Basic implementation

import { bindable, BindingMode, customAttribute } from 'aurelia';

@customAttribute({
  name: 'rectsize',
  hasDynamicOptions: true
})
export class RectSize {

  public static inject = [INode];

  @bindable({ mode: BindingMode.fromView })
  public value!: ResizeObserverSize;

  @bindable()
  public boxSize!: 'border-box' | 'content-box';

  private observer!: ResizeObserver;

  constructor(
    private readonly element: HTMLElement
  ) {

  }

  public binding(): void {
    let observer = this.observer;
    if (observer === void 0) {
      observer = this.observer = this.createObserver();
    }
    observer.observe(this.element, { box: 'border-box' });
  }

  public unbinding(): void {
    this.observer.disconnect();
    this.observer = (void 0)!;
  }

  private createObserver(): ResizeObserver {
    return new ResizeObserver((entries) => {
      this.handleResize(entries[0]);
    });
  }

  /**
   * @internal
   */
  private handleResize(entry: ResizeObserverEntry): void {
    this.value = this.boxSize === 'content-box' ? entry.contentBoxSize : entry.borderBoxSize;
  }
}

Working with polyfill

As ResizeObserver is still new and experimental API, it's not widely supported in all browsers. Fortunately there are a few polyfills available. For example:

• older spec, with only contentRect: https://github.com/que-etc/resize-observer-polyfill

• newer spec, with supports for box model options: https://github.com/juggle/resize-observer

To make the attribute work seamlessly with any polyfill users want to choose, we can adjust the way we get the ResizeObserver constructor, as shown in the following example:

export class RectSize {

+  /**
+   * Allow user to ponyfill Resize observer via this static field
+   */
+  public static ResizeObserver: ResizeObserver;


+  private createObserver(): ResizeObserver {
+    const ResizeObserverCtor = this.getResizeObserver();
-    return new ResizeObserver((entries) => {
+    return new ResizeObserverCtor((entries) => {
+      this.handleResize(entries[0]);
+    });
+  }


+  private getResizeObserver(): ResizeObserver {
+    return RectSize.ResizeObserver || window.ResizeObserver;
+  }

And now, our user can switch to any polyfill as the following example:

import { RectSize } from './some-path'
import { ResizeObserver } from 'some-resize-observer-polyfill'

RectSize.ResizeObserver = ResizeObserver;

Example usage

For the above implementation, the usage would be:

<form rectsize.bind="formSize">
  ...
</form>
or
<form rectsize="value.bind: formSize; box-model: content-box;">
  ...
</form>

Special note:

For the polyfill at https://github.com/que-etc/resize-observer-polyfill , you cannot use box-model for observation option, as it follows an older spec of ResizeObserver. Though it is a mature polyfill, and works well so if you want to use it, slightly modify the above implementation:

  private handleResize(entry: ResizeObserverEntry): void {
-    this.value = this.boxSize === 'content-box' ? entry.contentBoxSize : entry.borderBoxSize;
+    this.value = entry.contentRect;
  }

Looping with repeat.for

Aurelia provides a repeat.for attribute that allows you to loop over data inside of your views. Think of repeat.for as the equivalent of a traditional for loop in Javascript. Common scenarios might include loop over an array of products for an online store or options for a select dropdown.

You will find numerous working code examples below showcasing how you can work with collections of data in Aurelia.

Looping over an array of strings

In the following example, we loop over an array inside of our view model called items and display the values in an unordered list. Notice how we use repeat.for on our list item? This tells Aurelia that for every value it finds, populate item with its value.

This type of functionality is convenient for situations where you want to display simple string values or populate select dropdowns where you only need the display value.

Looping over an array of objects

More commonly when working with an array of data, you will be working with an array of objects. You still loop over the array as we did in the previous example, however, we treat the populated item variable as an object, referencing the property names in view.

Looping with numeric ranges

Sometimes you might want to display a list of values in successive order, a range of numbers of 1 to 10. The repeater allows you to do this with ease. In the following example, there is nothing defined inside of the view model, just the view. All we did was write repeat.for="i of 10" and then displayed the value, counting it down with each iteration.

Expression syntax

• The Ampersand & represents a BindingBehavior (instead of Bitwise AND)

• The Bar | represents a ValueConverter (instead of a Bitwise OR)

Non-expression syntax (statements, declarations, function and class definitions) is not supported.

As an overview of various expressions that are possible, the following list is for illustrative purposes and not exhaustive (and not necessarily recommended, either), but should give you a fairly good idea of what you can do.

