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‎src/pages/storey/container-impl.mdx

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@@ -6,8 +6,10 @@ import { Callout } from "nextra/components";
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# Implementing new containers
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Storey provides a set of built-in containers, but you're not limited to just those. For special needs, you can go ahead and create your own containers that will
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play along nicely with the rest of the Storey "ecosystem". If it's appropriate, your container could for example allow for nesting other containers - like [`Map`].
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Storey provides a set of built-in containers, but you're not limited to just those. For special
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needs, you can go ahead and create your own containers that will play along nicely with the rest of
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the Storey "ecosystem". If it's appropriate, your container could for example allow for nesting
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other containers - like [`Map`].
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# MyMap
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}
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```
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No magic here. The `prefix` field is used when this collection is a top-level collection - it's a single-byte key that creates a subspace for this collection's internal data.
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No magic here. The `prefix` field is used when this collection is a top-level collection - it's a
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single-byte key that creates a subspace for this collection's internal data.
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The `phantom` field allows us to use the type parameter `V` without actually storing any values of that type.
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The `phantom` field allows us to use the type parameter `V` without actually storing any values of
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that type.
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The `V` type parameter is the type of the **container** inside the map. If you've followed the [`Map`] documentation,
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you'll know that `Map` is a composable container - it holds another container inside.
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The `V` type parameter is the type of the **container** inside the map. If you've followed the
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[`Map`] documentation, you'll know that `Map` is a composable container - it holds another container
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inside.
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The constructor is simple - it just initializes the fields.
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Whew. Let's break this down.
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The `MyMapAccess` struct is our accessor. It's a facade that's used to actually
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access the data in the collection given a `Storage` instance - this is usually a subspace of the "root" storage backend.
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The [`Storable`] trait is the main trait a container must implement. It's currently a mix of things. The associated types tell the framework:
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| Associated type | Details |
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|-------------------|-------------------------------------------------------------------------|
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| `Kind` | We put `NonTerminal` here to signify our container creates subkeys rather than just saving data at the root. |
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| `Accessor` | The accessor type. `MyMapAccess` in our case. |
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| `Key` | The key type to be returned by iterators. Here a tuple of `u32` and the key type of the inner container. |
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| `KeyDecodeError` | The error to be returned on invalid key data. Here we don't care, so we use `()`. In production, use a proper error type. |
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| `Value` | The value type to be returned by iterators. Here it's delegated to the inner container. |
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| `ValueDecodeError`| The error type for invalid value data. Delegated to the inner container. |
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The methods:
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| Method | Function |
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|----------------|-----------------------------------------------------------------------------|
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| `access_impl` | Produces an accessor given a storage abstraction (usually representing a "slice" of the underlying storage.) |
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| `decode_key` | Used for iteration. This is how the framework knows how to decode this map's keys given raw data from the storage backend. |
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| `decode_value` | Used for iteration. This is how the framework knows how to decode values given raw data. |
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`MyMap::access` is an access method in cases where you're using the container as a top-level container.
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There's one thing we're missing for this to actually by useful. We need some methods
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for the accessor.
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The `MyMapAccess` struct is our accessor. It's a facade that's used to actually access the data in
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the collection given a `Storage` instance - this is usually a subspace of the "root" storage
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backend.
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The [`Storable`] trait is the main trait a container must implement. It's currently a mix of things.
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The associated types tell the framework: | Associated type | Details |
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|-------------------|-------------------------------------------------------------------------| |
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`Kind` | We put `NonTerminal` here to signify our container creates subkeys rather than just saving
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data at the root. | | `Accessor` | The accessor type. `MyMapAccess` in our case. | | `Key` | The key
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type to be returned by iterators. Here a tuple of `u32` and the key type of the inner container. | |
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`KeyDecodeError` | The error to be returned on invalid key data. Here we don't care, so we use `()`.
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In production, use a proper error type. | | `Value` | The value type to be returned by iterators.
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Here it's delegated to the inner container. | | `ValueDecodeError`| The error type for invalid value
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data. Delegated to the inner container. |
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The methods: | Method | Function |
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|----------------|-----------------------------------------------------------------------------| |
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`access_impl` | Produces an accessor given a storage abstraction (usually representing a "slice" of
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the underlying storage.) | | `decode_key` | Used for iteration. This is how the framework knows how
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to decode this map's keys given raw data from the storage backend. | | `decode_value` | Used for
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iteration. This is how the framework knows how to decode values given raw data. |
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`MyMap::access` is an access method in cases where you're using the container as a top-level
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container.
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There's one thing we're missing for this to actually by useful. We need some methods for the
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accessor.
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```rust template="storage" {2, 59-78}
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use storey::containers::{NonTerminal, Storable};
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}
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```
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What does this new code do? It provides a way to create accessors for the inner container based on the `u32` key.
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The [`StorageBranch`] type is a helper that creates a "subspace" of the storage backend. It's used to create a "slice" of the storage granted to `MyMap`.
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What does this new code do? It provides a way to create accessors for the inner container based on
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the `u32` key. The [`StorageBranch`] type is a helper that creates a "subspace" of the storage
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backend. It's used to create a "slice" of the storage granted to `MyMap`.
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Time to see this in action.
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@@ -238,4 +247,4 @@ To be continued...
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[`Item`]: /storey/containers/item
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[`Map`]: /storey/containers/map
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[`Storable`]: https://docs.rs/storey/latest/storey/containers/trait.Storable.html
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[`StorageBranch`]: https://docs.rs/storey/latest/storey/storage/struct.StorageBranch.html
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[`StorageBranch`]: https://docs.rs/storey/latest/storey/storage/struct.StorageBranch.html

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