fix(forms): add setDisabledState handler for base value accessor

enables [disabled] for form controls

Author: sis0k0

Diff for: nativescript-angular/value-accessors/base-value-accessor.ts

@@ -1,6 +1,7 @@
 import { ControlValueAccessor } from "@angular/forms";
+import { View } from "tns-core-modules/ui/core/view";
 
-export class BaseValueAccessor<TView> implements ControlValueAccessor {
+export class BaseValueAccessor<TView extends View> implements ControlValueAccessor {
     private pendingChangeNotification: number = 0;
     onChange = (_) => { };
     onTouched = () => {};
@@ -19,7 +20,13 @@ export class BaseValueAccessor<TView> implements ControlValueAccessor {
         };
     }
 
-    registerOnTouched(fn: () => void): void { this.onTouched = fn; }
+    registerOnTouched(fn: () => void): void {
+        this.onTouched = fn;
+    }
+
+    setDisabledState(isDisabled: boolean): void {
+        this.view.isEnabled = !isDisabled;
+     };
 
     writeValue(_: any) { }
 }