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https://github.com/paritytech/scale-decode
Decode SCALE bytes into custom types using a scale-info type registry and a custom Visitor impl.
https://github.com/paritytech/scale-decode
Last synced: 3 months ago
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Decode SCALE bytes into custom types using a scale-info type registry and a custom Visitor impl.
- Host: GitHub
- URL: https://github.com/paritytech/scale-decode
- Owner: paritytech
- License: apache-2.0
- Created: 2022-07-27T15:00:18.000Z (almost 2 years ago)
- Default Branch: main
- Last Pushed: 2024-02-16T11:44:17.000Z (5 months ago)
- Last Synced: 2024-03-16T13:48:24.118Z (4 months ago)
- Language: Rust
- Size: 177 KB
- Stars: 6
- Watchers: 7
- Forks: 3
- Open Issues: 4
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- License: LICENSE
- Codeowners: CODEOWNERS
Lists
- awesome-substrate - `scale-decode` - Decode SCALE bytes into arbitrary custom types by implementing a visitor trait. (Tools)
README
# scale-decode
This crate makes it easy to decode SCALE encoded bytes into a custom data structure with the help of a `TypeResolver` (one of which
is a `scale_info::PortableRegistry`). By using this type information to guide decoding (instead of just trying to decode bytes based
on the shape of the target type), it's possible to be much more flexible in how data is decoded and mapped to some target type.
The main trait used to decode types is a `Visitor` trait (example below). By implementing this trait, you can describe how to
take SCALE decoded values and map them to some custom type of your choosing (whether it is a dynamically shaped type or some
static type you'd like to decode into). Implementations of this `Visitor` trait exist for many existing Rust types in the standard
library.
There also exists an `IntoVisitor` trait, which is implemented on many existing rust types and maps a given type to some visitor
implementation capable of decoding into it.
Finally, a wrapper trait, `DecodeAsType`, is auto-implemented for all types that have an `IntoVisitor` implementation,
and whose visitor errors can be turned into a standard `crate::Error`.
For custom structs and enums, one can use the `DecodeAsType` derive macro to have a `DecodeAsType` implementation automatically
generated.
Here's an example of implementing `Visitor` to decode bytes into a custom `Value` type:
```rust
use std::marker::PhantomData;
use scale_decode::TypeResolver;
use scale_decode::visitor::{
self,
types::{Array, BitSequence, Composite, Sequence, Str, Tuple, Variant},
TypeIdFor,
};
// A custom type we'd like to decode into:
#[derive(Debug, PartialEq)]
enum Value {
Bool(bool),
Char(char),
U8(u8),
U16(u16),
U32(u32),
U64(u64),
U128(u128),
U256([u8; 32]),
I8(i8),
I16(i16),
I32(i32),
I64(i64),
I128(i128),
I256([u8; 32]),
Sequence(Vec),
Composite(Vec<(String, Value)>),
Tuple(Vec),
Str(String),
Array(Vec),
Variant(String, Vec<(String, Value)>),
BitSequence(scale_bits::Bits),
}
// Implement the `Visitor` trait to define how to go from SCALE
// values into this type. Here, we are choosing to be generic over
// how types are resolved, which is a good default choice when creating
// a visitor unless you rely on a specific type resolver/ID type.
