com.facebook.litho.annotations

Annotation Type Prop

@Retention(value=RUNTIME)
public @interface Prop

Inputs to a Component are its props. The props for a given Component are the union of all arguments annotated with Prop in your spec methods. The same prop can be defined and accessed in multiple lifecycle methods. The annotation processor will validate that the props are being used with the correct type and name.

For each unique prop defined on the spec, the annotation processor adds a setter method on the Component's Builder that has the same name as the prop. By default props are required but can be marked as optional(). A prop can also be constrained further by setting it's resType().

The parent component sets the props when it creates the component in it's OnCreateLayout method. The props cannot be updated throughout the lifecycle of the component unless they are marked as dynamic(). If the layout need to be updated, the parent has to create a new component and set new props. The props should be immutable since the layout can be calculated on multiple threads. Immutability of the props ensures that no thread safety issues can occur in the component hierarchy.

Creating Props:

 @LayoutSpec
 public class HeaderSpec {

   @OnCreateLayout
   static Component onCreateLayout(
     ComponentContext c,
     @Prop MyTitles title,
     @Prop(varArg = imageUrl) List<String> urls,
     @Prop(optional = true) boolean isSelected) {
     if (urls.isEmpty()) {
       return null;
     }

     return Column.create(c)
       .paddingDip(YogaEdge.ALL, 8)
       .backgroundColor(isSelected ? Color.WHITE : Color.GREEN)
       .child(
         Image.create(c)
           .url(urls.get(0))
           .marginDip(YogaEdge.BOTTOM, 4)
       )
       .child(
         Text.create(c)
           .text(title.getTitle())
           .textSizeSp(16)
           .marginDip(YogaEdge.BOTTOM, 4)
       )
       .child(
         Text.create(c)
           .text(title.getSubtitle())
           .textSizeSp(12)
       )
       .build();
   }
 }

Notice how imageUrl, title and isSelected are used to set properties on different components within the layout.

Setting Props:

 @LayoutSpec
 public class MyComponent {

   @OnCreateLayout
   static Component onCreateLayout(ComponentContext c) {

     return Header.create(c)
       .title(new MyTitles("title", "subtitle"))
       .imageUrl("https://example.com/image.jpg")
       .build();
   }
 }

See Also:

ResType, PropDefault, OnCreateLayout, State, Param

Optional Element Summary

Optional Elements

Modifier and Type	Optional Element and Description
java.lang.String	docString
boolean	dynamic EXPERIMENTAL.
boolean	isCommonProp
boolean	optional Declares that this prop can be omitted by the caller of the component.
boolean	overrideCommonPropBehavior This may only be set to true if isCommonProp() is also set to true.
ResType	resType Marks this prop as one that corresponds to a specific Android resource type, and therefore generates various helper methods to initialize it.
java.lang.String	varArg Declares this prop supports a variable arguments, and provide utility methods to add values to the prop.

Element Detail

optional

public abstract boolean optional

Declares that this prop can be omitted by the caller of the component. When a prop is omitted it will take its default value. Its default value will be the standard default initialization value according to the Java standard (e.g. 0, false, null) or the value of a constant declared in spec which is annotated with PropDefault and has the same name and type as the prop.

Returns:

true iff the prop is optional otherwise false.

Default:

false

resType

public abstract ResType resType

Marks this prop as one that corresponds to a specific Android resource type, and therefore generates various helper methods to initialize it.

For example, a CharSequence prop named "title" may be marked as ResType.STRING. This will make the component have not only method "title(CharSequence)" but also various methods that enable initializing the prop from a resource or attribute:

   titleRes(@StringRes int resId)
   titleRes(@StringRes int resId, Object... formatArgs)
   titleAttr(@AttrRes int attrResId, @StringRes int defResId)
   titleAttr(@AttrRes int attrResId)
 

Returns:

The resource type.

Default:

com.facebook.litho.annotations.ResType.NONE

docString

public abstract java.lang.String docString

Returns:

The documentation for the annotated parameter.

Default:

""

varArg

public abstract java.lang.String varArg

Declares this prop supports a variable arguments, and provide utility methods to add values to the prop.

For example, having @Prop(varArg="name") List<CharSequence> names would generate a setter name method which can be called multiple times to add a set of names.

The prop must be a parameterized list. It is effectively always optional(), and has an empty list (immutable) as a default value.

   MyComponent.create(c)
     .name("A")
     .name("B")
     .name("C")
 

Returns:

The declared name of the setter method.

Default:

""

isCommonProp

public abstract boolean isCommonProp

Returns:

true if the name of the prop conflicts with a common prop.

Default:

false

overrideCommonPropBehavior

public abstract boolean overrideCommonPropBehavior

This may only be set to true if isCommonProp() is also set to true. If true, then the behavior of it's common props will be overridden. If false, then the common prop will applied by the framework level as normal as well as any behavior that the component declares within the spec.

Returns:

true if the framework should not apply the common prop.

Default:

false

dynamic

public abstract boolean dynamic

EXPERIMENTAL. This is a part of new experimental API. There is no guarantee that this API will work or that it will remain the same.

Marks a Prop as dynamic, so that a DynamicValue object could be passed to a Component that the Prop belongs to, which makes it possible to update the actual value of the Prop bypassing LayoutState and MountState.

Only Props that DO NOT AFFECT LAYOUT could be marked as dynamic.

Additionally, for every dynamic Prop, a ComponentSpec must contain OnBindDynamicValue method, that applies the actual value to the mounted content.

See Also:

OnBindDynamicValue

Default:

false