Add debugNetworkImageHttpClientProvider (#32857)

Currently, the fact that NetworkImage uses a static HttpClient
makes it impossible to properly test, as a mock in one test will
be reused in another test. This change fixes that.

https://github.com/flutter/flutter/issues/32374

packages/flutter/lib/src/painting/debug.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:io';
+
 import 'package:flutter/foundation.dart';
 
 /// Whether to replace all shadows with solid color blocks.
@@ -11,6 +13,22 @@ import 'package:flutter/foundation.dart';
 /// version to version (or even from run to run).
 bool debugDisableShadows = false;
 
+/// Signature for a method that returns an [HttpClient].
+///
+/// Used by [debugNetworkImageHttpClientProvider].
+typedef HttpClientProvider = HttpClient Function();
+
+/// Provider from which [NetworkImage] will get its [HttpClient] in debug builds.
+///
+/// If this value is unset, [NetworkImage] will use its own internally-managed
+/// [HttpClient].
+///
+/// This setting can be overridden for testing to ensure that each test receives
+/// a mock client that hasn't been affected by other tests.
+///
+/// This value is ignored in non-debug builds.
+HttpClientProvider debugNetworkImageHttpClientProvider;
+
 /// Returns true if none of the painting library debug variables have been changed.
 ///
 /// This function is used by the test framework to ensure that debug variables
@@ -24,7 +42,8 @@ bool debugDisableShadows = false;
 /// test framework itself overrides this value in some cases.)
 bool debugAssertAllPaintingVarsUnset(String reason, { bool debugDisableShadowsOverride = false }) {
   assert(() {
-    if (debugDisableShadows != debugDisableShadowsOverride) {
+    if (debugDisableShadows != debugDisableShadowsOverride ||
+        debugNetworkImageHttpClientProvider != null) {
       throw FlutterError(reason);
     }
     return true;

packages/flutter/lib/src/painting/image_provider.dart

@@ -12,6 +12,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
 import 'binding.dart';
+import 'debug.dart';
 import 'image_cache.dart';
 import 'image_stream.dart';
 
@@ -510,7 +511,18 @@ class NetworkImage extends ImageProvider<NetworkImage> {
     );
   }
 
-  static final HttpClient _httpClient = HttpClient();
+  // Do not access this field directly; use [_httpClient] instead.
+  static final HttpClient _sharedHttpClient = HttpClient();
+
+  static HttpClient get _httpClient {
+    HttpClient client = _sharedHttpClient;
+    assert(() {
+      if (debugNetworkImageHttpClientProvider != null)
+        client = debugNetworkImageHttpClientProvider();
+      return true;
+    }());
+    return client;
+  }
 
   Future<ui.Codec> _loadAsync(NetworkImage key) async {
     assert(key == this);