aiya000 · July 8, 2019 05:15 · Jul 8, 2019 · Jul 5, 2019 · Jul 4, 2019 · Jul 4, 2019
diff --git a/Observable.ts b/Observable.ts
@@ -1,23 +1,12 @@
 import * as Untyped from 'tns-core-modules/data/observable'
 import deprecated from 'deprecated-decorator'  // npm install --save-dev deprecated-decorator
-
-/**
- * ValueTypes<{foo: number, bar: string}> = number | string
- */
-type ValueTypes<T> = T extends { [k: string]: infer I }
-  ? I
-  : never
-
-/**
- * Field<'foo', {foo: number, bar: string}> = 'foo'
- */
-type Field<X, K extends string, T> = K extends keyof X
-  ? X[K] extends T ? K : never
-  : never
+import { Field } from '@/data/conditional-types'
 
 /**
  * Don't dirty your hands.
  * You must use this instead of [[Untyped.Observable]].
+ *
+ * This description is [here](http://aiya000.github.io/posts/2019-07-04-recover-nativescript-type-unsafe-observable.html).
  */
 export default class Observable<X extends object> extends Untyped.Observable {
   constructor() {
@@ -39,8 +28,8 @@ export default class Observable<X extends object> extends Untyped.Observable {
   /**
    * A type safety get()
    */
-  public take<K extends string, T>(key: Field<X, K, T>): T {
-    return super.get(key)
+  public take<K extends string, T>(key: Field<X, K, T>): T | null {
+    return super.get(key) || null
   }
 
   @deprecated('take')

diff --git a/conditional-types.ts b/conditional-types.ts
@@ -0,0 +1,10 @@
+/**
+ * Declare a type T of a property K via K of a type X.
+ *
+ * - Field<{foo: number, bar: string}, 'foo', number> = 'foo'
+ * - Field<{foo: number, bar: string}, 'bar', string> = 'bar'
+ * - Field<{foo: number, bar: string}, 'foo', string> = never
+ */
+export type Field<X, K extends string, T> = K extends keyof X
+  ? T extends X[K] ? K : never
+  : never
diff --git a/Observable.ts b/Observable.ts
@@ -11,31 +11,40 @@ type ValueTypes<T> = T extends { [k: string]: infer I }
 /**
  * Field<'foo', {foo: number, bar: string}> = 'foo'
  */
-type Field<K extends string, T> = K extends keyof T
-  ? K
+type Field<X, K extends string, T> = K extends keyof X
+  ? X[K] extends T ? K : never
   : never
 
 /**
  * Don't dirty your hands.
  * You must use this instead of [[Untyped.Observable]].
- *
- * Unfortunately, still this is a little type unsafe.
- * Please see the comment of [https://github.com/aiya000/workspace/blob/7cc4c5e53a74036e9c2339cc39d6ee0f47f8a315/TypeScript/ConditionalTypes/type-untyped-method.ts].
  */
-export default class Observable<T extends object> extends Untyped.Observable {
+export default class Observable<X extends object> extends Untyped.Observable {
   constructor() {
     super()
   }
 
   /**
    * A typed safety set()
    */
-  public assign<K extends string>(name: Field<K, T>, value: ValueTypes<T>): void {
+  public assign<K extends string, T>(name: Field<X, K, T>, value: T): void {
     super.set(name, value)
   }
 
   @deprecated('assign')
   public set(name: string, value: any): void {
     super.set(name, value)
   }
+
+  /**
+   * A type safety get()
+   */
+  public take<K extends string, T>(key: Field<X, K, T>): T {
+    return super.get(key)
+  }
+
+  @deprecated('take')
+  public get(name: string): any {
+    super.get(name)
+  }
 }
diff --git a/using.ts b/using.ts
@@ -1,3 +1,13 @@
-const x: Observable<{value?: number}> = new Observable<{value?: number}>()
-x.assign('value', 10)
-// x.assign('value', 'a')  // 2345: Argument of type '"a"' is not assignable to parameter of type 'number | undefined'.
+const p = new Observable<{ x: number, y: string }>()
+p.assign('x', 10)
+p.assign('y', 'poi')
+// 2345: Argument of type '"y"' is not assignable to parameter of type 'never'.
+// p.assign('y', 10)
+
+const x: number = p.take('x')
+const y: string = p.take('y')
+
+// 2345: Argument of type '"y"' is not assignable to parameter of type 'never'.
+// const e: number = p.take('y')
+
+// Use {x?: number, y?: string} if you are careful to forget initializing :D
diff --git a/using.ts b/using.ts
@@ -0,0 +1,3 @@
+const x: Observable<{value?: number}> = new Observable<{value?: number}>()
+x.assign('value', 10)
+// x.assign('value', 'a')  // 2345: Argument of type '"a"' is not assignable to parameter of type 'number | undefined'.
diff --git a/Observable.ts b/Observable.ts
@@ -18,6 +18,9 @@ type Field<K extends string, T> = K extends keyof T
 /**
  * Don't dirty your hands.
  * You must use this instead of [[Untyped.Observable]].
+ *
+ * Unfortunately, still this is a little type unsafe.
+ * Please see the comment of [https://github.com/aiya000/workspace/blob/7cc4c5e53a74036e9c2339cc39d6ee0f47f8a315/TypeScript/ConditionalTypes/type-untyped-method.ts].
  */
 export default class Observable<T extends object> extends Untyped.Observable {
   constructor() {

diff --git a/Observable.ts b/Observable.ts
@@ -0,0 +1,38 @@
+import * as Untyped from 'tns-core-modules/data/observable'
+import deprecated from 'deprecated-decorator'  // npm install --save-dev deprecated-decorator
+
+/**
+ * ValueTypes<{foo: number, bar: string}> = number | string
+ */
+type ValueTypes<T> = T extends { [k: string]: infer I }
+  ? I
+  : never
+
+/**
+ * Field<'foo', {foo: number, bar: string}> = 'foo'
+ */
+type Field<K extends string, T> = K extends keyof T
+  ? K
+  : never
+
+/**
+ * Don't dirty your hands.
+ * You must use this instead of [[Untyped.Observable]].
+ */
+export default class Observable<T extends object> extends Untyped.Observable {
+  constructor() {
+    super()
+  }
+
+  /**
+   * A typed safety set()
+   */
+  public assign<K extends string>(name: Field<K, T>, value: ValueTypes<T>): void {
+    super.set(name, value)
+  }
+
+  @deprecated('assign')
+  public set(name: string, value: any): void {
+    super.set(name, value)
+  }
+}