feat(context_extensions): add byFormFactor<T>

Author: Jens Becker

README.md

@@ -99,7 +99,12 @@ All features with links to their page in the documentation:
       Returns the correct [FormFactor](https://pub.dev/documentation/fleasy/latest/fleasy/FormFactor-class.html) based on the [FormFactorBreakpoints](https://pub.dev/documentation/fleasy/latest/fleasy/FormFactorBreakpoints-class.html).
     - #### [screenSize](https://pub.dev/documentation/fleasy/latest/fleasy/AdpativeContextExtensions/screenSize.html)
       Returns the correct [ScreenSize](https://pub.dev/documentation/fleasy/latest/fleasy/ScreenSize-class.html) based on the [FormFactorBreakpoints](https://pub.dev/documentation/fleasy/latest/fleasy/FormFactorBreakpoints-class.html). 
-     
+    - #### [byFormFactor<T>](https://pub.dev/documentation/fleasy/latest/fleasy/AdpativeContextExtensions/byFormFactor.html)
+      Returns onWatch, onHandset, onTablet or onDesktop depending on the current [formFactor](https://pub.dev/documentation/fleasy/latest/fleasy/AdpativeContextExtensions/formFactor.html).
+      Example:
+      ```dart
+      double width = context.byFormFactor<double>(onHandset: 10, onTablet: 25, onDesktop: 50);
+      ```   
  
 - ### Extensions on `BuildContext` (Navigation helpers):
     - #### [pushPage(...)](https://pub.dev/documentation/fleasy/latest/fleasy/NavigationContextExtensions/pushPage.html)

lib/src/extensions/context_extensions.dart

@@ -93,6 +93,27 @@ extension AdpativeContextExtensions on BuildContext {
                 ? ScreenSize.normal
                 : ScreenSize.small;
   }
+
+  /// Returns onWatch, onHandset, onTablet or onDesktop depending on the current [formFactor].
+  ///
+  /// Example:
+  /// ```dart
+  /// double width = context.byFormFactor<double>(onHandset: 10, onTablet: 25, onDesktop: 50);
+  /// ```
+  T byFormFactor<T>({
+    T? onWatch,
+    required T onHandset,
+    required T onTablet,
+    required T onDesktop,
+  }) {
+    return formFactor.isWatch
+        ? onWatch ?? onHandset
+        : formFactor.isHandset
+            ? onHandset
+            : formFactor.isTablet
+                ? onTablet
+                : onDesktop;
+  }
 }
 
 extension NavigationContextExtensions on BuildContext {