fix(forms): default to unsetValue for value accessors

Author: sis0k0

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

@@ -1,5 +1,7 @@
 import { ControlValueAccessor } from "@angular/forms";
-import { View } from "tns-core-modules/ui/core/view";
+import { View, unsetValue } from "tns-core-modules/ui/core/view";
+
+import { isBlank } from "../lang-facade";
 
 export class BaseValueAccessor<TView extends View> implements ControlValueAccessor {
     private pendingChangeNotification: number = 0;
@@ -28,5 +30,9 @@ export class BaseValueAccessor<TView extends View> implements ControlValueAccess
         this.view.isEnabled = !isDisabled;
     }
 
-    writeValue(_: any) { }
+    writeValue(_: any) {}
+
+    protected normalizeValue(value: any): any {
+        return isBlank(value) ? unsetValue : value;
+    }
 }

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

@@ -34,6 +34,7 @@ export class CheckedValueAccessor extends BaseValueAccessor<Switch> { // tslint:
     }
 
     writeValue(value: any): void {
-        this.view.checked = value;
+        const normalized = super.normalizeValue(value);
+        this.view.checked = normalized;
     }
 }

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

@@ -35,6 +35,7 @@ export class DateValueAccessor extends BaseValueAccessor<DatePicker> { // tslint
     }
 
     writeValue(value: any): void {
-        this.view.date = value;
+        const normalized = super.normalizeValue(value);
+        this.view.date = normalized;
     }
 }

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

@@ -34,6 +34,7 @@ export class NumberValueAccessor extends BaseValueAccessor<Slider> { // tslint:d
     }
 
     writeValue(value: any): void {
-        this.view.value = value;
+        const normalized = super.normalizeValue(value);
+        this.view.value = normalized;
     }
 }

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

@@ -51,7 +51,8 @@ export class SelectedIndexValueAccessor extends BaseValueAccessor<SelectableView
     private viewInitialized: boolean;
 
     writeValue(value: any): void {
-        this.value = value;
+        const normalized = super.normalizeValue(value);
+        this.value = normalized;
 
         if (this.viewInitialized) {
             this.view.selectedIndex = this.value;

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

@@ -48,6 +48,7 @@ export class TextValueAccessor extends BaseValueAccessor<TextView> { // tslint:d
     }
 
     writeValue(value: any): void {
-        this.view.text = value;
+        const normalized = super.normalizeValue(value);
+        this.view.text = normalized;
     }
 }

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

@@ -36,6 +36,7 @@ export class TimeValueAccessor extends BaseValueAccessor<TimePicker> { // tslint
     }
 
     writeValue(value: any): void {
-        this.view.time = value;
+        const normalized = super.normalizeValue(value);
+        this.view.time = normalized;
     }
 }