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Available on Android, iOS, macOS, and JVM targets

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The Problem: Losing File Access

Modern operating systems use security measures like sandboxing, which means your app can lose access to files selected by the user once it restarts. A standard file path might become invalid. This is especially true on Android (for files outside your app’s private storage) and on sandboxed apps on macOS and iOS.

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The Solution: Bookmark Data

BookmarkData is a feature that creates a persistent, secure reference to a file. You can save this reference and use it later to reliably regain access to the file, even after your app has been closed and reopened. FileKit handles the complex platform-specific implementations for you.

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The Basic Workflow

The process involves two main steps: creating and saving a bookmark, and later loading and resolving it. 
// 1. User picks a file
val userPickedFile: PlatformFile = // ...from a file picker

// 2. Create and save its bookmark data
val bookmark = userPickedFile.bookmarkData()
MyPreferences.save("last_file_bookmark", bookmark.bytes)

// --- App restarts ---

// 3. Load the saved bookmark data
val savedBytes = MyPreferences.load("last_file_bookmark")

// 4. Restore the PlatformFile from the bookmark
if (savedBytes != null) {
    val restoredFile = PlatformFile.fromBookmarkData(savedBytes)
    // Now you can work with the restoredFile
}

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Complete Example

Here’s a more complete, practical example using a simple object to manage the bookmark. 
// A simple manager for a single bookmarked file
object BookmarkManager {
    private val bookmarkFile = FileKit.filesDir / "bookmark.bin"

    suspend fun save(file: PlatformFile) {
        try {
            val bookmark = file.bookmarkData()
            bookmarkFile.write(bookmark.bytes)
        } catch (e: Exception) {
            // Handle exceptions, e.g., log the error
            println("Error saving bookmark: ${e.message}")
        }
    }

    suspend fun load(): PlatformFile? {
        if (!bookmarkFile.exists()) return null
        
        return try {
            val bytes = bookmarkFile.readBytes()
            val file = PlatformFile.fromBookmarkData(bytes)
            
            // Best practice: verify the file still exists
            if (file.exists()) {
                file
            } else {
                // The file was moved or deleted, so clean up the stale bookmark
                clear()
                null
            }
        } catch (e: Exception) {
            // Bookmark is invalid or corrupted, clean it up
            clear()
            null
        }
    }
    
    suspend fun clear() {
        try {
            if (bookmarkFile.exists()) {
                bookmarkFile.delete()
            }
        } catch (e: Exception) {
            println("Error clearing bookmark: ${e.message}")
        }
    }
}

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Platform-Specific Behavior

FileKit abstracts away the details, but here’s what happens on each platform:
  • Android: For standard file paths, the path itself is stored. For content:// URIs from the system picker, FileKit requests persistent URI permissions and stores the URI string. This ensures long-term access.
  • iOS & macOS: Uses the native security-scoped bookmark system. This is crucial for sandboxed apps. FileKit automatically starts and stops access to the security-scoped resource when you use the restored PlatformFile.
  • JVM: Stores the file’s absolute path. This is usually sufficient for desktop apps which are not typically sandboxed.

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Handling Invalid Bookmarks

A bookmark is not a guarantee. It can become invalid if the original file is deleted, moved, or if permissions change.
Always handle restoration failures gracefully. A bookmark can become invalid if:
  • The user moves or deletes the file.
  • The user revokes file permissions for your app.
  • System security policies change.
  • The app is uninstalled and reinstalled (on some platforms).
Your code should anticipate that restoring from a bookmark might fail. 
suspend fun loadFileSafely(): PlatformFile? {
    return try {
        val bytes = MyStorage.getBookmarkBytes() ?: return null
        val file = PlatformFile.fromBookmarkData(bytes)

        // The most important check: does the file still exist?
        if (file.exists()) {
            file
        } else {
            // The file is gone. Clean up the invalid bookmark.
            MyStorage.deleteBookmark()
            null
        }
    } catch (e: Exception) {
        // The bookmark data is corrupted or invalid for other reasons.
        // Clean it up to prevent future errors.
        MyStorage.deleteBookmark()
        null
    }
}
This defensive approach ensures your app doesn’t crash from a stale bookmark and can self-heal by clearing invalid data.
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