Add boolean version of setAttr (#240)

* vdom: Add setAttr proc with boolean val

* README: Tweak description of boolean attributes

* Use cstring(nil) approach in boolean setAttr

karax/vdom.nim

@@ -253,6 +253,19 @@ proc setAttr*(n: VNode; key: kstring; val: kstring = "") =
     n.attrs.add key
     n.attrs.add val
 
+proc setAttr*(n: VNode, key: kstring, val: bool) =
+  when defined(js):
+    n.setAttr(key, if val: cstring"" else: cstring(nil))
+  else:
+    if val:
+      n.setAttr(key, "")
+    else:
+      for i in countup(0, n.attrs.len-2, 2):
+        if n.attrs[i] == key:
+          n.attrs.delete i+1
+          n.attrs.delete i
+          break
+
 proc getAttr*(n: VNode; key: kstring): kstring =
   for i in countup(0, n.attrs.len-2, 2):
     if n.attrs[i] == key: return n.attrs[i+1]

readme.md

@@ -348,6 +348,25 @@ setError password, password & " must not be empty"
 ```
 There are likely more elegant solutions to this problem.
 
+## Boolean attributes
+
+Some HTML attributes don't have meaningful values; instead, they are treated like
+a boolean whose value is `false` when the attribute is not set, and `true` when
+the attribute is set to any value. Some examples of these attributes are `disabled`
+and `contenteditable`.
+
+In Karax, these attributes can be set/cleared with a boolean value:
+
+```nim
+proc submitButton(dataIsValid: bool): VNode =
+  buildHtml(tdiv):
+    button(disabled = not dataIsValid):
+      if dataIsValid:
+        text "Submit"
+      else:
+        text "Cannot submit, data is invalid!"
+```
+
 ## Routing