aurelia-binding-functions

An Aurelia plugin that allows you to create bi-directional BindingFunctions in a similar way to ValueConverters or BindingBehaviors.

How to install this plugin?

1. In your project install the plugin via jspm with following command

jspm install npm:aurelia-binding-functions

2. Make Aurelia load the plugin by adding the following line to the configure function in the main.js file of your src folder

  export function configure(aurelia) {
    aurelia.use
      .standardConfiguration()
      .developmentLogging();

+   aurelia.use.plugin('aurelia-binding-functions');

    aurelia.start().then(a => a.setRoot());
  }

3. If you use TypeScript or use Visual Studio Code the type declarations for aurelia-binding-functions should be visible automatically.

Using the plugin

You may create a BindingFunction the same way as you would BindingBehaviors or ValueConverters.

The simplest implementation for a one-way binding might look as follows:

// async-binding-function.ts //

export class AsyncBindingFunction implements BindingFunction {
  connect(callScope: CallScope, binding: Binding, scope: Scope) {
    // get the value of the first argument passed to our CallScope, 
    // e.g. the property from async(property)
    const promise = callScope.args[0].evaluate(scope, binding.lookupFunctions, true) as Promise<any> & {promiseResult:any}
    
    // make sure the binding is updated 
    // once the property "promiseResult" changes on the "promise"
    binding.observeProperty(promise, 'promiseResult')
    
    // set the "promiseResult" property once the Promise resolves
    if (promise.promiseResult === undefined && typeof promise.then === 'function') {
      promise.then(value => {
        promise.promiseResult = value
      })
    }
  }
  
  evaluate(callScope: CallScope, scope: Scope, lookupFunctions, mustEvaluate: boolean) {
    const promise = callScope.args[0].evaluate(scope, lookupFunctions, true) as Promise<any> & {promiseResult:any}
    // return the value of "promiseResult" property 
    // or undefined if the value of the argument is not set 
    return promise ? promise.promiseResult : undefined
  }
}

Now the BindingFunction can be used inside bindings prefixed by @, i.e. @async():

<require from="./async-binding-function"></require>

<h2>${ @async(somePromise) }</h2>

A BindingFunction can implement the following methods:

export interface BindingFunction {
  /**
   * invoked by Aurelia to either: 
   *  - retrieve the current value of the binding
   *  - trigger a call (e.g. by click.delegate)
   */
  evaluate(bindingFunctionScope: BindingFunctionScope, scope: Scope, lookupFunctions, mustEvaluate: boolean): any
  
  /**
   * invoked if the binding is used as a source of values
   * (as opposed to being used to trigger changes, like in click.delegate)
   * this is invoked by Aurelia after bind() and every time the binding is recomputed
   */
  connect?(bindingFunctionScope: BindingFunctionScope, binding: Binding, scope: Scope): void
  
  /**
   * when the binding is two-way, invoked every time new values are fed into the binding by Aurelia
   */
  assign?(bindingFunctionScope: BindingFunctionScope, scope: Scope, value: any, lookupFunctions: any): void
  
  /**
   * invoked when the binding is bound
   */
  bind?(bindingFunctionScope: BindingFunctionScope, binding: Binding, scope: Scope, lookupFunctions: any): void
  
  /**
   * invoked when the binding is unbound
   */
  unbind?(bindingFunctionScope: BindingFunctionScope, binding: Binding, scope: Scope): void
}

For a one-time binding you only need to implement the evaluate() method. A one-way binding will require you to also implement connect(), while a two-way binding requires you to also implement assign().

ScopeFunctions

If you want to create lower-level, global, arbitraitly named Expressions, you may also use ScopeFunctions:

import {ParserImplementation} from 'aurelia-binding';

export function configure(aurelia) {
  let parser = aurelia.container.get(ParserImplementation);
  parser.registerScopeFunction('@custom', CustomExpression);
}

Where CustomExpression is a class that implements Expression. For references see ast.js.

Dependencies

• aurelia-templating
• aurelia-binding
• aurelia-loader
• aurelia-metadata

Used By

This library isn't used by Aurelia. It is an optional plugin.

Platform Support

This library can be used in the browser as well as on the server.

Building The Code

To build the code, follow these steps.

1. Ensure that NodeJS is installed. This provides the platform on which the build tooling runs.
2. From the project folder, execute the following command:

npm install

3. Ensure that Gulp is installed. If you need to install it, use the following command:

npm install -g gulp

4. To build the code, you can now run:

gulp build

5. You will find the compiled code in the dist folder, available in three module formats: AMD, CommonJS and ES6.

6. See gulpfile.js for other tasks related to generating the docs and linting.

Running The Tests

To run the unit tests, first ensure that you have followed the steps above in order to install all dependencies and successfully build the library. Once you have done that, proceed with these additional steps:

1. Ensure that the Karma CLI is installed. If you need to install it, use the following command:

npm install -g karma-cli

2. Ensure that jspm is installed. If you need to install it, use the following commnand:

npm install -g jspm

3. Install the client-side dependencies with jspm:

jspm install

4. You can now run the tests with this command:

karma start