Macro wrap_download_image_callback 

macro_rules! wrap_download_image_callback {
    ($ vis : vis struct $ name : ident ; impl DownloadImageCallback { $ ($ (# [$ attrs_name : meta]) * fn $ method_name : ident (& $ self : ident $ (, $ arg_name : ident : $ arg_type : ty) * $ (,) ?) $ (-> $ return_type : ty) ? { $ ($ body : tt) * }) * }) => { ... };
    ($ vis : vis struct $ name : ident $ (< $ ($ generic_type : ident : $ first_generic_type_bound : tt $ (+ $ generic_type_bound : tt) *) , + $ (,) ? >) ? { $ ($ field_vis : vis $ field_name : ident : $ field_type : ty) , * $ (,) ? } impl DownloadImageCallback { $ ($ (# [$ attrs_name : meta]) * fn $ method_name : ident (& $ self : ident $ (, $ arg_name : ident : $ arg_type : ty) * $ (,) ?) $ (-> $ return_type : ty) ? { $ ($ body : tt) * }) * }) => { ... };
}

Implement the WrapDownloadImageCallback trait for the specified struct. You can declare more members for your struct, and in the impl DownloadImageCallback block you can override default methods implemented by the ImplDownloadImageCallback trait.

Example

wrap_download_image_callback! {
    struct MyDownloadImageCallback {
        payload: String,
    }

    impl DownloadImageCallback {
        // ...
    }
}

fn make_my_struct() -> DownloadImageCallback {
    MyDownloadImageCallback::new("payload".to_string())
}