Support conversion/resizing of image files

BlazorInputFile/FileListEntryImpl.cs

@@ -1,5 +1,6 @@
 using System;
 using System.IO;
+using System.Threading.Tasks;
 
 namespace BlazorInputFile
 {
@@ -34,6 +35,11 @@ public Stream Data
             }
         }
 
+        public async Task<IFileListEntry> ToImageFileAsync(string format, int maxWidth, int maxHeight)
+        {
+            return await Owner.ConvertToImageFileAsync(this, format, maxWidth, maxHeight);
+        }
+
         internal void RaiseOnDataRead()
         {
             OnDataRead?.Invoke(this, null);

BlazorInputFile/IFileListEntry.cs

@@ -1,5 +1,6 @@
 using System;
 using System.IO;
+using System.Threading.Tasks;
 
 namespace BlazorInputFile
 {
@@ -17,6 +18,8 @@ public interface IFileListEntry
 
         Stream Data { get; }
 
+        Task<IFileListEntry> ToImageFileAsync(string format, int maxWidth, int maxHeight);
+
         event EventHandler OnDataRead;
     }
 }

BlazorInputFile/InputFile.razor

@@ -40,6 +40,16 @@
             : new RemoteFileListEntryStream(JSRuntime, inputFileElement, file, MaxMessageSize, MaxBufferSize);
     }
 
+    internal async Task<FileListEntryImpl> ConvertToImageFileAsync(FileListEntryImpl file, string format, int maxWidth, int maxHeight)
+    {
+        var imageFile = await JSRuntime.InvokeAsync<FileListEntryImpl>("BlazorInputFile.toImageFile", inputFileElement, file.Id, format, maxWidth, maxHeight);
+
+        // So that method invocations on the file can be dispatched back here
+        imageFile.Owner = (InputFile)(object)this;
+
+        return imageFile;
+    }
+
     void IDisposable.Dispose()
     {
         thisReference?.Dispose();

BlazorInputFile/wwwroot/inputfile.js

@@ -1,14 +1,14 @@
 (function () {
     window.BlazorInputFile = {
         init: function init(elem, componentInstance) {
-            var nextFileId = 0;
+            elem._blazorInputFileNextFileId = 0;
 
             elem.addEventListener('change', function handleInputFileChange(event) {
                 // Reduce to purely serializable data, plus build an index by ID
                 elem._blazorFilesById = {};
                 var fileList = Array.prototype.map.call(elem.files, function (file) {
                     var result = {
-                        id: ++nextFileId,
+                        id: ++elem._blazorInputFileNextFileId,
                         lastModified: new Date(file.lastModified).toISOString(),
                         name: file.name,
                         size: file.size,
@@ -34,6 +34,41 @@
             });
         },
 
+        toImageFile(elem, fileId, format, maxWidth, maxHeight) {
+            var originalFile = getFileById(elem, fileId);
+            return createImageBitmap(originalFile.blob)
+                .then(function (imageBitmap) {
+                    return new Promise(function (resolve) {
+                        var desiredWidthRatio = Math.min(1, maxWidth / imageBitmap.width);
+                        var desiredHeightRatio = Math.min(1, maxHeight / imageBitmap.height);
+                        var chosenSizeRatio = Math.min(desiredWidthRatio, desiredHeightRatio);
+
+                        var canvas = document.createElement('canvas');
+                        canvas.width = Math.round(imageBitmap.width * chosenSizeRatio);
+                        canvas.height = Math.round(imageBitmap.height * chosenSizeRatio);
+                        canvas.getContext('2d').drawImage(imageBitmap, 0, 0, canvas.width, canvas.height);
+                        return canvas.toBlob(resolve, format);
+                    });
+                })
+                .then(function (resizedImageBlob) {
+                    var result = {
+                        id: ++elem._blazorInputFileNextFileId,
+                        lastModified: originalFile.lastModified,
+                        name: originalFile.name, // Note: we're not changing the file extension
+                        size: resizedImageBlob.size,
+                        type: format,
+                        relativePath: originalFile.relativePath
+                    };
+
+                    elem._blazorFilesById[result.id] = result;
+
+                    // Attach the blob data itself as a non-enumerable property so it doesn't appear in the JSON
+                    Object.defineProperty(result, 'blob', { value: resizedImageBlob });
+
+                    return result;
+                });
+        },
+
         readFileData: function readFileData(elem, fileId, startOffset, count) {
             var readPromise = getArrayBufferFromFileAsync(elem, fileId);
 

samples/Sample.Core/App.razor

@@ -3,6 +3,7 @@
     <NavLink href="" Match="NavLinkMatch.All">Single file</NavLink>
     <NavLink href="multi">Multiple files</NavLink>
     <NavLink href="dragdrop-viewer">Drag/drop viewer</NavLink>
+    <NavLink href="image">Image file</NavLink>
     <NavLink href="native">Native upload</NavLink>
 </nav>
 <main>

samples/Sample.Core/Pages/ImageFile.razor

@@ -0,0 +1,37 @@
+@page "/image"
+
+<h1>Image file</h1>
+
+<p>A single file input that reads the input as an image in a chosen format with specified maximum dimensions.</p>
+
+<InputFile OnChange="HandleSelection" />
+
+<p>@status</p>
+
+@if (!string.IsNullOrEmpty(imageDataUri))
+{
+    <img src="@imageDataUri" />
+}
+
+@code {
+    string status;
+    string imageDataUri;
+
+    async Task HandleSelection(IFileListEntry[] files)
+    {
+        var rawFile = files.FirstOrDefault();
+        if (rawFile != null)
+        {
+            // Load as an image file in memory
+            var format = "image/jpeg";
+            var imageFile = await rawFile.ToImageFileAsync(format, 640, 480);
+            var ms = new MemoryStream();
+            await imageFile.Data.CopyToAsync(ms);
+
+            // Make a data URL so we can display it
+            imageDataUri = $"data:{format};base64,{Convert.ToBase64String(ms.ToArray())}";
+
+            status = $"Finished loading {ms.Length} bytes from {imageFile.Name}";
+        }
+    }
+}