Switching from .NET MAUI Blazor to WebView Control for Document Scanning

Xiao Ling - Jul 29 - - Dev Community

In .NET MAUI development, both BlazorWebView and WebView are used to display web content, but they serve different purposes and are designed for different scenarios. The BlazorWebView is specifically designed to host Blazor components in a .NET MAUI application, allowing you to reuse Blazor components and share code between web and native applications. The WebView is a general-purpose control for displaying web content, including web pages, HTML strings, and local HTML files. In this article, we will explore how to transition a .NET MAUI Blazor document scanner application to a .NET MAUI application using the WebView control, implementing the document scanning logic in JavaScript and HTML, and enabling interoperation between C# and JavaScript to scan documents and save images.

Prerequisites

  1. Install Dynamsoft Service: This service is necessary for communicating with TWAIN, SANE, ICA, ESCL, and WIA scanners on Windows and macOS.
  2. Request a Free Trial License: Obtain a 30-day free trial license for Dynamic Web TWAIN to get started.

Step 1: Create a New .NET MAUI Project with WebView Control

  1. In Visual Studio or Visual Studio Code, create a new .NET MAUI project.

  2. Open the MainPage.xaml file and replace the existing code with the following XAML to add a WebView control:

    <?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MauiWebView.MainPage">

    <ScrollView>
        <StackLayout
            VerticalOptions="FillAndExpand"
HorizontalOptions="FillAndExpand">
            <WebView
    x:Name="WebView"
    VerticalOptions="FillAndExpand"
    HorizontalOptions="FillAndExpand" Navigating="OnWebViewNavigated"/>

        </StackLayout>
    </ScrollView>

</ContentPage>

  3. Open the MainPage.xaml.cs file and add the following code to set the source of the WebView and handle the Navigating event:

    namespace MauiWebView
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
            LoadHtmlFile();
        }

        private void LoadHtmlFile()
        {
            WebView.Source = "index.html";

        }

        private async void OnWebViewNavigated(object sender, WebNavigatingEventArgs e)
        {
            if (e.Url.StartsWith("invoke://callcsharpfunction"))
            {
                // TODO: Implement interop between C# and JavaScript
            }
        }
    }

}

    Exaplanation:

    • The LoadHtmlFile method sets the Source property of the WebView control to load the index.html file.
    • The OnWebViewNavigated method is triggered when the WebView navigates to a new URL. It checks if the URL starts with invoke://callcsharpfunction and, if so, allows for C# and JavaScript interop.

Step 2: Load Static HTML, JavaScript, and CSS Files into the WebView Control

In a .NET MAUI project, you can load static HTML, JavaScript, and CSS files located in the Resources/Raw folder into the WebView. Ensure that the MauiAsset build action is included in the .csproj file:

<ItemGroup>
    <MauiAsset Include="Resources\Raw\**" LogicalName="%(RecursiveDir)%(Filename)%(Extension)" />
</ItemGroup>
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.NET MAUI HTML, JavaScript and CSS files

We create a similar UI layout as the previous Blazor document scanner application in the index.html file.

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <title>Dynamsoft RESTful API Example</title>
    <link rel="stylesheet" href="main.css">
</head>

<body>
    <div id="loading-indicator" class="loading-indicator">
        <div class="spinner"></div>
    </div>

    <h2>Document Scanner</h2>
    <div class="row">
        <div>
            <label>
                Get a License key from <a href="https://www.dynamsoft.com/customer/license/trialLicense?product=dwt"
                                          target="_blank">here</a>.
            </label>
            <input type="text" placeholder="licenseKey" id="inputText" class="license-input">
            <br />
        </div>

    </div>

    <div class="container">
        <div class="image-tool">
            <h3>Acquire Image</h3>
            <button class="btn btn-primary" id="query-devices-button">Get Devices</button>

            <div>
                <label for="sourceSelect">Source: </label>
                <select id="sources" class="form-control"></select>
            </div>

            <div>
                <label for="pixelTypeSelect">Pixel Type: </label>
                <select id="pixelTypeSelectId" class="form-control">
                    <option>B &amp; W</option>
                    <option>Gray</option>
                    <option>Color</option>
                </select>
            </div>

