如何在Xamarin Forms中生成PDF第一页的缩略图

Question

我正在使用Syncfusion PDF viewer for Xamarin Forms来显示一组PDF文件，但在Xamarin中似乎无法将PDF转换为PNG（提取PDF文件的第一页（封面）以在轮播中向用户展示）。请参阅https://www.syncfusion.com/kb/9112/how-to-convert-pdf-to-png。我想知道是否有一种方法可以在Xamarin平台上将PDF转换为PNG，或者是否应该在服务器端进行转换。谢谢。

- Crusty Applesniffer

---

关于将pdf文件转换为png，请查看https://dev59.com/XGAg5IYBdhLWcg3wLIXS - Cherry Bu - MSFT

我知道如何使用ImageMagik将PDF转换为png。由于我正在使用Xamarin Forms，我的问题是关于在设备上（即时）完成此操作的方法，包括Android/iOS智能手机/平板电脑，或者我必须在Web服务器端执行此操作并提供下载链接到设备。 - Crusty Applesniffer

Answer 1

通过使用PdfRenderer、CGPDFDocument和PdfDocument类，您可以在不使用Syncfusion PDF Viewer控件的情况下将PDF页面导出为图像。

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();
}
}
}
}
}

我在Syncfusion工作。

- Karthikk