Implementing loaders/savers

Principle of operation: module qualification

When FileIO detects that a file or stream should be handled by a particular package, it will try to call private methods in that package for processing the request. For example, suppose you have created a package called MyFileFormat to handle files of a particular format; then load("somefile.myfmt") for a suitable file will cause FileIO to:

  • attempt to load your package MyFileFormat using Base.require(id::PkgId), where a PkgId combines the name and UUID that you supplied via add_format
  • call MyFileFormat.load(file) where file is File.

A crucial point is that MyFileFormat.load does not extend FileIO.load: it is a private function defined in module MyFileFormat. This is important for ensuring that single formats can be supported by multiple packages; if two or more packages specialized File.load for file::File{format"MYFORMAT"}), then

using Pkg1, Pkg2   # two packages both inappropriately extending FileIO.load

would cause all such loads to be handled by Pkg2, but

using Pkg2, Pkg1

would cause them to be handled by Pkg1. This would make loading incredibly brittle. For that reason, it is essential to keep load private to your package and let FileIO call it by module-qualification.

The same applies to save, loadstreaming, and savestreaming.

If you run into a naming conflict with the load and save functions (for example, you already have another function in your package that has one of these names), you can instead name your loaders fileio_load, fileio_save etc. Note that you cannot mix and match these styles: either all your loaders have to be named load, or all of them should be called fileio_load, but you cannot use both conventions in one module.

All-at-once I/O: implementing load and save

In your package, write code like the following:

module MyFileFormat

using FileIO

# Again, this is a *private* `load` function, do not extend `FileIO.load`!
function load(f::File{format"PNG"})
    open(f) do s
        skipmagic(s)  # skip over the magic bytes
        # You can just call the `load(::Stream)` method below...
        ret = load(s)
        # ...or implement everything here instead

# You can support streams and add keywords:
function load(s::Stream{format"PNG"}; keywords...)
    # s is already positioned after the magic bytes
    # Do the stuff to read a PNG file
    chunklength = read(s, UInt32)

function save(f::File{format"PNG"}, data)
    open(f, "w") do s
        # Don't forget to write the magic bytes!
        write(s, magic(format"PNG"))
        # Do the rest of the stuff needed to save in PNG format

end # module MyFileFormat

load(::File) and save(::File) should close any streams they open. (If you use the do syntax, this happens for you automatically even if the code inside the do scope throws an error.) Conversely, load(::Stream) and save(::Stream) should not close the stream argument.

Implementing streaming I/O

loadstreaming and savestreaming use the same query mechanism, but return a decoded stream that users can read or write. You should also implement a close method on your reader or writer type. Just like with load and save, if the user provided a filename, your close method should be responsible for closing any streams you opened in order to read or write the file. If you are given a Stream, your close method should only do the clean up for your reader or writer type, not close the stream.

struct WAVReader

function, frames::Int)
    # read and decode audio samples from

function Base.close(reader::WAVReader)
    # do whatever cleanup the reader needs
    reader.ownstream && close(

# FileIO has fallback functions that make these work using `do` syntax as well,
# and will automatically call `close` on the returned object.
loadstreaming(f::File{format"WAV"}) = WAVReader(open(f), true)
loadstreaming(s::Stream{format"WAV"}) = WAVReader(s, false)

If you choose to implement loadstreaming and savestreaming in your package, you can easily add save and load methods in the form of:

function save(q::Formatted{format"WAV"}, data, args...; kwargs...)
    savestreaming(q, args...; kwargs...) do stream
        write(stream, data)

function load(q::Formatted{format"WAV"}, args...; kwargs...)
    loadstreaming(q, args...; kwargs...) do stream

where Formatted is the abstract supertype of File and Stream.