APL Base Component Properties (APL 1.1)

(This is not the most recent version of APL. Use the Other Versions option to see the documentation for the most recent version of APL)

A component is a primitive element that displays on the viewport.

In APL, components are primitive visual components rendered by the client. For example, a Text component displays text on the screen. All APL components share a set of base properties.

Properties

All components support the following properties. See below the table for more information about the meaning of the columns.

The Default column lists the default values of properties. All REQUIRED properties must be set or the component will not inflate. A value of <none> means the property does not have a default value. The interpretation of <none> is property-dependent. For example, not setting the speech property prevents a component from running the SpeakItem command. Not setting maxHeight allows the component to grow arbitrarily tall, which is equivalent to setting maxHeight to a very large number.

The Styled column identifies properties that you can set from a style. Directly specifying a component property overrides any style-defined values. In the following example the Text component always has an opacity of 0.5 regardless of what might be defined in myTextStyle:

    {
      "type": "Text",
      "opacity": 0.5,
      "style": "myTextStyle"
    }

The Dynamic column identifies properties that you can change dynamically with the SetValue command. Setting a dynamic property with SetValue overrides any style values. There is no mechanism to unset a dynamic property.

When the component is the target of an event, the following values are reported in event.target:

{
    "event": {
        "target": {
            "disabled": Boolean,
            "height": Number,   
            "id": ID,           
            "uid": UID,         
            "opacity": Number,  
            "width": Number     
        }
    }
}

Component-specific values are added to event.target.

accessibilityLabel

A text string used by a screen reader in accessibility mode.

bind

The bind property of a component extends the data-binding context for the component and its children. The bind property also specifies an ordered set of data-bindings that extend the current context. Bindings are ordered, and later bindings may use previous definitions.

bind properties

Each binding object in the binding array contains the following properties

bind evaluation

The bind property evaluates after the when property and before any other properties. In other words, the when expression cannot rely on the new bindings, but other properties of this component may rely on the bindings. For example:

{
"type": "Text",
"bind": [
{ "name": "foo", "value": "27" },
{ "name": "bar", "value": "${foo + 23}" }
],
"text": "The value of bar is ${bar}"
}

In this example, foo and bar augment the data-binding context. The final text will be "The value of bar is 2723" because the addition of a string and an integer results in an implicit cast to string.

checked

The checked property sets the checked state of the component. You can use component states when you define styles.

Use the SetValue command to change the checked state on a component.

Set the inheritParentState property to true on child components to inherit the checked state of the parent.

disabled

The disabled property sets the disabled state of the component. You can use component states when you define styles.

Use the SetValue command to change the disabled state on a component.

Set the inheritParentState property to true on child components to inherit the disabled state of the parent.

display

The display property of a component controls whether or not it appears on the screen and how it affects the layout calculation. Set to one of the following values:

Data-binding context, ordinals, and indices used when constructing components are not affected by the display property. For example, in a container with three children, if the first child is set to display=none, it is still be data-bound with index=0 and the next child has index=1.

The following table may be useful when understanding the different ways to hide and show content:

The Focusable? column is for accepting keyboard focus. The Touchable? column is for responding to touch events. Note that when display is set to invisible or none, the touch events pass through the component as if it was not there and are processed by the next component down in the stacking order. The normal display components do not pass touch events through, but they only respond to touch events if they are not disabled.

The descendants of a component with display==invisible are not drawn. In other words, an invisible component will not have visible descendants. Similarly, when a component has display==none, it and all of its descendants are removed from the display hierarchy and take no space on the screen.

entity

Opaque entity data. Alexa devices pass the entity data back to the skill as context.

height and width

The properties width, height, minWidth, minHeight, maxWidth, and maxHeight are dimensional properties.

Minimum width and height values default to 0, which means the component can disappear. Maximum width and height properties default to none, which indicates that the component can scale arbitrarily. If unspecified, the width and height values will revert to the component's natural size.

id

A identifier you can define for this instance of the component. This is used to locate a particular component in the view hierarchy. Although recommended, the id is not required to be unique.

inheritParentState

Replaces the component state with the state of the parent of the component. Used for components that need to visually change appearance based on the state of the parent. For example, you may want a Text component inside of a TouchWrapper to change color when the TouchWrapper is pressed. By setting inheritParentState, the Text component changes state whenever the TouchWrapper changes state.

onMount

Commands to run when the component is first displayed. Mount commands are used for visual transitions between screens. The event.source.value is left unset during mount.

The onMount command is triggered at document load time. Component onMount commands always run even if the component itself is invisible or otherwise not displayed on the screen.

The event generated has the form:

{
  "event": {
    "source": {
      "type": "nameOfComponent",
      "handler": "Mount",
      "id": "id of the component",
      "uid": "Runtime-generated unique ID of the component",
      "value": null
    }
  }
}

The event type property is set to the name of the component; for example, "Frame", "Container", "TouchWrapper", etc.

opacity

Applies a uniform opacity to this component and any child components. The opacity is a number in the range [0,1]; numbers outside of this range are clipped to the range. The actual displayed opacity of a component is the product of this value and all the ancestor opacity values. For example, if the current component's opacity is 0.5 and its parent opacity is 0.8, then the actual displayed opacity of the component is 0.4. Setting the opacity to 0 hides the component, but doesn't remove it from the component hierarchy.

paddingBottom, paddingLeft, paddingRight, and paddingTop

Padding values are dimensions that add space around a component. APL uses the border-box model for calculating the total width and height of an element. Default for each is 0.

speech

The speech data is opaque data received from the Alexa ssmlToSpeech transformer. This is opaque data used when speaking the contents of a component.

See Data Sources and Transformers for more about the ssmlToSpeech and textToSpeech transformers.

style

A named style to use to set properties on the component.

transform

The transformation array is an array of transformation values applied to the component. For example:

"transform": [ { "rotate": 30 }, { "scaleX": 1.5 }, { "translateX": 10 }]

Each individual transform is an object with a single property and associated value. Transformations are applied from right to left (following the web standard); in the above example the component is first shifted 10 dp to the right, scaled up by 50%, and then rotated about its center 30 degrees. Each element in the transformation array is one of the following:

Transformation properties from the same group may be gathered into a single object. For example, the following are equivalent:

[ { "scaleX": 3.0, "scaleY": 2.0} ]
[ { "scale": 3.0, "scaleY": 2.0 } ]
[ { "scaleX": 3.0 }, { "scaleY": 2.0 } ]
[ { "scale": 2.0 }, { "scaleX": 1.5 } ]

Note that transformations are cumulative. The final example scales the X-axis by 1.5, then scales both axes by 2.0, giving a final scaling of { "scaleX": 3.0, "scaleY": 2.0 }

The component scales and rotates about the center of the component.

The translation values support both absolute dimensions (such as "10px", "20dp") as well as relative dimensions (such as 30%). The relative dimensions are calculated in terms of the unscaled component width and height. For example, to rotate a component 45 degrees about its bottom-right corner:

"transform": [
  { "translateX": "50%", "translateY": "50%" }
  { "rotate": 45 },
  { "translateX": "-50%", "translateY": "-50%" }
]

type

Specifies the particular component to inflate. It can be one of the primitive components listed in this section or can be a named layout.

when

If the when expression is true, inflate the component. If false, ignore the component and all child components.

Last updated: Dec 18, 2024