[url_lancher] Don't use canLaunchUrl in Link (flutter#4400)

The pattern of calling `canLaunchUrl` when there's no fallback is an anti-pattern, and the README explicitly recommends against it; on some platforms, `canLaunchUrl` is unreliable in some cases, and/or requires extra permissions that `launchUrl` does not.

This reworks the launch call to just try, and handle failure, instead of trying to pre-check.

Author: stuartmorgan-g

packages/url_launcher/url_launcher/CHANGELOG.md

@@ -1,5 +1,6 @@
-## NEXT
+## 6.1.12
 
+* Removes the use of `canLaunchUrl` in `Link`, to avoid issues on platforms where `canLaunchUrl` is unreliable or requires permissions.
 * Updates minimum supported macOS version to 10.14.
 * Fixes stale ignore: prefer_const_constructors.
 * Updates minimum supported SDK version to Flutter 3.10/Dart 3.0.

packages/url_launcher/url_launcher/README.md

@@ -17,7 +17,7 @@ To use this plugin, add `url_launcher` as a [dependency in your pubspec.yaml fil
 ### Example
 
 <?code-excerpt "basic.dart (basic-example)"?>
-``` dart
+```dart
 import 'package:flutter/material.dart';
 import 'package:url_launcher/url_launcher.dart';
 
@@ -72,7 +72,7 @@ element must be added to your manifest as a child of the root element.
 Example:
 
 <?code-excerpt "../../android/app/src/main/AndroidManifest.xml (android-queries)" plaster="none"?>
-``` xml
+```xml
 <!-- Provide required visibility configuration for API level 30 and above -->
 <queries>
   <!-- If your app checks for SMS support -->

packages/url_launcher/url_launcher/lib/src/link.dart

@@ -121,14 +121,18 @@ class DefaultLinkDelegate extends StatelessWidget {
 
     // At this point, we know that the link is external. So we use the
     // `launchUrl` API to open the link.
-    if (await canLaunchUrl(url)) {
-      await launchUrl(
+    bool success;
+    try {
+      success = await launchUrl(
         url,
         mode: _useWebView
             ? LaunchMode.inAppWebView
             : LaunchMode.externalApplication,
       );
-    } else {
+    } on PlatformException {
+      success = false;
+    }
+    if (!success) {
       FlutterError.reportError(FlutterErrorDetails(
         exception: 'Could not launch link $url',
         stack: StackTrace.current,

packages/url_launcher/url_launcher/pubspec.yaml

@@ -3,7 +3,7 @@ description: Flutter plugin for launching a URL. Supports
   web, phone, SMS, and email schemes.
 repository: https://github.com/flutter/packages/tree/main/packages/url_launcher/url_launcher
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+url_launcher%22
-version: 6.1.11
+version: 6.1.12
 
 environment:
   sdk: ">=3.0.0 <4.0.0"

packages/url_launcher/url_launcher/test/link_test.dart

@@ -64,7 +64,9 @@ void main() {
         )
         ..setResponse(true);
       await followLink!();
-      expect(mock.canLaunchCalled, isTrue);
+      // Calling canLaunch just to pre-check launch is an anti-pattern, since
+      // canLaunch doesn't always work, so ensure that it's not called.
+      expect(mock.canLaunchCalled, isFalse);
       expect(mock.launchCalled, isTrue);
     });
 
@@ -93,7 +95,9 @@ void main() {
         )
         ..setResponse(true);
       await followLink!();
-      expect(mock.canLaunchCalled, isTrue);
+      // Calling canLaunch just to pre-check launch is an anti-pattern, since
+      // canLaunch doesn't always work, so ensure that it's not called.
+      expect(mock.canLaunchCalled, isFalse);
       expect(mock.launchCalled, isTrue);
     });