Added method to register boolean filters (#160)

* added method to register boolean filters

* parametrised negative filter name

* Update Extension.swift

* Update CHANGELOG.md

* renamed registerBooleanFilter to registerFilter

* updated docs

CHANGELOG.md

@@ -19,6 +19,9 @@
 - You can now use parentheses in boolean expressions to change operator precedence.  
   [Ilya Puchka](https://github.com/ilyapuchka)
   [#165](https://github.com/stencilproject/Stencil/pull/165)
+- Added method to add boolean filters with their negative counterparts.  
+  [Ilya Puchka](https://github.com/ilyapuchka)
+  [#160](https://github.com/stencilproject/Stencil/pull/160)
 
 ### New Features
 

Sources/Extension.swift

@@ -18,6 +18,15 @@ open class Extension {
       return SimpleNode(token: token, handler: handler)
     })
   }
+  
+  /// Registers boolean filter with it's negative counterpart
+  public func registerFilter(name: String, negativeFilterName: String, filter: @escaping (Any?) throws -> Bool?) {
+    filters[name] = .simple(filter)
+    filters[negativeFilterName] = .simple {
+      guard let result = try filter($0) else { return nil }
+      return !result
+    }
+  }
 
   /// Registers a template filter with the given name
   public func registerFilter(_ name: String, filter: @escaping (Any?) throws -> Any?) {

Tests/StencilTests/FilterSpec.swift

@@ -22,6 +22,24 @@ class FilterTests: XCTestCase {
         let result = try template.render(Context(dictionary: context, environment: Environment(extensions: [repeatExtension])))
         try expect(result) == "Kyle Kyle"
       }
+        
+      $0.it("allows you to register boolean filters") {
+        let repeatExtension = Extension()
+        repeatExtension.registerFilter(name: "isPositive", negativeFilterName: "isNotPositive") { (value: Any?) in
+          if let value = value as? Int {
+            return value > 0
+          }
+          return nil
+        }
+        
+        let result = try Template(templateString: "{{ value|isPositive }}")
+          .render(Context(dictionary: ["value": 1], environment: Environment(extensions: [repeatExtension])))
+        try expect(result) == "true"
+        
+        let negativeResult = try Template(templateString: "{{ value|isNotPositive }}")
+          .render(Context(dictionary: ["value": -1], environment: Environment(extensions: [repeatExtension])))
+        try expect(negativeResult) == "true"
+      }
 
       $0.it("allows you to register a custom filter which accepts single argument") {
         let template = Template(templateString: """

docs/custom-template-tags-and-filters.rst

@@ -48,6 +48,17 @@ Registering custom filters with arguments:
       return value
     }
 
+Registering custom boolean filters:
+
+.. code-block:: swift
+
+    ext.registerFilter("ordinary", negativeFilterName: "odd") { (value: Any?) in
+        if let value = value as? Int {
+            return myInt % 2 == 0
+        }
+        return nil
+    }
+
 Custom Tags
 -----------