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LoadAsync(string,int,object) Method

Summary

Loads the specified page from an image file using default options asynchronously.

Syntax
C#
VB
Objective-C
C++
public RasterImage LoadAsync( 
   string fileName, 
   int pageNumber, 
   object userState 
) 
Public Overloads Function LoadAsync( _ 
   ByVal fileName As String, _ 
   ByVal pageNumber As Integer, _ 
   ByVal userState As Object _ 
) As RasterImage 
- (void)loadFileAsync:(NSString *)file  
           pageNumber:(NSInteger)pageNumber  
           completion:(void (^)(LTRasterImage * __nullable image, NSError * __nullable error))completion 
public: 
RasterImage^ LoadAsync(  
   String^ fileName, 
   int pageNumber, 
   Object^ userState 
)  

Parameters

fileName
A String containing the name of the image file to load.

pageNumber
The 1-based page number.

userState
A user-defined object that is passed to the method invoked when the asynchronous operation completes.

Return Value

The RasterImage object that this method loads. See remarks for the usage of this object.

Remarks

NOTE: This topic is part of RasterCodecs Async support using the .NET System.ComponentMode.AsyncOperation model. For .NET async/await, use Task<CodecsImageInfo> LoadAsync(ILeadStream stream, int bitsPerPixel).

The file can be in any supported image file format and bits per pixel, whether compressed or uncompressed.

This method will load a single page from a multi-page file. The resulting image will have the same bits/pixel and color order value of the image as it was stored in the file.

LEADTOOLS will attempt to load corrupted files so you can see at least a portion of the image. For these images, the load methods succeed, but LoadStatus will return an error code.

Use the CodecsLoadOptions class to set up other load option parameters before calling this method.

Support for 12 and 16-bit grayscale images is only available in the Document/Medical Imaging editions.

For supported formats, refer to Summary of All Supported Image File Formats.

LEADTOOLS loads all PDF files as Raster PDF uncompressed RasterImageFormat.RasPdf, regardless of the compression and color space used when saving the file.

The RasterCodecs class supports loading image files asynchronously using the LoadAsync methods. When calling any of these methods, the caller thread will not be blocked and the method will return instantly with an instance of RasterImage that is in a loading status (RasterImage.IsLoading set to true). You should not use the other properties of this object while the object is in loading status.

When the RasterCodecs object finishes loading the image, the various properties of the RasterImage will be populated with the image information and data and the RasterImage.IsLoading property will be set to false.

Do not poll the RasterImage.IsLoading property to determine whether the image has finished loading. Instead, subscribe to the LoadAsyncCompleted event to get notification when the LoadAsync operation has completed and whether any errors occurred.

The LoadAsyncCompleted event data will also contain the same object returned from LoadAsync so you do not have to keep the original object in your application.

Example

This example will query the number of pages from a file and then load them.

C#
VB
using Leadtools; 
using Leadtools.Codecs; 
using Leadtools.ImageProcessing; 
using Leadtools.ImageProcessing.Color; 
using Leadtools.Svg; 
using LeadtoolsExamples.Common; 
 
class LoadPagesFromFileAsyncTestData 
{ 
   public int PageCount; 
   public int PageNumber; 
} 
 
private static void LoadPagesFromFileAsyncTest(string fileName) 
{ 
   // Create a new RasterCodecs instance, we will dispose it when the images finish loading 
   RasterCodecs rasterCodecs = new RasterCodecs(); 
 
   // Create the data 
   LoadPagesFromFileAsyncTestData data = new LoadPagesFromFileAsyncTestData(); 
 
   // Get the number of pages 
   data.PageCount = rasterCodecs.GetTotalPages(fileName); 
   Console.WriteLine("Number of pages in the file is {0}", data.PageCount); 
 
   // Load all pages asynchrounsly 
   data.PageNumber = 1; 
 
