Add decoding hooks

src/hooks.rs

@@ -0,0 +1,45 @@
+//! This module provides a way to register decoding hooks for image formats not directly supported
+//! by this crate.
+
+use std::{
+    collections::HashMap,
+    io::{BufReader, Read, Seek},
+    sync::RwLock,
+};
+
+use crate::{ImageDecoder, ImageFormat, ImageResult};
+
+pub(crate) trait ReadSeek: Read + Seek {}
+impl<T: Read + Seek> ReadSeek for T {}
+
+pub(crate) static DECODING_HOOKS: RwLock<Option<HashMap<ImageFormat, DecodingHook>>> =
+    RwLock::new(None);
+
+/// A wrapper around a type-erased trait object that implements `Read` and `Seek`.
+pub struct BoxReadSeek<'a>(pub(crate) Box<dyn ReadSeek + 'a>);
+impl<'a> Read for BoxReadSeek<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        self.0.read(buf)
+    }
+}
+impl<'a> Seek for BoxReadSeek<'a> {
+    fn seek(&mut self, pos: std::io::SeekFrom) -> std::io::Result<u64> {
+        self.0.seek(pos)
+    }
+}
+
+/// A function to produce an `ImageDecoder` for a given image format.
+pub type DecodingHook = Box<
+    dyn for<'a> Fn(BufReader<BoxReadSeek<'a>>) -> ImageResult<Box<dyn ImageDecoder + 'a>>
+        + Send
+        + Sync,
+>;
+
+/// Register a new decoding hook or replace an existing one.
+pub fn register_decoding_hook(format: ImageFormat, hook: DecodingHook) {
+    let mut hooks = DECODING_HOOKS.write().unwrap();
+    if hooks.is_none() {
+        *hooks = Some(HashMap::new());
+    }
+    hooks.as_mut().unwrap().insert(format, hook);
+}

src/image_reader/image_reader_type.rs

@@ -4,6 +4,7 @@ use std::path::Path;
 
 use crate::dynimage::DynamicImage;
 use crate::error::{ImageFormatHint, UnsupportedError, UnsupportedErrorKind};
+use crate::hooks::{BoxReadSeek, DECODING_HOOKS};
 use crate::image::ImageFormat;
 use crate::{ImageDecoder, ImageError, ImageResult};
 
@@ -178,9 +179,16 @@ impl<'a, R: 'a + BufRead + Seek> ImageReader<R> {
             #[cfg(feature = "qoi")]
             ImageFormat::Qoi => Box::new(qoi::QoiDecoder::new(reader)?),
             format => {
+                let hooks = DECODING_HOOKS.read().unwrap();
+                if let Some(hooks) = hooks.as_ref() {
+                    if let Some(hook) = hooks.get(&format) {
+                        return hook(BufReader::new(BoxReadSeek(Box::new(reader))));
+                    }
+                }
+
                 return Err(ImageError::Unsupported(
                     ImageFormatHint::Exact(format).into(),
-                ))
+                ));
             }
         })
     }

src/lib.rs

@@ -287,6 +287,7 @@ mod buffer_;
 mod buffer_par;
 mod color;
 mod dynimage;
+pub mod hooks;
 mod image;
 mod image_reader;
 pub mod metadata;