Fetcher.key() blocks UI thread

Describe the bug

Currently the key() method of Fetchers is called from the UI thread, which results in a StrictMode violation for the FileFetchers. As it uses the .lastModified() method.

https://github.com/coil-kt/coil/blob/master/coil-base/src/main/java/coil/fetch/FileFetcher.kt#L14

To Reproduce

• Enable StrictMode in your application
• Load an Image via coil via the normal view.load() syntax
• StrictMode will complain about this violation
• The violation is minimal, but in various cases this can get problematic

Expected behavior

• method should also be called from a IO dispatcher so it will not block the UI thread

Logs/Screenshots

2019-09-12 18:15:20.539 16670-16670 D/StrictMode: StrictMode policy violation; ~duration=1 ms: android.os.strictmode.DiskReadViolation
        at android.os.StrictMode$AndroidBlockGuardPolicy.onReadFromDisk(StrictMode.java:1556)
        at java.io.UnixFileSystem.getLastModifiedTime(UnixFileSystem.java:289)
        at java.io.File.lastModified(File.java:939)
        at coil.fetch.FileFetcher.key(FileFetcher.kt:14)
        at coil.fetch.FileFetcher.key(FileFetcher.kt:12)
        at coil.RealImageLoader.computeCacheKey(RealImageLoader.kt:276)
        at coil.RealImageLoader.access$computeCacheKey(RealImageLoader.kt:78)
        at coil.RealImageLoader$execute$2$deferred$1.invokeSuspend(RealImageLoader.kt:202)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:241)
        at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:68)
        at kotlinx.coroutines.DispatchedKt.resumeCancellable(Dispatched.kt:464)
        at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:26)
        at kotlinx.coroutines.CoroutineStart.invoke(CoroutineStart.kt:109)
        at kotlinx.coroutines.AbstractCoroutine.start(AbstractCoroutine.kt:154)
        at kotlinx.coroutines.BuildersKt__Builders_commonKt.launch(Builders.common.kt:54)
        at kotlinx.coroutines.BuildersKt.launch(Unknown Source:1)
        at kotlinx.coroutines.BuildersKt__Builders_commonKt.launch$default(Builders.common.kt:47)
        at kotlinx.coroutines.BuildersKt.launch$default(Unknown Source:1)
        at coil.RealImageLoader.load(RealImageLoader.kt:137)

Library version 0.7.0

[mikepenz]

Not sure how I missed that one.

Anyways from my perspective you may close this. You might want to note somewhere that people who use the FileLoader will face the IO on main thread, and that they can use the key() method to work around this problem

[colinrtwhite]

Sounds good, I’ll add a note to FileFetcher.