RealmInputScreen: Offer nicer experience for copied URL

Prompted by Alya's comment on CZO [1]:

> One would have to play with pasting behavior to make sure
> something reasonable happens if you are trying to paste in a whole
> URL, including the `https` part.

[1] https://chat.zulip.org/#narrow/stream/48-mobile/topic/can't.20paste.20org.20URL/near/1327170

Fixes: zulip#5228

src/start/RealmInputScreen.js

@@ -2,6 +2,7 @@
 import React, { useState, useCallback } from 'react';
 import type { Node } from 'react';
 import { Keyboard } from 'react-native';
+import Clipboard from '@react-native-clipboard/clipboard';
 
 import type { RouteProp } from '../react-navigation';
 import type { AppNavigationProp } from '../nav/AppNavigator';
@@ -15,6 +16,7 @@ import ZulipButton from '../common/ZulipButton';
 import { tryParseUrl } from '../utils/url';
 import * as api from '../api';
 import { navigateToAuth } from '../actions';
+import { useClipboardHasURL } from '../@react-native-clipboard/clipboard';
 
 type Props = $ReadOnly<{|
   navigation: AppNavigationProp<'realm-input'>,
@@ -69,6 +71,24 @@ export default function RealmInputScreen(props: Props): Node {
     button: { marginTop: 8 },
   };
 
+  const tryCopiedUrl = useCallback(async () => {
+    // The copied string might not be a valid realm URL:
+    // - It might not be a URL because useClipboardHasURL is subject to
+    //   races (and Clipboard.getString is itself async).
+    // - It might not be a valid Zulip realm that the client can connect to.
+    //
+    // So…
+    const url = await Clipboard.getString();
+
+    // …let the user see what string is being tried and edit it if it fails…
+    setRealmInputValue(url);
+
+    // …and run it through our usual validation.
+    await tryRealm(url);
+  }, [tryRealm]);
+
+  const clipboardHasURL = useClipboardHasURL();
+
   return (
     <Screen
       title="Welcome"
@@ -99,6 +119,32 @@ export default function RealmInputScreen(props: Props): Node {
         onPress={handleInputSubmit}
         disabled={tryParseUrl(realmInputValue) === undefined}
       />
+      {clipboardHasURL === true && (
+        // We prepopulate the input with https:// to help when you're not
+        // copy-pasting. (Not everyone has memorized that sequence of
+        // characters.)
+        //
+        // When you *do* want to copy-paste, though, prepopulating with
+        // https:// can be annoying -- you don't want to end up with
+        // https://https://chat.zulip.org.
+        //
+        // To solve that, it's risky to fuss around with the input value
+        // while the user is focused on it, or overcomplicate the input
+        // field (like by breaking it into two parts). I think these kinds
+        // of solutions risk leaving some users confused or dissatisfied.
+        //
+        // Instead, we try to recognize when the user has copied a URL, and
+        // let them use it without making them enter it into the input.
+        //
+        // TODO(?): Instead, use a FAB that persists while
+        //   clipboardHasURL !== true && !progress
+        <ZulipButton
+          style={styles.button}
+          text="Use copied URL"
+          progress={progress}
+          onPress={tryCopiedUrl}
+        />
+      )}
     </Screen>
   );
 }

static/translations/messages_en.json

@@ -48,6 +48,7 @@
   "Welcome": "Welcome",
   "Enter your Zulip server URL:": "Enter your Zulip server URL:",
   "e.g. zulip.example.com": "e.g. zulip.example.com",
+  "Use copied URL": "Use copied URL",
   "Subscriptions": "Subscriptions",
   "Search": "Search",
   "Log in": "Log in",