[Snapshots] Cannot snapshot two previews in different files with same package

[rbro112]

We've had a client present an edge case that causes snapshots to fail when two different preview functions live in separate files with the same package/name:

// Snapshot1.kt
package com.example

@Preview
@Composable
fun MyPreview() {...}

// Snapshot2.kt
package com.example

@Preview
@Composable
fun MyPreview() {...}

We use the FQN of the composable function as the primary key (with preview details), meaning in both cases the key is com.example.MyPreview. Depending on which is invoked first, the second will result in a crash as a snapshot with that key has already been saved.

[rbro112]

The best workaround for this currently is one of three options:

1. Ensure the fully qualified name (package.method name) is unique for each preview function. This would mean moving one function to a different package.
2. Renaming one function, resulting in unique FQNs for both methods.
3. Modifying the @Preview annotation to have different parameters, as Emerge keys off a combination of FQN and preview parameters.

We're looking into use the file as an additional key in the future to solve this, but there's currently no ETA on that change.

[rbro112]

We've also seen cases where this bug can prevent other snapshot tests from running due to early exits from snapshot runs. We'd encourage the above workaround for any errors where a stacktrace like the following is present:

java.lang.IllegalStateException: File with name {snapshot}.png already exists.
    at com.emergetools.snapshots.SnapshotSaver.saveFile(SnapshotSaver.kt:194)
    at com.emergetools.snapshots.SnapshotSaver.saveImage(SnapshotSaver.kt:113)
    at com.emergetools.snapshots.SnapshotSaver.save(SnapshotSaver.kt:58)
    at com.emergetools.snapshots.EmergeSnapshots.take(EmergeSnapshots.kt:48)
    at com.emergetools.snapshots.compose.ComposeSnapshotterKt.snapshotComposable$lambda$3(ComposeSnapshotter.kt:77)
    at com.emergetools.snapshots.compose.ComposeSnapshotterKt.$r8$lambda$Zxh5rzaAh_iTEohgph_dgwGCc4w(Unknown Source:0)
    at com.emergetools.snapshots.compose.ComposeSnapshotterKt$$ExternalSyntheticLambda0.run(Unknown Source:6)
    at android.os.Handler.handleCallback(Handler.java:958)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loopOnce(Looper.java:205)
    at android.os.Looper.loop(Looper.java:294)
    at android.app.ActivityThread.main(ActivityThread.java:8177)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:971)

[rbro112]

We've added a better error state in Emerge's snapshotting UI to handle this case as an immediate fix. We'll plan to take the file name into account in the snapshot's key in an upcoming update.