No issue: improve allowUndo

This patch fixes a few issues:
- it was an extension on a CoroutineScope, but that was quite misleading
since the Main dispatcher would be always used regardless of what dispatcher
the owning CoroutineScope was configured with.
- timing was reliant on exact value of the undocumented Snackbar.LENGTH_LONG duration
- coroutine cancellation relied on cooperation of the 'operation' suspend function,
which we can't depend on

New 'allowUndo' fully controls its timing, doesn't imply a dispatcher to its consumers,
and doesn't rely on cooperation of passed-in suspend blocks for cancellation to work.

app/src/main/java/org/mozilla/fenix/components/FenixSnackbar.kt

@@ -56,6 +56,7 @@ class FenixSnackbar private constructor(
     companion object {
         const val LENGTH_LONG = Snackbar.LENGTH_LONG
         const val LENGTH_SHORT = Snackbar.LENGTH_SHORT
+        const val LENGTH_INDEFINITE = Snackbar.LENGTH_INDEFINITE
 
         private const val minTextSize = 12
         private const val maxTextSize = 18

app/src/main/java/org/mozilla/fenix/ext/CoroutineScope.kt

@@ -1,35 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-package org.mozilla.fenix.ext
-
-import android.view.View
-import kotlinx.coroutines.CoroutineScope
-import kotlinx.coroutines.Dispatchers.Main
-import kotlinx.coroutines.delay
-import kotlinx.coroutines.launch
-import org.mozilla.fenix.components.FenixSnackbar
-
-fun CoroutineScope.allowUndo(
-    view: View,
-    message: String,
-    undoActionTitle: String,
-    onCancel: suspend () -> Unit = {},
-    operation: suspend () -> Unit
-) {
-    val undoJob = launch(Main) {
-        delay(UNDO_DELAY)
-        operation.invoke()
-    }
-    FenixSnackbar.make(view, FenixSnackbar.LENGTH_LONG)
-        .setText(message)
-        .setAction(undoActionTitle) {
-            undoJob.cancel()
-            launch(Main) {
-                onCancel.invoke()
-            }
-        }.show()
-}
-
-internal const val UNDO_DELAY = 3000L

app/src/main/java/org/mozilla/fenix/home/HomeFragment.kt

@@ -54,7 +54,6 @@ import org.mozilla.fenix.R
 import org.mozilla.fenix.collections.CreateCollectionViewModel
 import org.mozilla.fenix.collections.SaveCollectionStep
 import org.mozilla.fenix.components.metrics.Event
-import org.mozilla.fenix.ext.allowUndo
 import org.mozilla.fenix.ext.requireComponents
 import org.mozilla.fenix.ext.urlToTrimmedHost
 import org.mozilla.fenix.home.sessioncontrol.CollectionAction
@@ -76,6 +75,7 @@ import org.mozilla.fenix.mvi.getManagedEmitter
 import org.mozilla.fenix.onboarding.FenixOnboarding
 import org.mozilla.fenix.settings.SupportUtils
 import org.mozilla.fenix.share.ShareTab
+import org.mozilla.fenix.utils.allowUndo
 import kotlin.coroutines.CoroutineContext
 import kotlin.math.roundToInt
 
@@ -552,7 +552,7 @@ class HomeFragment : Fragment(), CoroutineScope, AccountObserver {
             useCases.removeAllTabsOfType.invoke(isPrivate)
         }
 
-        CoroutineScope(Dispatchers.Main).allowUndo(
+        allowUndo(
             view!!, getString(R.string.snackbar_tabs_deleted),
             getString(R.string.snackbar_deleted_undo), {
                 deleteAllSessionsJob = null
@@ -594,7 +594,7 @@ class HomeFragment : Fragment(), CoroutineScope, AccountObserver {
                 }
         }
 
