handle inherit in style resolution and add regression coverage

Author: Nadeem Shadan

src/StyleContextStack.js

@@ -1,5 +1,7 @@
 import { isString, isValue } from './helpers/variableType';
 
+const isInheritValue = value => isString(value) && value.toLowerCase() === 'inherit';
+
 /**
  * Used for style inheritance and style overrides
  */
@@ -127,7 +129,9 @@ class StyleContextStack {
 			}
 
 			if (isValue(style[property])) {
-				return style[property];
+				if (!isInheritValue(style[property])) {
+					return style[property];
+				}
 			}
 
 			if (style.extends) {
@@ -153,12 +157,16 @@ class StyleContextStack {
 						return value;
 					}
 				} else if (isValue(item[property])) { // style-overrides-object
+					if (isInheritValue(item[property])) {
+						continue;
+					}
 					return item[property];
 				}
 			}
 		}
 
-		return this.defaultStyle && this.defaultStyle[property];
+		let defaultValue = this.defaultStyle && this.defaultStyle[property];
+		return isInheritValue(defaultValue) ? undefined : defaultValue;
 	}
 
 	/**
@@ -172,7 +180,9 @@ class StyleContextStack {
 		let value;
 
 		if (isValue(item[property])) { // item defines this property
-			return item[property];
+			if (!isInheritValue(item[property])) {
+				return item[property];
+			}
 		}
 
 		if (!styleContextStack) {

tests/unit/StyleContextStack.spec.js

@@ -195,6 +195,37 @@ describe('StyleContextStack', function () {
 			assert.equal(StyleContextStack.getStyleProperty({ font: 'ZapfDingbats', bold: true }, null, 'font', 'Arial'), 'ZapfDingbats');
 		});
 
+		it('should resolve "inherit" from node style overrides', function () {
+			var stack = new StyleContextStack({
+				parent: {
+					color: 'blue',
+				},
+				child: {
+					extends: 'parent'
+				}
+			}, {
+				color: 'black'
+			});
+
+			assert.equal(StyleContextStack.getStyleProperty({ style: 'child', color: 'inherit' }, stack, 'color', 'red'), 'blue');
+		});
+
+		it('should resolve "inherit" declared in named style definitions', function () {
+			var stack = new StyleContextStack({
+				parent: {
+					color: 'blue',
+				},
+				child: {
+					color: 'inherit',
+					extends: 'parent'
+				}
+			}, {
+				color: 'black'
+			});
+
+			assert.equal(StyleContextStack.getStyleProperty({ style: 'child' }, stack, 'color', 'red'), 'blue');
+		});
+
 		it('should process extends styles', function () {
 			var stack = new StyleContextStack({
 				header: {