Primary Expressions

Identifiers

• foo - The foo variable in the current view-model

• ßɑṙ - The ßɑṙ variable in the current view-model

Identifiers with special meaning in Aurelia

• $this - The current view-model

• $parent - The parent view-model

Primitive literals

• true - The literal value true

• false - The literal value false

• null - The literal value null

• undefined - The literal value undefined

String literals and escape sequences

• 'foo' or "foo" - The literal string foo

• '\n' - The literal string [NEWLINE]

• '\t' - The literal string [TAB]

• '\'' - The literal string '

• '\\' - The literal string \

• '\\n' - The literal string

• '\u0061' - The literal string a

Template literals

• `foo` - Equivalent to 'foo'

• `foo\${bar}baz\${qux}quux` - Equivalent to 'foo'+bar+'baz'+qux+'quux'

Numeric literals

• 42 - The literal number 42

• 42. or 42.0 - The literal number 42.0

• .42 or 0.42 - The literal number 0.42

• 42.3 - The literal number 42.3

• 10e3 or 10E3 - The literal number 1000

Array literals

• [] - An empty array

• [1,2,3] - An array containing the literal numbers 1, 2 and 3

• [foo, bar] - An array containing the variables foo and bar

• [[]] - An array containing an empty array

Object literals

• {} - An empty object

• {foo} or {foo,bar} - ES6 shorthand notation, equivalent to {'foo':foo} or {'foo':foo,'bar':bar}

• {42:42} - Equivalent to {'42':42}

Unary Expressions

foo here represents any valid primary expression or unary expression.

• +foo or +1 - Equivalent to foo or 1 (the + unary operator is always ignored)

• -foo or -1 - Equivalent to 0-foo or 0-1

• !foo - Negates foo

• typeof foo - Returns the primitive type name of foo

• void foo - Evaluates foo and returns undefined

Binary Expressions (from highest to lowest precedence)

a and b here represent any valid primary, unary or binary expression.

• a*b or a/b or a%b - Multiplicative

• a+b or a-b - Additive

• a<b or a>b or a<=b or a>=b or a in b or a instanceof b - Relational

• a==b or a!=b or a===b or a!==b - Equality

• a&&b - Logical AND

• a||b - Logical OR

Conditional Expressions

foo etc here represent any valid primary, unary, binary or conditional expression.

• foo ? bar : baz

• foo ? bar : baz ? qux : quux

Assignment Expressions

foo here must be an assignable expression (a simple accessor, a member accessor or an indexed member accessor). bar can any valid primary, unary, binary, conditional or assignment expression.

• foo = bar

• foo = bar = baz

Lambda Expressions

Lambda expressions can be used to call methods like Array#filter, Array#sort, but can also be passed in as an argument to a method on your ViewModel or even invoked as an IIFE.

• () => 42

• x => x.name (e.g. ${items.map(x => x.name).join(', ')})

• (...args) => args.join(', ')

• (a => b => c => a + b + c)(1)(2)(3) (renders '6')

• items.reduce((sum, x) => sum + x, 0)

Member and Call Expressions

Member expressions with special meaning in Aurelia:

• $parent.foo - Access the foo variable in the parent view-model

• $parent.$parent.foo - Access the foo variable in the parent's parent view-model

• $this - Access the current view-model (equivalent to simply this inside the view-model if it's an ES class)

Normal member and call expressions:

foo here represents any valid member, call, assignment, conditional, binary, unary or primary expression (provided the expression as a whole is also valid JavaScript).

• foo.bar - Member accessor

• foo['bar'] - Keyed member accessor

• foo() - Function call

• foo.bar() - Member function call

• foo['bar']() - Keyed member function call

Tagged template literals:

foo here should be a function that can be called. The string parts of the template are passed as an array to the first argument and the expression parts are passed as consecutive arguments.

• foo`bar` - Equivalent to foo(['bar'])

• foo`bar\${baz}qux` - Equivalent to foo(['bar','qux'], baz)

• foo`bar\${baz}qux\${quux}corge` - Equivalent to foo(['bar','qux','corge'],baz,quux)

• foo`\${bar}\${baz}\${qux}` - Equivalent to foo(['','','',''],bar,baz,qux)

Binding Behaviors and Value Converters

These are not considered to be a part of normal expressions and must always come at the end of an expression (though multiple can be chained). Furthermore, BindingBehaviors must come after ValueConverters. (note: BindingBehavior and ValueConverter are abbreviated to BB and VC for readability)

Valid BB expressions:

• foo & bar & baz - Applies the BB bar to the variable foo, and then applies the BB baz to the result of that.

• foo & bar:'baz' - Applies the BB bar to the variable foo, and passes the literal string 'baz' as an argument to the BB

• foo & bar:baz:qux - Applies the BB bar to the variable foo, and passes the variables baz and qux as arguments to the BB

• 'foo' & bar - Applies the BB bar to the literal string 'foo'

Valid VC expressions (likewise):

• foo | bar | baz

• foo | bar:'baz'

• foo | bar:baz:qux

• 'foo' | bar

Combined BB and VC expressions:

• foo | bar & baz

• foo | bar:42:43 & baz:'qux':'quux'

• foo | bar | baz & qux & quux

Invalid combined BB and VC expressions (BB must come at the end):

• foo & bar | baz

• foo | bar & baz | qux