            <div>
                <label for="resolutionSelect">Resolution: </label>
                <select id="resolutionSelectId" class="form-control">
                    <option>100</option>
                    <option>150</option>
                    <option>200</option>
                    <option>300</option>
                </select>
            </div>


            <div>
                <input class="form-check-input" type="checkbox" id="showUICheckId">
                <label class="form-check-label" for="showUICheck">Show UI</label>
            </div>
            <div>
                <input class="form-check-input" type="checkbox" id="adfCheckId">
                <label class="form-check-label" for="adfCheck">ADF</label>
            </div>
            <div>
                <input class="form-check-input" type="checkbox" id="duplexCheckId">
                <label class="form-check-label" for="duplexCheck">Duplex</label>
            </div>

            <button class="btn btn-primary mt-3" id="scan-button">Scan Now</button>
            <button class="btn btn-primary mt-2" id="save-button">Save</button>

            <h3>Image Tools</h3>
            <div class="image-tools">
                <button id="delete-button" style="border:none; background:none; padding:0;">
                    <img src="images/delete.png" alt="Click Me" style="width: 64px; height: 64px;" />
                </button>
                <button id="rotate-left-button" style="border:none; background:none; padding:0;">
                    <img src="images/rotate_left.png" alt="Click Me" style="width: 64px; height: 64px;" />
                </button>
                <button id="rotate-right-button" style="border:none; background:none; padding:0;">
                    <img src="images/rotate_right.png" alt="Click Me" style="width: 64px; height: 64px;" />
                </button>
            </div>
        </div>

        <div class="image-display">
            <div class="full-img">
                <img id="document-image" src="images/default.png" class="scanned-image">
            </div>

            <div class="row">
                <div class="thumb-bar" id="thumb-bar">
                    <div class="thumb-box" id="thumb-box"> 
                    </div>
                </div>
            </div>
        </div>
    </div>

    <script src="main.js"></script>
</body>

</html>
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Step 3: Implement Document Scanning in JavaScript

With the environment ready, the next step is to implement the relevant functions in JavaScript.

Get Devices

Enumerate the available scanners.

const ScannerType = {
    // TWAIN scanner type, represented by the value 0x10
    TWAINSCANNER: 0x10,

    // WIA scanner type, represented by the value 0x20
    WIASCANNER: 0x20,

    // 64-bit TWAIN scanner type, represented by the value 0x40
    TWAINX64SCANNER: 0x40,

    // ICA scanner type, represented by the value 0x80
    ICASCANNER: 0x80,

    // SANE scanner type, represented by the value 0x100
    SANESCANNER: 0x100,

    // eSCL scanner type, represented by the value 0x200
    ESCLSCANNER: 0x200,

    // WiFi Direct scanner type, represented by the value 0x400
    WIFIDIRECTSCANNER: 0x400,

    // WIA-TWAIN scanner type, represented by the value 0x800
    WIATWAINSCANNER: 0x800
};

let queryDevicesButton = document.getElementById("query-devices-button");
queryDevicesButton.onclick = async () => {
    let scannerType = ScannerType.TWAINSCANNER | ScannerType.TWAINX64SCANNER;
    let devices = await getDevices(host, scannerType);
    let select = document.getElementById("sources");
    select.innerHTML = '';
    for (let i = 0; i < devices.length; i++) {
        let device = devices[i];
        let option = document.createElement("option");
        option.text = device['name'];
        option.value = JSON.stringify(device);
        select.add(option);
    };
}

async function getDevices(host, scannerType) {
    devices = [];
    let url = host + '/DWTAPI/Scanners'
    if (scannerType != null) {
        url += '?type=' + scannerType;
    }

    try {
        let response = await fetch(url);

        if (response.ok) {
            let devices = await response.json();
            return devices;
        }
    } catch (error) {
        console.log(error);
    }
    return [];
}
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Explanation

  • The getDevices function sends a GET request to the RESTful API endpoint /DWTAPI/Scanners to fetch the available scanners. The scanner type is specified by the scannerType parameter.

