Add several SavedStateHandle delegates.

app/src/main/java/org/thoughtcrime/securesms/util/SavedStateHandleExtensions.kt

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2023 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.util
+
+import androidx.lifecycle.SavedStateHandle
+import kotlin.properties.ReadWriteProperty
+import kotlin.reflect.KProperty
+
+/**
+ * Nullable SavedStateHandle delegate, which does not accept a default.
+ */
+fun <T> SavedStateHandle.delegate(key: String): ReadWriteProperty<Any?, T?> {
+  return NullableSavedStateHandleDelegate(this, key)
+}
+
+/**
+ * Non-null SavedStateHandle delegate with a default value. Recommended when the default
+ * value has to be created on-demand. The default is NOT written to the SavedStateHandle.
+ */
+fun <T> SavedStateHandle.delegate(key: String, default: () -> T): ReadWriteProperty<Any?, T> {
+  return DefaultSavedStateHandleDelegate(this, key, default)
+}
+
+/**
+ * Convenience function for non-null SavedStateHandle delegate. Recommended when working
+ * with primitive or pre-constructed objects. The default is NOT written to the SavedStateHandle.
+ */
+fun <T> SavedStateHandle.delegate(key: String, default: T): ReadWriteProperty<Any?, T> {
+  return DefaultSavedStateHandleDelegate(this, key) { default }
+}
+
+private class NullableSavedStateHandleDelegate<T>(
+  private val handle: SavedStateHandle,
+  private val key: String
+) : ReadWriteProperty<Any?, T?> {
+  override operator fun getValue(thisRef: Any?, property: KProperty<*>): T? {
+    return handle[key]
+  }
+
+  override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T?) {
+    handle[key] = value
+  }
+}
+
+private class DefaultSavedStateHandleDelegate<T>(
+  private val handle: SavedStateHandle,
+  private val key: String,
+  default: () -> T
+) : ReadWriteProperty<Any?, T> {
+
+  private val lazyDefault by lazy { default() }
+
+  override operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
+    return handle[key] ?: lazyDefault
+  }
+
+  override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
+    handle[key] = value
+  }
+}