-        CoroutineScope(Dispatchers.Main).allowUndo(
+        allowUndo(
             view!!, getString(R.string.snackbar_tab_deleted),
             getString(R.string.snackbar_deleted_undo), {
                 deleteSessionJob = null

app/src/main/java/org/mozilla/fenix/library/bookmarks/BookmarkFragment.kt

@@ -44,12 +44,12 @@ import org.mozilla.fenix.R
 import org.mozilla.fenix.components.FenixSnackbar
 import org.mozilla.fenix.components.metrics.Event
 import org.mozilla.fenix.components.metrics.MetricController
-import org.mozilla.fenix.ext.allowUndo
 import org.mozilla.fenix.ext.components
 import org.mozilla.fenix.ext.urlToTrimmedHost
 import org.mozilla.fenix.mvi.ActionBusFactory
 import org.mozilla.fenix.mvi.getAutoDisposeObservable
 import org.mozilla.fenix.mvi.getManagedEmitter
+import org.mozilla.fenix.utils.allowUndo
 import kotlin.coroutines.CoroutineContext
 
 @SuppressWarnings("TooManyFunctions", "LargeClass")
@@ -261,7 +261,7 @@ class BookmarkFragment : Fragment(), CoroutineScope, BackHandler, AccountObserve
                         getManagedEmitter<BookmarkChange>()
                             .onNext(BookmarkChange.Change(currentRoot - it.item.guid))
 
-                        CoroutineScope(IO).allowUndo(
+                        allowUndo(
                             view!!,
                             getString(R.string.bookmark_deletion_snackbar_message, it.item.url.urlToTrimmedHost()),
                             getString(R.string.bookmark_undo_deletion), { refreshBookmarks() }
@@ -335,7 +335,7 @@ class BookmarkFragment : Fragment(), CoroutineScope, BackHandler, AccountObserve
                 val selectedBookmarks = getSelectedBookmarks()
                 getManagedEmitter<BookmarkChange>().onNext(BookmarkChange.Change(currentRoot - selectedBookmarks))
 
-                CoroutineScope(IO).allowUndo(
+                allowUndo(
                     view!!, getString(R.string.bookmark_deletion_multiple_snackbar_message),
                     getString(R.string.bookmark_undo_deletion), { refreshBookmarks() }
                 ) {

app/src/main/java/org/mozilla/fenix/utils/Undo.kt

@@ -0,0 +1,66 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.utils
+
+import android.view.View
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import org.mozilla.fenix.components.FenixSnackbar
+import java.util.concurrent.atomic.AtomicBoolean
+
+internal const val UNDO_DELAY = 3000L
+
+/**
+ * Runs [operation] after giving user time (see [UNDO_DELAY]) to cancel it.
+ * In case of cancellation, [onCancel] is executed.
+ *
+ * Execution of suspend blocks happens on [Dispatchers.Main].
+ *
+ * @param view A [View] used to determine a parent for the [FenixSnackbar].
+ * @param message A message displayed as part of [FenixSnackbar].
+ * @param undoActionTitle Label for the action associated with the [FenixSnackbar].
+ * @param onCancel A suspend block to execute in case of cancellation.
+ * @param operation A suspend block to execute if user doesn't cancel via the displayed [FenixSnackbar].
+ */
+fun allowUndo(
+    view: View,
+    message: String,
+    undoActionTitle: String,
+    onCancel: suspend () -> Unit = {},
+    operation: suspend () -> Unit
+) {
+    val mainScope = CoroutineScope(Dispatchers.Main)
+
+    // By using an AtomicBoolean, we achieve memory effects of reading and
+    // writing a volatile variable.
+    val requestedUndo = AtomicBoolean(false)
+
+    // Launch an indefinite snackbar.
+    val snackbar = FenixSnackbar
+        .make(view, FenixSnackbar.LENGTH_INDEFINITE)
+        .setText(message)
+        .setAction(undoActionTitle) {
+            requestedUndo.set(true)
+            mainScope.launch {
+                onCancel.invoke()
+            }
+        }
+
+    // If user engages with the snackbar, it'll get automatically dismissed.
+    snackbar.show()
+
+    // Wait a bit, and if user didn't request cancellation, proceed with
+    // requested operation and hide the snackbar.
+    mainScope.launch {
+        delay(UNDO_DELAY)
+
+        if (!requestedUndo.get()) {
+            snackbar.dismiss()
+            operation.invoke()
+        }
+    }
+}