struct ValueVisitor(PhantomData);
impl ValueVisitor {
fn new() -> Self {
Self(PhantomData)
}
}
impl visitor::Visitor for ValueVisitor {
type Value<'scale, 'resolver> = Value;
type Error = visitor::DecodeError;
type TypeResolver = R;
fn visit_bool<'scale, 'resolver>(
self,
value: bool,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::Bool(value))
}
fn visit_char<'scale, 'resolver>(
self,
value: char,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::Char(value))
}
fn visit_u8<'scale, 'resolver>(
self,
value: u8,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::U8(value))
}
fn visit_u16<'scale, 'resolver>(
self,
value: u16,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::U16(value))
}
fn visit_u32<'scale, 'resolver>(
self,
value: u32,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::U32(value))
}
fn visit_u64<'scale, 'resolver>(
self,
value: u64,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::U64(value))
}
fn visit_u128<'scale, 'resolver>(
self,
value: u128,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::U128(value))
}
fn visit_u256<'scale, 'resolver>(
self,
value: &'scale [u8; 32],
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::U256(*value))
}
fn visit_i8<'scale, 'resolver>(
self,
value: i8,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::I8(value))
}
fn visit_i16<'scale, 'resolver>(
self,
value: i16,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::I16(value))
}
fn visit_i32<'scale, 'resolver>(
self,
value: i32,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::I32(value))
}
fn visit_i64<'scale, 'resolver>(
self,
value: i64,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::I64(value))
}
fn visit_i128<'scale, 'resolver>(
self,
value: i128,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::I128(value))
}
fn visit_i256<'scale, 'resolver>(
self,
value: &'scale [u8; 32],
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::I256(*value))
}
fn visit_sequence<'scale, 'resolver>(
self,
value: &mut Sequence<'scale, 'resolver, Self::TypeResolver>,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
let mut vals = vec![];
while let Some(val) = value.decode_item(ValueVisitor::new()) {
let val = val?;
vals.push(val);
}
Ok(Value::Sequence(vals))
}
fn visit_composite<'scale, 'resolver>(
self,
value: &mut Composite<'scale, 'resolver, Self::TypeResolver>,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
let mut vals = vec![];
for item in value.by_ref() {
let item = item?;
let val = item.decode_with_visitor(ValueVisitor::new())?;
let name = item.name().unwrap_or("").to_owned();
vals.push((name, val));
}
Ok(Value::Composite(vals))
}
fn visit_tuple<'scale, 'resolver>(
self,
value: &mut Tuple<'scale, 'resolver, Self::TypeResolver>,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
let mut vals = vec![];
while let Some(val) = value.decode_item(ValueVisitor::new()) {
let val = val?;
vals.push(val);
}
Ok(Value::Tuple(vals))
}
fn visit_str<'scale, 'resolver>(
self,
value: &mut Str<'scale>,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
Ok(Value::Str(value.as_str()?.to_owned()))
}
fn visit_variant<'scale, 'resolver>(
self,
value: &mut Variant<'scale, 'resolver, Self::TypeResolver>,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
let mut vals = vec![];
let fields = value.fields();
for item in fields.by_ref() {
let item = item?;
let val = item.decode_with_visitor(ValueVisitor::new())?;
let name = item.name().unwrap_or("").to_owned();
vals.push((name, val));
}
Ok(Value::Variant(value.name().to_owned(), vals))
}
fn visit_array<'scale, 'resolver>(
self,
value: &mut Array<'scale, 'resolver, Self::TypeResolver>,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
let mut vals = vec![];
while let Some(val) = value.decode_item(ValueVisitor::new()) {
let val = val?;
vals.push(val);
}
Ok(Value::Array(vals))
}
fn visit_bitsequence<'scale, 'resolver>(
self,
value: &mut BitSequence<'scale>,
_type_id: &TypeIdFor,
) -> Result, Self::Error> {
let bools: Result = value.decode()?.collect();
Ok(Value::BitSequence(bools?))
}
}
```
This can then be passed to a decode function like so:
```rust
let value: Value = scale_decode::visitor::decode_with_visitor(scale_bytes, &type_id, &types, ValueVisitor::new())?;
```
Where `scale_bytes` are the bytes you'd like to decode, `type_id` is the type stored in the `types` registry
that you'd like to try and decode the bytes into, and `types` is a `scale_type_resolver::TypeResolver` containing
information about the various types in use (a concrete implementation of this is `scale_info::PortableRegistry`).
If we were to then write an `IntoVisitor` implementation like so:
```rust
impl scale_decode::IntoVisitor for Value {
type AnyVisitor = ValueVisitor;
fn into_visitor() -> Self::AnyVisitor {
ValueVisitor::new()
}
}
```
We can then also decode via tha automatic `DecodeAsType` impl like so:
```rust
use scale_decode::DecodeAsType;
let value = Value::decode_as_type(scale_bytes, type_id, types)?;
```
With an `IntoVisitor` impl, you'd also benefit from being able to decode things like `Vec`,
`(Value, bool)`, `Arc` and so on in the same way.