如何在 Xamarin Forms 中生成 PDF 第一页的缩略图

Posted 2023-03-28

【中文标题】如何在 Xamarin Forms 中生成 PDF 第一页的缩略图

【英文标题】：How to generate the Thumbnail of the PDF first page in Xamarin Forms

【发布时间】：2019-04-29 12:42:44

【问题描述】：

我正在使用 Xamarin Forms 的 Syncfusion PDF 查看器来显示 PDF 文件的集合，并且似乎从 PDF 转换为 PNG（提取 PDF 文件的第一页（封面）以显示它，到用户，进入轮播）在 Xamarin 中不起作用 [参见 https://www.syncfusion.com/kb/9112/how-to-convert-pdf-to-png]

我想知道是否有办法在 Xamarin 平台上将 PDF 转换为 PNG，或者我是否应该在服务器端进行转换。

谢谢。

【问题讨论】：

关于pdf转png请看***.com/questions/23905169/…

我知道如何将 PDF 转换为 png（使用 ImageMagik）。当我使用 Xamarin Forms 时，我的问题是关于在设备（即时）、android/ios 智能手机/平板电脑上执行此操作，或者我是否必须在 Web 服务器端执行此操作并提供设备的下载链接.

【参考方案1】：

您可以通过使用 PdfRenderer、CGPDFDocument 和 PdfDocument 类将 PDF 页面导出为图像，而无需使用 Syncfusion PDF 查看器控件。

Xamarin.Forms.Android：

//initialize PDFRenderer by passing PDF file from location.
PdfRenderer renderer = new PdfRenderer(GetSeekableFileDescriptor()); 
int pageCount = renderer.PageCount;
for(int i=0;i<pageCount;i++)

// Use `openPage` to open a specific page in PDF.
Page page =  renderer.OpenPage(i); 
//Creates bitmap
Bitmap bmp = Bitmap.CreateBitmap(page.Width, page.Height, Bitmap.Config.Argb8888); 
//renderes page as bitmap, to use portion of the page use second and third parameter
page.Render(bmp, null, null, PdfRenderMode.ForDisplay);
//Save the bitmap
SaveImage(bmp);
page.Close();


//Method to retrieve PDF file from the location
private ParcelFileDescriptor GetSeekableFileDescriptor()

ParcelFileDescriptor fileDescriptor = null;
try

string root = Android.OS.Environment.ExternalStorageDirectory.ToString()+ "/Syncfusion/sample.pdf";
fileDescriptor = ParcelFileDescriptor.Open(new Java.IO.File(root),ParcelFileMode.ReadOnly
);

catch (FileNotFoundException e)



return fileDescriptor;

Xamarin.Forms.iOS：

public void ConvertToImage(Stream fileStream)  //Pass PDF stream


MemoryStream stream = new MemoryStream();
// Create memory stream from file stream.
fileStream.CopyTo(stream);
// Create data provider from bytes.
CGDataProvider provider = new CGDataProvider(stream.ToArray());
try

//Load a PDF file.
m_pdfDcument = new CGPDFDocument(provider);

catch (Exception)


//Get PDF's page and convert as image.
using (CGPDFPage pdfPage = m_pdfDcument.GetPage(2))

//initialise image context.
UIGraphics.BeginImageContext(pdfPage.GetBoxRect(CGPDFBox.Media).Size);
// get current context.
CGContext context = UIGraphics.GetCurrentContext();
context.SetFillColor(1.0f, 1.0f, 1.0f, 1.0f);
// Gets page's bounds.
CGRect bounds = new CGRect(pdfPage.GetBoxRect(CGPDFBox.Media).X, pdfPage.GetBoxRect(CGPDFBox.Media).Y, pdfPage.GetBoxRect(CGPDFBox.Media).Width, pdfPage.GetBoxRect(CGPDFBox.Media).Height);
if (pdfPage != null)

context.FillRect(bounds);
context.TranslateCTM(0, bounds.Height);
context.ScaleCTM(1.0f, -1.0f);
context.ConcatCTM(pdfPage.GetDrawingTransform(CGPDFBox.Crop, bounds, 0, true));
context.SetRenderingIntent(CGColorRenderingIntent.Default);
context.InterpolationQuality = CGInterpolationQuality.Default;
// Draw PDF page in the context.
context.DrawPDFPage(pdfPage);
// Get image from current context.
pdfImage = UIGraphics.GetImageFromCurrentImageContext();
UIGraphics.EndImageContext();


// Get bytes from UIImage object.
using (var imageData = pdfImage.AsPNG())

imageBytes = new byte[imageData.Length];
System.Runtime.InteropServices.Marshal.Copy(imageData.Bytes, imageBytes, 0, Convert.ToInt32(imageData.Length));
//return bytes;

//Create image from bytes.
imageStream = new MemoryStream(imageBytes);
//Save the image. It is a custom method to save the image
Save("PDFtoImage.png", "image/png", imageStream);

Xamarin.Forms.UWP

public async void ConvertToImage(Stream fileStream) //Pass PDF stream

StorageFile file = null;
//Creates file picker to choose PDF file.
FileOpenPicker filePicker = new FileOpenPicker();

filePicker.FileTypeFilter.Add(".pdf");

filePicker.ViewMode = PickerViewMode.Thumbnail;

filePicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;

filePicker.SettingsIdentifier = "picker1";

filePicker.CommitButtonText = "Open Pdf File";
//Open file picker option
file = await filePicker.PickSingleFileAsync();

// Load selected PDF file from the file picker.
PdfDocument pdfDocument = await PdfDocument.LoadFromFileAsync(file);

if (pdfDocument != null && pdfDocument.PageCount > 0)

for (int pageIndex = 0; pageIndex < pdfDocument.PageCount; pageIndex++)

//Get page from a PDF file.
var pdfPage = pdfDocument.GetPage((uint)pageIndex);

if (pdfPage != null)

//Create temporary folder to store images.
StorageFolder tempFolder = ApplicationData.Current.TemporaryFolder;
//Create image file.
StorageFile destinationFile = await KnownFolders.CameraRoll.CreateFileAsync(Guid.NewGuid().ToString() + ".jpg");

if (destinationFile != null)

IRandomAccessStream randomStream = await destinationFile.OpenAsync(FileAccessMode.ReadWrite);
//Crerate PDF rendering options
PdfPageRenderOptions pdfPageRenderOptions = new PdfPageRenderOptions();

pdfPageRenderOptions.DestinationWidth = (uint)(300);
// Render the PDF's page as stream.
await pdfPage.RenderToStreamAsync(randomStream, pdfPageRenderOptions);

await randomStream.FlushAsync();
//Dispose the random stream
randomStream.Dispose();
//Dispose the PDF's page.
pdfPage.Dispose();

我为 Syncfusion 工作。