App Context

+++ title = "AppContext & prelude" description = "The AppContext struct field-by-field, and everything use loco_rs::prelude::* brings into scope." date = 2021-05-01T18:10:00+00:00 updated = 2021-05-01T18:10:00+00:00 draft = false weight = 6 sort_by = "weight" template = "docs/page.html"

[extra] lead = "" toc = true top = false +++

AppContext is the cloneable, axum-State-compatible struct that carries every shared resource of a Loco app (DB connection, cache, queue, mailer, storage, config, and an open-ended DI slot). loco_rs::prelude is the single-import surface app code pulls in instead of naming individual loco_rs and axum paths. Both are declared in src/app.rs and src/prelude.rs.

AppContext

Defined at src/app.rs:253-273:

#[derive(Clone, FromRef)]
pub struct AppContext {
    pub environment: Environment,
    #[cfg(feature = "with-db")]
    pub db: DatabaseConnection,
    pub queue_provider: Option<Arc<bgworker::Queue>>,
    pub config: Config,
    pub mailer: Option<EmailSender>,
    pub storage: Arc<Storage>,
    pub cache: Arc<cache::Cache>,
    pub shared_store: Arc<SharedStore>,
}

Derives

#[derive(Clone, FromRef)] (src/app.rs:253). FromRef (from axum) auto-generates impl FromRef<AppContext> for <FieldType> for each field, so a handler can extract a single field directly — e.g. State<DatabaseConnection> or State<Arc<cache::Cache>> — instead of always taking the whole State<AppContext>.

Fields

db is the only field that is compiled out (not just None-able) when its feature (with-db) is disabled — every other field is unconditionally present, with Option/empty-default standing in for "not configured."

SharedStore — the generic DI slot

shared_store: Arc<SharedStore> (src/app.rs:33-245, 272) is a small heterogeneous store for services that don't have a dedicated AppContext field. Its API:

• insert<T: 'static + Send + Sync>(&self, val: T) (:62)
• remove<T>(&self) -> Option<T> (:103)
• get_ref<T>(&self) -> Option<RefGuard<'_, T>> (:151) — borrowed access via a Deref<Target = T> guard
• get<T: Clone + 'static + Send + Sync>(&self) -> Option<T> (:195) — cloning access
• contains<T>(&self) -> bool (:221)

To read a stashed value from inside a handler, use the extractor of the same name: controller::extractor::shared_store::SharedStore<T>(pub T), which implements FromRequestParts<AppContext> and returns Error::InternalServerError if T was never inserted (src/controller/extractor/shared_store.rs:6-29).

Naming note: app::SharedStore (the container held on AppContext) and the extractor controller::extractor::shared_store::SharedStore<T> (the axum extractor) are two distinct types that share a name. Both are reachable through the prelude — see below — so disambiguate by context: the field type is the store, the tuple-struct-with-generic is the extractor.

loco_rs::prelude

use loco_rs::prelude::*; (src/prelude.rs) is the standard single import for app code (controllers, models, workers, tasks). It re-exports, unconditionally unless noted:

Async / axum plumbing

• async_trait::async_trait
• axum::debug_handler
• axum::extract::{Form, Multipart, Path, Query, State}
• axum::response::{IntoResponse, Response}
• axum::routing::{delete, get, head, options, patch, post, put, trace}
• axum_extra::extract::cookie

Third-party helpers

• chrono::NaiveDateTime as DateTime
• include_dir::{include_dir, Dir}
• serde_json::json as data — sugar so controller/view code can write data!({"item": ..}) instead of json!
• validator::Validate

Core Loco types

• app::{AppContext, Initializer}
• bgworker::{BackgroundWorker, Queue}
• errors::Error and Result (the crate's Result<T, Error> alias)
• mailer (the module itself) and mailer::Mailer
• task::{self, Task, TaskInfo}
• validation::{self, Validatable, ValidatorTrait}

Controller layer

• controller::{bad_request, not_found, unauthorized} — error-response constructor fns
• controller::format — the response-builder module (format::json, format::render(), ...)
• controller::middleware::format::{Format, RespondTo} — content-negotiation extractor/enum
• controller::middleware::remote_ip::RemoteIP — computed-client-IP extractor
• controller::extractor::shared_store::SharedStore — the DI extractor (see above)
• controller::extractor::validate::{JsonValidate, JsonValidateWithMessage} — validating-body extractors
• controller::views::{engines::TeraView, ViewEngine, ViewRenderer}
• controller::{Json, Routes}

Feature-gated

• #[cfg(feature = "auth")] controller::extractor::auth (the whole auth extractor module: JWT, JWTWithUser, ApiToken, UserClaims, token-extraction helpers)
• #[cfg(feature = "with-db")]:
  • Sea-ORM traits and types: ActiveModelBehavior, ActiveModelTrait, ActiveValue, ColumnTrait, ConnectionTrait, DatabaseConnection, DbErr, EntityTrait, IntoActiveModel, ModelTrait, QueryFilter, Set, TransactionTrait
  • Sea-ORM scalar re-exports: sea_orm::prelude::{Date, DateTimeUtc, DateTimeWithTimeZone, Decimal, Uuid}
  • model::{query, Authenticable, ModelError, ModelResult} — plus a nested pub mod model { pub use crate::model::query; }, so query is reachable both as loco_rs::prelude::query and as loco_rs::prelude::model::query
• #[cfg(feature = "testing")] crate::testing::prelude::* — pulled in only for apps/tests built with the testing feature

Source: src/prelude.rs (verified against HEAD at authoring time).