check for property via selector before using value(forKey:)

CHANGELOG.md

@@ -4,7 +4,9 @@
 
 ### Bug Fixes
 
- _None_
+- Now accessing undefined keys in NSObject does not cause runtime crash and instead renders empty string.
+  [Ilya Puchka](https://github.com/ilyapuchka)
+  [#234](https://github.com/stencilproject/Stencil/pull/234)
 
 ### Breaking Changes
 

Sources/Variable.swift

@@ -104,7 +104,9 @@ public struct Variable : Equatable, Resolvable {
         #if os(Linux)
           return nil
         #else
+          if object.responds(to: Selector(bit)) {
             current = object.value(forKey: bit)
+          }
         #endif
       } else if let value = current {
         current = Mirror(reflecting: value).getValue(for: bit)

Tests/StencilTests/VariableSpec.swift

@@ -148,6 +148,12 @@ func testVariable() {
       let result = try variable.resolve(context) as? String
       try expect(result) == "Foo"
     }
+
+    $0.it("does not crash on KVO") {
+      let variable = Variable("object.fullname")
+      let result = try variable.resolve(context) as? String
+      try expect(result).to.beNil()
+    }
 #endif
 
     $0.it("can resolve a value via reflection") {