extend skipHook to skip object fields based on values

README.md

@@ -277,6 +277,42 @@ Gives us:
 "{"a":1,"b":0.5}"
 ```
 
+Also, `skipHook` can be used at runtime to skip fields based on their current value. For example:
+```nim
+type
+  Boo = ref object
+    hoo: string
+    x: seq[string]
+    y: seq[int]
+    i: int
+    b: bool
+
+  Woo = object
+    boo: Boo
+
+proc skipHook(v: Boo, key: string): bool =
+  ## Skip `nil` fields
+  result = v == nil
+
+proc skipHook(v: string, key: string): bool =
+  ## skip all fields that contains empty strings
+  result = v.len == 0
+
+proc skipHook[T](v: seq[T], key: string): bool =
+  # skip all fields that contains empty sequences
+  result = v.len == 0
+
+let w = Woo(boo: Boo())
+assert w.toJson() == """{"boo":{"i":0,"b":false}}"""
+
+let w2 = Woo()
+assert w2.toJson() == "{}"
+
+let b = Boo()
+assert b.toJson() == 
+  """{"i":0,"b":false}"""
+```
+
 ## Static writing with `toStaticJson`.
 
 Sometimes you have some json, and you want to write it in a static way. There is a special function for that:

src/jsony.nim

@@ -827,6 +827,24 @@ proc dumpHook*(s: var string, v: object) =
           s.dumpKey(k)
           s.dumpHook(e)
           inc i
+      elif compiles(skipHook(type(v), e, k)):
+        if skipHook(type(v), e, k):
+          discard
+        else:
+          if i > 0:
+            s.add ','
+          s.dumpKey(k)
+          s.dumpHook(e)
+          inc i
+      elif compiles(skipHook(e, k)):
+        if skipHook(e, k):
+          discard
+        else:
+          if i > 0:
+            s.add ','
+          s.dumpKey(k)
+          s.dumpHook(e)
+          inc i
       else:
         if i > 0:
           s.add ','

tests/test_skipHook.nim

@@ -1,5 +1,4 @@
 import jsony
-
 type
   Conn = object
     id: int
@@ -15,3 +14,45 @@ proc skipHook(T: typedesc[Foo], key: static string): bool =
 let v = Foo(a:1, password: "12345", b:0.6, conn: Conn(id: 1))
 doAssert v.toJson() ==
   """{"a":1,"b":0.6}"""
+
+type
+  Boo = ref object
+    hoo: string
+    x: seq[string]
+    y: seq[int]
+    i: int
+    b: bool
+
+  Woo = object
+    boo: Boo
+
+  Moo = object
+    boo: Boo
+
+proc skipHook(T: typedesc[Woo], v: Boo, key: string): bool =
+  result = v == nil
+
+proc skipHook(v: string, key: string): bool =
+  result = v.len == 0
+
+proc skipHook[T](v: seq[T], key: string): bool =
+  result = v.len == 0
+
+let w = Woo(boo: Boo())
+doAssert w.toJson() == 
+  """{"boo":{"i":0,"b":false}}"""
+let w2 = Woo()
+doAssert w2.toJson() ==
+  """{}"""
+let b = Boo()
+doAssert b.toJson() == 
+  """{"i":0,"b":false}"""
+
+block:
+  let m = Moo() # not covered by `skipHook`
+  doAssert m.toJson() ==
+    """{"boo":null}"""
+block:
+  let m = Moo(boo: Boo(hoo: "just moo"))
+  doAssert m.toJson() ==
+    """{"boo":{"hoo":"just moo","i":0,"b":false}}"""