Support loading resource files in the Android Studio preview

[Quyunshuo]

I am using AsyncImage to load a webp image in a resource file, but I have no way to preview it, is there any way to provide a preview without affecting the logic? For example, are there any tools attributes similar to those in xml?

To Reproduce
Load a resource file

Logs/Screenshots

Version
version: 2.5.0

[colinrtwhite]

This is currently intended behaviour. Coil only loads the placeholder in the preview environment - mostly since we can guarantee that the placeholder isn't a URL or another data type that can't be loaded in the preview environment. That said, we should re-evaluate this behaviour and likely make it possible to load known-loadable data types (resource ints, bitmaps) in previews.

I'm thinking we could support something like this:

AsyncImage(
    model = if (LocalInspectionMode.current) {
        "https://example.com/image.jpg"
    } else {
        R.drawable.preview_placeholder
    },
    contentDescription = null,
)

[Quyunshuo]

Looking forward to the follow-up

[colinrtwhite]

Let's keep this open to track.

[colinrtwhite]

This is now more important given androidx's new screenshot testing library sets isPreview = true and doesn't set a Dispatchers.Main implementation. This means FakeImageEngine can't easily be used in that environment.

As a stopgap I think it probably makes sense to introduce a LocalAsyncImagePainterPreviewHandler to allow overriding AsyncImagePainter's preview behaviour.