Acquire Image

Scan documents from the selected scanner by specifying the pixel type, resolution, and other settings.

let scanButton = document.getElementById("scan-button");
scanButton.onclick = async () => {
    let select = document.getElementById("sources");
    let device = select.value;

    if (device == null || device.length == 0) {
        alert('Please select a scanner.');
        return;
    }

    let inputText = document.getElementById("inputText").value;
    let license = inputText.trim();

    if (license == null || license.length == 0) {
        alert('Please input a valid license key.');
    }

    let parameters = {
        license: license,
        device: JSON.parse(device)['device'],
    };

    let showUICheck = document.getElementById("showUICheckId");

    let pixelTypeSelect = document.getElementById("pixelTypeSelectId");

    let resolutionSelect = document.getElementById("resolutionSelectId");

    let adfCheck = document.getElementById("adfCheckId");

    let duplexCheck = document.getElementById("duplexCheckId");

    parameters.config = {
        IfShowUI: showUICheck.checked,
        PixelType: pixelTypeSelect.selectedIndex,
        Resolution: parseInt(resolutionSelect.value),
        IfFeederEnabled: adfCheck.checked,
        IfDuplexEnabled: duplexCheck.checked,
    };


    let jobId = await scanDocument(host, parameters);
    let images = await getImages(host, jobId);

    for (let i = 0; i < images.length; i++) {
        let url = images[i];

        let img = document.getElementById('document-image');
        img.src = url;
        data.unshift(url);

        let option = document.createElement("option");
        option.selected = true;
        option.text = url;
        option.value = url;

        let thumbnails = document.getElementById("thumb-box");
        let newImage = document.createElement('img');
        newImage.setAttribute('src', url);
        if (thumbnails != null) {
            thumbnails.insertBefore(newImage, thumbnails.firstChild);
            newImage.addEventListener('click', e => {
                if (e != null && e.target != null) {
                    let target = e.target;
                    img.src = target.src;
                    selectedThumbnail = target;
                }
            });
        }

        selectedThumbnail = newImage;
    }

}

async function scanDocument(host, parameters, timeout = 30) {
    let url = host + '/DWTAPI/ScanJobs?timeout=' + timeout;

    try {
        let response = await fetch(url, {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify(parameters)
        });

        if (response.ok) {
            let jobId = await response.text();
            return jobId;
        }
        else {
            return '';
        }
    } catch (error) {
        alert(error);
        return '';
    }
}

async function getImages(host, jobId) {
    let images = [];
    let url = host + '/DWTAPI/ScanJobs/' + jobId + '/NextDocument';

    while (true) {
        try {

            let response = await fetch(url);

            if (response.status == 200) {
                const arrayBuffer = await response.arrayBuffer();
                const blob = new Blob([arrayBuffer], { type: response.type });
                const imageUrl = URL.createObjectURL(blob);

                images.push(imageUrl);
            }
            else {
                break;
            }

        } catch (error) {
            console.error('No more images.');
            break;
        }
    }

    return images;
}
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Explanation

  • The scanDocument function sends a POST request to the RESTful API endpoint /DWTAPI/ScanJobs to start a scanning job. The parameters include the license key, device name, and scanning settings.
  • The getImages function sends a GET request to the RESTful API endpoint /DWTAPI/ScanJobs/{jobId}/NextDocument to fetch scanned images. The images are stored in a blob object and displayed in the image display area.

Rotate Image

Rotate the scanned image by -90 or 90 degrees.

let rotateLeftButton = document.getElementById("rotate-left-button");
rotateLeftButton.onclick = () => {
    let img = document.getElementById('document-image');
    img.src = rotateImage('document-image', -90);
    selectedThumbnail.src = img.src;
}

let rotateRightButton = document.getElementById("rotate-right-button");
rotateRightButton.onclick = () => {
    let img = document.getElementById('document-image');
    img.src = rotateImage('document-image', 90);
    selectedThumbnail.src = img.src;
}

    function rotateImage (imageId, angle) {
    const image = document.getElementById(imageId);
    const canvas = document.createElement('canvas');
    const context = canvas.getContext('2d');
    const imageWidth = image.naturalWidth;
    const imageHeight = image.naturalHeight;

    // Calculate the new rotation
    let rotation = 0;
    rotation = (rotation + angle) % 360;

