is returned.
     def next
+      # Keep going until either:
+      # a. an example is found.
+      # b. the stack is empty.
       until @stack.empty?
+        # Retrieve the next "thing".
+        # This could be an `Example`,
+        # or a group.
         item = advance
+        # Return the item if it's an example.
+        # Otherwise, advance and check the next one.
         return item if item.is_a?(Example)
       end
+      # Nothing left to iterate.
       stop
     end
 
+    # Restart the iterator at the beginning.
     def rewind
+      # Same code as `#initialize`, but return self.
       iter = @group.each.as(Iterator(ExampleComponent))
       @stack = [iter]
       self
     end
 
+    # Retrieves the top of the stack.
     private def top
       @stack.last
     end
 
+    # Retrieves the next "thing" from the tree.
+    # This method will return an `Example` or "something else."
     private def advance
-      item = top.next
-      case (item)
+      # Get the iterator from the top of the stack.
+      # Advance the iterator and check what the next item is.
+      case (item = top.next)
       when ExampleGroup
+        # If the next thing is a group,
+        # we need to traverse its branch.
+        # Push its iterator onto the stack and return.
         @stack.push(item.each)
       when Iterator::Stop
+        # If a stop instance is encountered,
+        # then the current group is done.
+        # Pop its iterator from the stack and return.
         @stack.pop
       else
+        # Found an example, return it.
         item
       end
     end