Support for DecoderOptions/ISpecializedDecoderOptions

[hey-red]

Since 3.0 release there is no way to configure DecoderOptions or other decoders specific options. OnBeforeLoadAsync is unusable in this case.

Maybe someone suggest a more elegant way to do this, but I'm did it in simple way:

// Add delegate to ImageSharpMiddlewareOptions
private Func<ImageCommandContext, DecoderOptions, Stream, bool, Task<FormattedImage?>> onLoadAsync = (_, _, _, _) 
=> Task.FromResult(default(FormattedImage));

public Func<ImageCommandContext, DecoderOptions, Stream, bool, Task<FormattedImage?>> OnLoadAsync
{
    get => this.onLoadAsync;

    set
    {
        Guard.NotNull(value, nameof(this.OnLoadAsync));
        this.onLoadAsync = value;
    }
}

// Change access modifier to public
public FormattedImage(Image image, IImageFormat format, bool keepOpen) {}

// ImageSharpMiddleware.cs
image = await this.options.OnLoadAsync(imageCommandContext, decoderOptions, inStream, requiresAlpha);
if (image is null)
{
    if (requiresAlpha)
    {
        image = await FormattedImage.LoadAsync<Rgba32>(decoderOptions, inStream);
    }
    else
    {
        image = await FormattedImage.LoadAsync(decoderOptions, inStream);
    }
}

and now I have full control on how to image should be decoded:

options.OnLoadAsync = async (ctx, decoderOptions, inStream, requiresAlpha) => { };

[JimBobSquarePants]

DAMMIT!

I had completely forgotten that the properties on DecoderOptions properties are init only. That makes it impossible to update.

I'll have to change the OnBeforeLoadAsync method to return a nullable instance of those options.

[JimBobSquarePants]

Event signature.

    /// <summary>
    /// Gets or sets the method that can be used to used to augment decoder options.
    /// This is called before the image is decoded and loaded for processing.
    /// </summary>
    public Func<ImageCommandContext, Configuration, Task<DecoderOptions?>> OnBeforeLoadAsync
    {
        get => this.onBeforeLoadAsync;

        set
        {
            Guard.NotNull(value, nameof(this.OnBeforeLoadAsync));
            this.onBeforeLoadAsync = value;
        }
    }

Override

services.AddImageSharp(options =>
{
    options.OnBeforeLoadAsync = (_, configuration) =>
    {
        DecoderOptions decoderOptions = new()
        {
            Configuration = configuration,
            MaxFrames = 1,
            SkipMetadata = true,
        };

        return Task.FromResult<DecoderOptions?>(decoderOptions);
    };
})

[hey-red]

Yes, this resolves problem with DecoderOptions 👍
But maybe you add a delegate that can hook Load methods(similar to my solution)?
I have some wrappers around native libraries and they may require more fine tuning.

[hey-red]

This also applies to all decoders with ISpecializedDecoderOptions.

[JimBobSquarePants]

I'm not sure I want to introduce support for specialized options in this library. It's really designed for general purpose web.

Your default implementation of ISpecializedDecoderOptions should be configured to use the most sensible general options.

[hey-red]

Ok. Understood. This is really very specific use case.