Add rememberRecipientField composable function for reading live recipient fields.

app/src/main/java/org/thoughtcrime/securesms/recipients/Composables.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.recipients
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.State
+import androidx.compose.runtime.derivedStateOf
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import kotlinx.coroutines.rx3.asFlow
+import kotlinx.coroutines.withContext
+import org.signal.core.util.concurrent.SignalDispatchers
+
+/**
+ * Allows composable UI to subscribe to changes for a specific field on a recipient.
+ */
+@Composable
+fun <T> rememberRecipientField(recipient: Recipient, toField: Recipient.() -> T): State<T> {
+  return rememberRecipientField(recipientId = recipient.id, initialData = recipient, toField = toField)
+}
+
+/**
+ * Allows composable UI to subscribe to changes for a specific field on a recipient.
+ */
+@Composable
+fun <T> rememberRecipientField(recipientId: RecipientId, toField: Recipient.() -> T): State<T> {
+  return rememberRecipientField(recipientId = recipientId, initialData = Recipient.UNKNOWN, toField = toField)
+}
+
+@Composable
+private fun <T> rememberRecipientField(recipientId: RecipientId, initialData: Recipient, toField: Recipient.() -> T): State<T> {
+  var recipientAndCounter by remember(recipientId) { mutableStateOf(initialData to 0L) }
+
+  LaunchedEffect(recipientId) {
+    withContext(SignalDispatchers.IO) {
+      Recipient.observable(recipientId)
+        .asFlow()
+        .collect {
+          recipientAndCounter = it to (recipientAndCounter.second + 1L)
+        }
+    }
+  }
+
+  return remember(recipientAndCounter) {
+    derivedStateOf { toField(recipientAndCounter.first) }
+  }
+}