   // Start with the first page, then load next when operation finishes 
   rasterCodecs.LoadAsyncCompleted += new EventHandler<CodecsLoadAsyncCompletedEventArgs>(rasterCodecs_LoadAsyncCompleted); 
   rasterCodecs.LoadAsync(fileName, data.PageNumber, data); 
} 
 
private static void rasterCodecs_LoadAsyncCompleted(object sender, CodecsLoadAsyncCompletedEventArgs e) 
{ 
   // A page is loaded, show info 
   LoadPagesFromFileAsyncTestData data = e.UserState as LoadPagesFromFileAsyncTestData; 
   Console.WriteLine("Page {0} loaded", data.PageNumber); 
 
   // If there is an error, show it 
   if (e.Error != null) 
      Console.WriteLine("Error {0}", e.Error.Message); 
 
   // Dispose the image 
   if (e.Image != null) 
      e.Image.Dispose(); 
 
   // Get the raster codecs instance 
   RasterCodecs rasterCodecs = sender as RasterCodecs; 
 
   if (data.PageNumber < data.PageCount) 
   { 
      // Load next 
      Console.WriteLine("Load next"); 
      data.PageNumber++; 
      rasterCodecs.LoadAsync(e.FileName, data.PageNumber, data); 
   } 
   else 
   { 
      // Finished loading, clean up 
      Console.WriteLine("Finished"); 
      rasterCodecs.LoadAsyncCompleted -= new EventHandler<CodecsLoadAsyncCompletedEventArgs>(rasterCodecs_LoadAsyncCompleted); 
      rasterCodecs.Dispose(); 
   } 
} 
Imports Leadtools 
Imports Leadtools.Codecs 
Imports Leadtools.ImageProcessing 
Imports Leadtools.ImageProcessing.Color 
Imports Leadtools.Drawing 
Imports Leadtools.Svg 
 
Class LoadPagesFromFileAsyncTestData 
   Public PageCount As Integer 
   Public PageNumber As Integer 
End Class 
 
Private Shared Sub LoadPagesFromFileAsyncTest(fileName As String) 
   ' Create a new RasterCodecs instance, we will dispose it when the images finish loading 
   Dim rasterCodecs As New RasterCodecs() 
 
   ' Create the data 
   Dim data As New LoadPagesFromFileAsyncTestData() 
 
   ' Get the number of pages 
   data.PageCount = rasterCodecs.GetTotalPages(fileName) 
   Console.WriteLine("Number of pages in the file is {0}", data.PageCount) 
 
   ' Load all pages asynchrounsly 
   data.PageNumber = 1 
 
   ' Start with the first page, then load next when operation finishes 
   AddHandler rasterCodecs.LoadAsyncCompleted, AddressOf rasterCodecs_LoadAsyncCompleted 
   rasterCodecs.LoadAsync(fileName, data.PageNumber, data) 
End Sub 
 
Private Shared Sub rasterCodecs_LoadAsyncCompleted(sender As Object, e As CodecsLoadAsyncCompletedEventArgs) 
   ' A page is loaded, show info 
   Dim data As LoadPagesFromFileAsyncTestData = CType(e.UserState, LoadPagesFromFileAsyncTestData) 
   Console.WriteLine("Page {0} loaded", data.PageNumber) 
 
   ' If there is an error, show it 
   If Not IsNothing(e.Error) Then 
      Console.WriteLine("Error {0}", e.Error.Message) 
   End If 
 
   ' Dispose the image 
   If Not IsNothing(e.Image) Then 
      e.Image.Dispose() 
   End If 
 
   ' Get the raster codecs instance 
   Dim rasterCodecs As RasterCodecs = CType(sender, RasterCodecs) 
 
   If data.PageNumber < data.PageCount Then 
      ' Load next 
      Console.WriteLine("Load next") 
      data.PageNumber = data.PageNumber + 1 
      rasterCodecs.LoadAsync(e.FileName, data.PageNumber, data) 
   Else 
      ' Finished loading, clean up 
      Console.WriteLine("Finished") 
      RemoveHandler rasterCodecs.LoadAsyncCompleted, AddressOf rasterCodecs_LoadAsyncCompleted 
      rasterCodecs.Dispose() 
   End If 
End Sub 

Requirements

Target Platforms

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