An annotation or form element in a PDF can specify an appearance stream, which allows the annotation to be rendered differently than its default annotation rendering. Each appearance stream is a self-contained PDF object that will be rendered inside the annotation rectangle. (See also: What Are Appearance Streams?)
Most appearance streams are just a visual representation of an annotation’s properties. This helps third-party PDF viewers that may be unable to directly generate a visual representation of those properties; these viewers can render the appearance stream instead. Appearance streams also ensure that the annotations in a document are displayed exactly as they were created in the source PDF editor, without subtle differences that could occur if the third-party PDF viewer had to recreate the annotations from raw annotation data.
However, sometimes appearance streams contain surprises. For example, square annotations may have an appearance stream with a photograph. There are a lot of edge cases out there, so one cannot disable appearance stream rendering and assume the annotation will look the same or even close to it.
Changing an annotation will remove existing appearance streams and rerender them using the PSPDFKit drawing engine. Some exceptions apply, such as stamp annotations containing an image or signature form elements.
We provide basic implementations of the
AssetAppearanceStreamGeneratorgenerates an appearance stream from the first page of a PDF stored in your app’s assets.
ℹ️ Note: Setting an appearance stream generator does not automatically remove the existing appearance stream. The appearance stream only gets generated when the annotation has been modified or when calling
It’s easy to set up an annotation to show a custom appearance stream. This is typically done with stamp annotations because they have few other properties. A stamp annotation used this way is usually called an image annotation. To learn more, see our guide on image annotations.
Custom appearance streams generated via
AppearanceStreamGenerator are saved into a document when saving. However, appearance streams need to be regenerated whenever annotations are modified after the document is reloaded.
It can be inconvenient to set appearance stream generators for all annotations that need to have custom appearances. A cleaner approach is to add global appearance stream generators via
AnnotationProvider. Global appearance stream generators added via this method have a chance to override appearance stream generation for all annotations in the document.
Your custom appearance stream generator will be asked if it wishes to generate an annotation’s appearance stream via
true, you are expected to return a
DataProvider instance to a PDF that should be used as the annotation’s appearance stream when
getDataProviderForAnnotation() is called.
ℹ️ Note: Syncing appearance streams is currently not supported by PSPDFKit Instant.