DesignKit: Update ElementFonts with new Fonts protocol.

DesignKit/Extensions/UIFont.swift

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+// 
+// Copyright 2021 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import UIKit
+
+public extension UIFont {
+    
+    // MARK: - Convenient methods
+    
+    /// Update current font with a SymbolicTraits
+    func vc_withTraits(_ traits: UIFontDescriptor.SymbolicTraits) -> UIFont {
+        guard let descriptor = fontDescriptor.withSymbolicTraits(traits) else {
+            return self
+        }
+        return UIFont(descriptor: descriptor, size: 0) // Size 0 means keep the size as it is
+    }
+    
+    /// Update current font with a given Weight
+    func vc_withWeight(weight: Weight) -> UIFont {
+        // Add the font weight to the descriptor
+        let weightedFontDescriptor = fontDescriptor.addingAttributes([
+            UIFontDescriptor.AttributeName.traits: [
+                UIFontDescriptor.TraitKey.weight: weight
+            ]
+        ])
+        return UIFont(descriptor: weightedFontDescriptor, size: 0)
+    }
+    
+    // MARK: - Shortcuts
+    
+    var vc_bold: UIFont {
+        return self.vc_withTraits(.traitBold)
+    }
+    
+    var vc_semiBold: UIFont {
+        return self.vc_withWeight(weight: .semibold)
+    }
+
+    var vc_italic: UIFont {
+        return self.vc_withTraits(.traitItalic)
+    }
+}