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FileReader

Namespace: SpawnDev.SpawnJS.JSObjects
Inherits: EventTarget
Source: JSObjects/FileReader.cs
MDN Reference: FileReader on MDN

The FileReader object lets web applications asynchronously read the contents of files (or raw data buffers) stored on the user's computer, using File or Blob objects to specify the file or data to read. https://developer.mozilla.org/en-US/docs/Web/API/FileReader

Constructors

Signature Description
FileReader(SpawnJSObjectReference _ref) Deserialization constructor
FileReader() The FileReader() constructor creates a new FileReader.

Properties

Property Type Access Description
Result SpawnJSObject? get The file's contents. This property is only valid after the read operation is complete, and the format of the data depends on which of the methods was used to initiate the read operation.

Methods

Method Return Type Description
ResultAs() T The file's contents. This property is only valid after the read operation is complete, and the format of the data depends on which of the methods was used to initiate the read operation.
ReadAsDataURL(Blob blob) void Starts reading the contents of the specified Blob, once finished, the result attribute contains a data: URL representing the file's data.
ReadAsArrayBuffer(Blob blob) void Starts reading the contents of the specified Blob, once finished, the result attribute contains an ArrayBuffer representing the file's data.
ReadAsText(Blob blob) void Starts reading the contents of the specified Blob, once finished, the result attribute contains the contents of the file as a text string. An optional encoding name can be specified.
ReadAsDataURLAsync(Blob blob) Task<string?> Starts reading the contents of the specified Blob, once finished, the result attribute contains a data: URL representing the file's data.
ReadAsTextAsync(Blob blob) Task<string?> Starts reading the contents of the specified Blob, once finished, the result attribute contains the contents of the file as a text string. An optional encoding name can be specified.
ReadAsArrayBufferAsync(Blob blob) Task<ArrayBuffer?> Starts reading the contents of the specified Blob, once finished, the result attribute contains an ArrayBuffer representing the file's data.

Events

Event Type Description
OnAbort ActionEvent<ProgressEvent> Fired when a read has been aborted, for example because the program called FileReader.abort().
OnError ActionEvent<ProgressEvent> Fired when the read failed due to an error.
OnLoad ActionEvent<ProgressEvent> Fired when a read has completed successfully.
OnLoadStart ActionEvent<ProgressEvent> Fired when a read has started.
OnLoadEnd ActionEvent<ProgressEvent> Fired when a read has completed, successfully or not.
OnProgress ActionEvent<ProgressEvent> Fired periodically as data is read.

Example

// Use the convenient static async methods (recommended)

// Read a Blob as a data URL string
string? dataUrl = await FileReader.ReadAsDataURLAsync(blob);

// Read a Blob as text
string? text = await FileReader.ReadAsTextAsync(blob);

// Read a Blob as an ArrayBuffer
using var buffer = await FileReader.ReadAsArrayBufferAsync(blob);

// Or use the instance-based event-driven approach for progress tracking
using var reader = new FileReader();

// Subscribe to events using named methods (required for proper cleanup)
reader.OnLoad += Reader_OnLoad;
reader.OnError += Reader_OnError;
reader.OnProgress += Reader_OnProgress;

// Start reading (triggers OnLoad/OnError when done)
reader.ReadAsText(blob);
// reader.ReadAsDataURL(blob);
// reader.ReadAsArrayBuffer(blob);

// Unsubscribe before disposing - every += must have a matching -=
reader.OnLoad -= Reader_OnLoad;
reader.OnError -= Reader_OnError;
reader.OnProgress -= Reader_OnProgress;

void Reader_OnLoad(ProgressEvent e)
{
    string result = reader.ResultAs<string>();
    Console.WriteLine($"Read complete: {result.Length} chars");
}

void Reader_OnError(ProgressEvent e) => Console.WriteLine("Error reading file");

void Reader_OnProgress(ProgressEvent e)
{
    if (e.LengthComputable)
    {
        double percent = (double)e.Loaded / e.Total * 100;
        Console.WriteLine($"Progress: {percent:F1}%");
    }
}