    // Adjust canvas size for rotation
    if (rotation === 90 || rotation === -270 || rotation === 270) {
        canvas.width = imageHeight;
        canvas.height = imageWidth;
    } else if (rotation === 180 || rotation === -180) {
        canvas.width = imageWidth;
        canvas.height = imageHeight;
    } else if (rotation === 270 || rotation === -90) {
        canvas.width = imageHeight;
        canvas.height = imageWidth;
    } else {
        canvas.width = imageWidth;
        canvas.height = imageHeight;
    }

    // Clear the canvas
    context.clearRect(0, 0, canvas.width, canvas.height);

    // Draw the rotated image on the canvas
    context.save();
    if (rotation === 90 || rotation === -270) {
        context.translate(canvas.width, 0);
        context.rotate(90 * Math.PI / 180);
    } else if (rotation === 180 || rotation === -180) {
        context.translate(canvas.width, canvas.height);
        context.rotate(180 * Math.PI / 180);
    } else if (rotation === 270 || rotation === -90) {
        context.translate(0, canvas.height);
        context.rotate(270 * Math.PI / 180);
    }
    context.drawImage(image, 0, 0);
    context.restore();

    return canvas.toDataURL();
}
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Delete Image

Delete all scanned images, including the main image and thumbnails, and reset the data array.

let deleteButton = document.getElementById("delete-button");
deleteButton.onclick = async () => {
    let img = document.getElementById('document-image');
    img.src = 'images/default.png';
    data = [];
    let thumbnails = document.getElementById("thumb-box");
    thumbnails.innerHTML = '';
}
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Step 4: Interop Between C# and JavaScript for Saving Images

Saving images directly in JavaScript is restricted due to security concerns. Therefore, we need to interoperate between C# and JavaScript to accomplish this task.

  1. Create a JavaScript function to convert the scanned image to a base64 string.

    function getBase64Image() {
    var img = document.getElementById('document-image');
    var canvas = document.createElement('canvas');
    canvas.width = img.naturalWidth;
    canvas.height = img.naturalHeight;
    var ctx = canvas.getContext('2d');
    ctx.drawImage(img, 0, 0);

    var dataURL = canvas.toDataURL('image/png'); 
    var base64 = dataURL.split(',')[1]; 
    return base64;
}

  2. When clicking the save button, set window.location.href to trigger the OnWebViewNavigated event handler of the WebView control.

    let saveButton = document.getElementById("save-button");
saveButton.onclick = async () => {
    window.location.href = 'invoke://CallCSharpFunction';    
}

  3. In the OnWebViewNavigated event handler, call EvaluateJavaScriptAsync to retrieve the base64 image data from JavaScript and save it to a file.

    private async void OnWebViewNavigated(object sender, WebNavigatingEventArgs e)
{
    if (e.Url.StartsWith("invoke://callcsharpfunction"))
    {
        var base64String = await WebView.EvaluateJavaScriptAsync("getBase64Image()");
        CallCSharpFunction(base64String);
    }
}

private void CallCSharpFunction(string base64String)
{
    if (!string.IsNullOrEmpty(base64String))
    {

        try
        {
            byte[] imageBytes = Convert.FromBase64String(base64String);

            var filePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), GenerateFilename());
            File.WriteAllBytes(filePath, imageBytes);
            DisplayAlert("Success", "Image saved to: " + filePath, "OK");

        }
        catch (Exception ex)
        {
            DisplayAlert("Error", ex.Message, "OK");
        }
    }
    else
    {
        DisplayAlert("Failure", "No image data found", "OK");
    }
}

private string GenerateFilename()
{
    DateTime now = DateTime.Now;
    string timestamp = now.ToString("yyyyMMdd_HHmmss");
    return $"image_{timestamp}.png";
}

Note: Do not pass the base64 string directly to the C# function via window.location.href, as the string may be too long and cause an error. Instead, return the base64 string when calling EvaluateJavaScriptAsync from the C# function.

Step 5: Run the .NET MAUI Document Scanner Application

Press F5 in Visual Studio or Visual Studio Code to run the .NET document scanner application on Windows or macOS.

.NET MAUI document scanner on Windows

Source Code

https://github.com/yushulx/dotnet-twain-wia-sane-scanner/tree/main/examples/MauiWebView

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