Add Json helpers

src/client/response.rs

@@ -3,7 +3,7 @@
 use std::error::Error;
 use std::fmt;
 
-use rustc_serialize::json::Json;
+use rustc_serialize::json::{self, Json};
 
 /// Response parsing.
 pub trait FromResponse: Sized {
@@ -54,3 +54,29 @@ impl fmt::Display for ParseError {
 impl Error for ParseError {
     fn description(&self) -> &str { "response parse error" }
 }
+
+/// JSON helper for response parsing.
+#[derive(Debug)]
+pub struct JsonHelper<'a>(pub &'a Json);
+
+impl<'a> JsonHelper<'a> {
+    /// Returns self as a `JsonObjectHelper` or fails with `ParseError::ExpectedType`.
+    pub fn as_object(&self) -> Result<JsonObjectHelper<'a>, ParseError>{
+        self.0.as_object()
+            .ok_or(ParseError::ExpectedType("object"))
+            .map(|o| JsonObjectHelper(o))
+    }
+}
+
+/// JSON object helper for response parsing.
+#[derive(Debug)]
+pub struct JsonObjectHelper<'a>(pub &'a json::Object);
+
+impl<'a> JsonObjectHelper<'a> {
+    /// Gets a field as a string or fails with `ParseError::ExpectedFieldType`.
+    pub fn get_string(&self, key: &'static str) -> Result<&'a str, ParseError> {
+        self.0.get(key)
+            .and_then(Json::as_string)
+            .ok_or(ParseError::ExpectedFieldType(key, "string"))
+    }
+}