Make IO operations go through Funcs

This is the same pattern used by `ConnectionManager` which allows us to
pass in mocked rothko interfaces for testing, but ensure that the rothko
dependency isn't loaded at runtime. However in GitHub.Exports we don't
have and (possibly) don't want a dependency on rothko so we need to work
out what to do there.

Author: Steven Kirk

src/GitHub.Exports/Services/UsageTracker.cs

@@ -2,8 +2,8 @@
 using System.ComponentModel.Composition;
 using System.Diagnostics;
 using System.Globalization;
-using System.IO;
 using System.Linq;
+using System.Text;
 using System.Windows.Threading;
 using GitHub.Models;
 using GitHub.Settings;
@@ -26,6 +26,11 @@ public class UsageTracker : IUsageTracker
         readonly DispatcherTimer timer;
         readonly string storePath;
 
+        Func<string, bool> fileExists;
+        Func<string, Encoding, string> readAllText;
+        Action<string, string, Encoding> writeAllText;
+        Action<string> dirCreate;
+
         [ImportingConstructor]
         public UsageTracker(
             IProgram program,
@@ -35,6 +40,11 @@ public UsageTracker(
         {
             var componentModel = serviceProvider.GetService(typeof(SComponentModel)) as IComponentModel;
 
+            fileExists = (path) => System.IO.File.Exists(path);
+            readAllText = (path, encoding) => System.IO.File.ReadAllText(path, encoding);
+            writeAllText = (path, content, encoding) => System.IO.File.WriteAllText(path, content, encoding);
+            dirCreate = (path) => System.IO.Directory.CreateDirectory(path);
+
             this.connectionManager = connectionManager;
             this.userSettings = userSettings;
             this.client = componentModel?.DefaultExportProvider.GetExportedValue<IMetricsService>();
@@ -119,8 +129,8 @@ public void IncrementUpstreamPullRequestCount()
 
         UsageStore LoadUsage()
         {
-            var result = File.Exists(storePath) ?
-                SimpleJson.DeserializeObject<UsageStore>(File.ReadAllText(storePath)) :
+            var result = fileExists(storePath) ?
+                SimpleJson.DeserializeObject<UsageStore>(readAllText(storePath, Encoding.UTF8)) :
                 new UsageStore { Model = new UsageModel() };
 
             result.Model.Lang = CultureInfo.InstalledUICulture.IetfLanguageTag;
@@ -132,9 +142,9 @@ UsageStore LoadUsage()
 
         void SaveUsage(UsageStore store)
         {
-            Directory.CreateDirectory(Path.GetDirectoryName(storePath));
+            dirCreate(System.IO.Path.GetDirectoryName(storePath));
             var json = SimpleJson.SerializeObject(store);
-            File.WriteAllText(storePath, json);
+            writeAllText(storePath, json, Encoding.UTF8);
         }
 
         void UpdateTimer(bool initialCall)