Modifying template parsing with AttributePattern

Sometimes developers want to simulate the situation they have experienced in other frameworks in Aurelia, like Angular or Vue binding syntax. Aurelia provides an API that allows you to change how it interprets templating syntax and even emulate other framework syntax with ease.

What is attributePattern?

attributePattern decorator in the form of extensibility feature in Aurelia. With it, we can introduce our own syntax to Aurelia's binding engine.

export function attributePattern(...patternDefs: AttributePatternDefinition[]): AttributePatternDecorator {
    // ...
}

export interface AttributePatternDefinition {
  pattern: string;
  symbols: string;
}

Its parameters are as follows

Parameter	Description
pattern	You define the pattern of your new syntax in terms of a very special keyword, PART. That's essentially the equivalent of this regex: (.+).
symbols	In symbols you put anything that should not be included in part extraction, anything that makes your syntax more readable but plays no role but separator e.g. in value.bind syntax, the symbols is . which sits there just in terms of more readability, and does not play a role in detecting parts of the syntax.

Consider the following example:

@attributePattern({ pattern: 'foo@PART', symbols: '@' })

foo@bar would give you the parts foo and bar, but if you omitted symbols, then it would give you the parts foo@ and bar.

This attribute should be on top of a class, and that class should have methods whose name matches the pattern property of each pattern you have passed to the attributePattern. Consider the following example:

// attr-patterns.ts

@attributePattern({ pattern: '[(PART)]', symbols: '[()]' })
export class AngularTwoWayBindingAttributePattern {

    public ['[(PART)]'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, parts[0], 'two-way');
    }

}

<!-- Angular two-way binding usage -->
<input [(value)]="message">

We have defined the Angular two-way binding pattern, [(PART)], the symbols are [()] which behaves as a syntax sugar for us; the public method defined in the body of the class has the same name as the pattern defined.

This method also accepts three parameters, rawName, rawValue, and parts.

Parameter	Description
rawName	Left-side of assignment.
rawValue	Right-side of assignment.
parts	The values of PARTs of your pattern without symbols.

• rawName: "[(value)]"

• rawValue: "message"

• parts: ["value"]

The ref binding command to create a reference to a DOM element. In Angular, this is possible with #. For instance, ref="uploadInput" has #uploadInput equivalent in Angular.

@attributePattern({ pattern: '#PART', symbols: '#' })
export class SharpRefAttributePattern {   
    public ['#PART'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, parts[0], 'element', 'ref');
    }
}

Given the above example and the implementation, the parameters would have values like the following:

<!-- #uploadInput="" -->
<input type="file" #uploadInput/>

• rawName: "#uploadInput"

• rawValue: "" , an empty string.

• parts: ["uploadInput"]

If we want to extend the syntax for ref.view-model="uploadVM", for example, we could just add another pattern to the existing class:

@attributePattern(
    { pattern: 'PART#PART', symbols: '#' }, // e.g. view-model#uploadVM
    { pattern: '#PART', symbols: '#' }      // e.g. #uploadInput
)
export class AngularSharpRefAttributePattern {
    public ['PART#PART'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, parts[1], parts[0], 'ref');
    }
    public ['#PART'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, parts[0], 'element', 'ref');
    }   
}

It is up to you to decide how each PART will be taken into play.

How to register an attributePattern?

You can register attributePattern in the following two ways:

Globally

Go to the main.ts or main.js and add the following code:

import {
  AngularTwoWayBindingAttributePattern
} from './attr-patterns';

Aurelia
  .register(
    AngularTwoWayBindingAttributePattern
  )
  .app(MyApp)
  .start();

Locally

You may want to use it in a specific part of your application. You can introduce it through dependencies.

Import from somewhere else:

import { AngularTwoWayBindingAttributePattern } from "./attr-patterns";

@customElement({ 
    name: 'foo',
    template, 
    /* HERE */
    dependencies: [
        AngularTwoWayBindingAttributePattern
    ] 
})

Define it inline:

@customElement({ 
    name: 'foo',
    template    
    /* HERE */
    dependencies: [
        AttributePattern.define(class { ['!PART'](n, v, [t]) { return new AttrSyntax(n, v, t, 'bind') } }
    ] 
})

Cheatsheet

// attr-patterns.ts

import { attributePattern, AttrSyntax } from '@aurelia/runtime-html';

// Angular

@attributePattern({ pattern: '#PART', symbols: '#' })
export class AngularSharpRefAttributePattern {
    public ['#PART'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, parts[0], 'element', 'ref');
    }
}

@attributePattern({ pattern: '[PART]', symbols: '[]' })
export class AngularOneWayBindingAttributePattern {
    public ['[PART]'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, parts[0], 'to-view' /*'bind'*/);
    }
}

@attributePattern({ pattern: '(PART)', symbols: '()' })
export class AngularEventBindingAttributePattern {
    public ['(PART)'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, parts[0], 'trigger');
    }
}

@attributePattern({ pattern: '[(PART)]', symbols: '[()]' })
export class AngularTwoWayBindingAttributePattern {
    public ['[(PART)]'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, parts[0], 'two-way');
    }
}

// Vue

@attributePattern({ pattern: '[PART]', symbols: ':' })
export class VueOneWayBindingAttributePattern {
    public [':PART'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, parts[0], 'to-view' /*'bind'*/);
    }
}

@attributePattern({ pattern: '@PART', symbols: '@' })
export class VueEventBindingAttributePattern {
    public ['@PART'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, parts[0], 'trigger');
    }
}

@attributePattern({ pattern: 'v-model', symbols: '' })
export class VueTwoWayBindingAttributePattern {
    public ['v-model'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, 'value', 'two-way');
    }
}

// Vue+

@attributePattern({ pattern: '::PART', symbols: '::' })
export class DoubleColonTwoWayBindingAttributePattern {
    public ['::PART'](rawName: string, rawValue: string, parts: string[]): AttrSyntax {
        return new AttrSyntax(rawName, rawValue, parts[0], 'two-way');
    }
}

// main.ts

import {
  AngularEventBindingAttributePattern,
  AngularOneWayBindingAttributePattern,
  AngularSharpRefAttributePattern,
  AngularTwoWayBindingAttributePattern,
  DoubleColonTwoWayBindingAttributePattern,
  VueEventBindingAttributePattern,
  VueOneWayBindingAttributePattern,
  VueTwoWayBindingAttributePattern
} from './attr-patterns';

Aurelia
  .register(
    AngularSharpRefAttributePattern,
    AngularOneWayBindingAttributePattern,
    AngularEventBindingAttributePattern,
    AngularTwoWayBindingAttributePattern,
    VueOneWayBindingAttributePattern,
    VueEventBindingAttributePattern,
    VueTwoWayBindingAttributePattern,
    DoubleColonTwoWayBindingAttributePattern
  )
  .app(MyApp)
  .start();