Add append file methods

src/PCLStorage.Abstractions/FileExtensions.cs

@@ -46,5 +46,65 @@ public static async Task WriteAllTextAsync(this IFile file, string contents)
                 }
             }
         }
+
+        /// <summary>
+        /// Opens a file, appends the specified string to the file, and then closes the file.
+        /// </summary>
+        /// <param name="file">The file to write to</param>
+        /// <param name="contents">The content to write to the file</param>
+        /// <returns>A task which completes when the write operation finishes</returns>
+        public static async Task AppendAllTextAsync(this IFile file, string contents)
+        {
+            using (var stream = await file.OpenAsync(FileAccess.ReadAndWrite).ConfigureAwait(false))
+            {
+                stream.Seek(stream.Length, SeekOrigin.Begin);
+                using (var sw = new StreamWriter(stream))
+                {
+                    await sw.WriteAsync(contents).ConfigureAwait(false);
+                }
+            }
+        }
+
+        /// <summary>
+        /// Appends lines to a file, and then closes the file.
+        /// </summary>
+        /// <param name="file">The file to write to</param>
+        /// <param name="contents">The content to write to the file</param>
+        /// <returns>A task which completes when the write operation finishes</returns>
+        public static async Task AppendAllLinesAsync(this IFile file, IEnumerable<string> contents)
+        {
+            using (var stream = await file.OpenAsync(FileAccess.ReadAndWrite).ConfigureAwait(false))
+            {
+                stream.Seek(stream.Length, SeekOrigin.Begin);
+                using (var sw = new StreamWriter(stream))
+                {
+                    foreach (var content in contents)
+                    {
+                        await sw.WriteLineAsync(content).ConfigureAwait(false);
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Appends lines to a file, and then closes the file.
+        /// </summary>
+        /// <param name="file">The file to write to</param>
+        /// <param name="contents">The content to write to the file</param>
+        /// <returns>A task which completes when the write operation finishes</returns>
+        public static async Task AppendAllLinesAsync(this IFile file, params string[] contents)
+        {
+            using (var stream = await file.OpenAsync(FileAccess.ReadAndWrite).ConfigureAwait(false))
+            {
+                stream.Seek(stream.Length, SeekOrigin.Begin);
+                using (var sw = new StreamWriter(stream))
+                {
+                    foreach (var content in contents)
+                    {
+                        await sw.WriteLineAsync(content).ConfigureAwait(false);
+                    }
+                }
+            }
+        }
     }
 }

test/PCLStorage.Test/FileTests.cs

@@ -35,6 +35,63 @@ public async Task GetFileThrowsWhenFileDoesNotExist()
             await ExceptionAssert.ThrowsAsync<FileNotFoundException>(async () => await folder.GetFileAsync(fileName));
         }
 
+        [TestMethod]
+        public async Task AppendFile()
+        {
+            //  Arrange
+            IFolder folder = TestFileSystem.LocalStorage;
+            string fileName = "fileToAppend.txt";
+
+            //  Act
+            IFile file = await folder.CreateFileAsync(fileName, CreationCollisionOption.FailIfExists);
+            await file.AppendAllTextAsync("Test");
+            var text = await file.ReadAllTextAsync();
+
+            //  Assert
+            Assert.AreEqual("Test", text);
+
+            // Act
+            await file.AppendAllTextAsync("Test");
+            text = await file.ReadAllTextAsync();
+
+            //  Assert
+            Assert.AreEqual("TestTest", text);
+
+
+            //  Cleanup
+            await file.DeleteAsync();
+        }
+
+        [TestMethod]
+        public async Task AppendLines()
+        {
+            //  Arrange
+            IFolder folder = TestFileSystem.LocalStorage;
+            string fileName = "fileToAppend.txt";
+            string expected = "Test" + Environment.NewLine + "Test" + Environment.NewLine;
+
+            //  Act
+            IFile file = await folder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);
+            await file.AppendAllLinesAsync("Test", "Test");
+            var text = await file.ReadAllTextAsync();
+
+            //  Assert
+            Assert.AreEqual(expected, text);
+
+            // Act
+            var lines = new List<string>(){"Test", "Test"};
+            await file.AppendAllLinesAsync(lines);
+            expected += expected;
+            text = await file.ReadAllTextAsync();
+
+            //  Assert
+            Assert.AreEqual(expected, text);
+
+
+            //  Cleanup
+            await file.DeleteAsync();
+        }
+
         [TestMethod]
         public async Task CreateFile()
         {