aurelia-typed-observable-plugin/README.md

A plugin for @observable and @bindable value coercion

# Aurelia-typed-observable-plugin

A plugin that provides enhanced `@bindable` and `@observable` decorators for Aurelia applications

  * Provides coercion support for decorator `@observable`, `@bindable`
  * Provides ability to create custom `@observable` with fluent syntax: `@observable.custom` / `@observable.custom()`
  * provide 4 base custom observables: `@observable.date`, `@observable.number`, `@observable.string`, `@observable.boolean`, 5 for `@bindable` with extra built-in: `@bindable.booleanAttr`

## Installation & Reminder

```
npm i aurelia-typed-observable-plugin
```

The decorators (`@observable` and `@bindable`) should be imported from this plugin, not the built in one of `aurelia`, like the following:

```js
// from Aurelia
import { bindable, observable } from 'aurelia-framework';
// from plugin
import { bindable, observable } from 'aurelia-typed-observable-plugin';
```

The two decorators are drop-in replacement for built-in decorators of `Aurelia`, no extra work needed to use them beside importing them from this plugin.
This plugin is an attempt to request for comment from anyone who interested in the features feature. It was originally intended to be part of the core, but
there is concern it would be hard to support down the road. You can find original PR at `aurelia-binding` (https://github.com/aurelia/binding/pull/623) and `aurelia-templating` (https://github.com/aurelia/templating/pull/558)

## Most common usecase

  * Boolean bindable properties to make custom elements behave like buit in boolean:

  ```js
    // view model
    export class VideoPlayer {

      // do it yourself
      @bindable({
        coerce(val) {
          if (val || val === '') {
            return true;
          }
          return false;
        }
      })
      playing

      // or use built in
      @bindable.booleanAttr
      playing
    }
  ```

  All of the following, will be converted to true, which matches native behavior.

  ```html
    <!-- app.html -->
    <template>
      <video-player playing></video-player>
      <video-player playing=''></video-player>
      <video-player playing='true'></video-player>
      <video-player playing='playing'></video-player>

      <!-- instead of specifying command to make it a boolean -->
      <video-player playing.bind='true'></video-player>
    </template>
  ```

## Usage

**With normal syntax**

```js
class MyViewModel {
  @observable({ coerce: 'number' }) numProp;
  @observable({ coerce: 'boolean' }) boolProp;
  @observable({ coerce: val => convertValue(val) }) customCoerceProp;
}
```

**Using metadata**

```js
import {metadata} from 'aurelia-metadata';

class MyViewModel {
  @observable
  @Reflect.metadata(metadata.propertyType, Number)
  num;
}

var instance = new MyViewModel();
instance.num = '4';
instance.num; // <====== 4
```

TypeScript users can achieve above result (metadata) by simpler code:

```ts
class MyViewModel {
  @observable num: number;
}

var instance = new MyViewModel();
instance.num = '4';
instance.num; // <===== 4 , with compile error though
```

#### ---- steps **For using metadata**:

  1. Enable emit decorator metadata, instruction: Instruction for the compiler to emit decorator metadata [TypeScript decorator doc](https://www.typescriptlang.org/docs/handbook/decorators.html)

  2. Enable property type flag for each deorator, as by default, it is off to avoid breaking your code when opt-ed in, like following:

      ```js
      import {
        usePropertyTypeForBindable,
        usePropertyTypeForObservable
      } from 'aurelia-typed-observable-plugin';

      usePropertyTypeForBindable(true);
      usePropertyTypeForObservable(true);
      ```

      They are only needed to be called once.


## Extension

All coerce type will be resolved to a string, which then is used to get the converter function in `coerceFunctions` export of this module. So, to extend or modify basic implementations:

```js
import {coerceFunctions} from 'aurelia-typed-observable-plugin';

// Modify built in
coerceFunctions.string = function(a) {
  return a === null || a === undefined ? '' : a.toString();
}

// Extend
coerceFunctions.point = function(a) {
  return a.split(' ').map(parseFloat).slice(0, 2);
}

// Usage of 'point' coerces defined above:
class MyLine {
  @observable({ coerce: 'point' }) point1;
  @observable({ coerce: 'point' }) point2;
}
```

For TS users or JS users who want to use metadata, to extend coerce mapping:

```ts
import {
  createTypedObservable
} from 'aurelia-typed-observable-plugin';

// use static class method
class Point {
  static coerce(value) {
    return new Point(value);
  }
}
mapCoerceFunction(Point, 'point');

// or just pass a 3rd parameter, fallback to static coerce method when 3rd param omitted:
mapCoerceFunction(Point, 'point', val => new Point(val));

// decorator usage:
// TypeScript
class MyLine {
  @observable point1: Point
  @observable point2: Point
}
// JavaScript
class MyLine {
  @observable
  @Reflect.metatata('design:type', Point)
  point1

  // or like this
  @observable({ coerce: 'point' })
  point2
}
```

## With fluent syntax

```js
class MyLine {
  @observable.number x1
  @observable.number() y1

  @observable.number() x2
  @observable.number y2
}

var line = new MyLine();

line.x1 = '15';
line.x1; // <======= 15
```

To built your own fluent syntax observable:

```js

import {
  coerceFunctions,
  createTypedObservable
} from 'aurelia-typed-observable-plugin'

coerceFunctions.point = function(value) {
  return value.split(' ').map(parseFloat);
}
createTypedObservable('point');

// usage:
class MyLine {
  @observable.point point1;
  @observable.point point2;
}
```