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// Copyright 2016 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
library fuchsia.modular;
// Exposes access to Context values, each of which are explicitly named by a
// 'topic'. Each topic has a known/pre-defined value representation.
// This interface is meant to be used in the scope of a single Fuchsia
// component.
interface ContextReader {
// When any conditions that would cause the resulting set of context values
// selected by |query| to change, a ContextUpdate is sent to |listener|.
// The current state of values selected by |query| are sent to |listener|
// immediately.
// The subscription will continue to stay active as long as the |listener|
// handle is not closed.
1: Subscribe(ContextQuery query, ContextListener listener);
// Returns the current context state matching |query|.
2: Get(ContextQuery query) -> (ContextUpdate update);
struct ContextQuery {
// A map of ContextSelector structs. The keys are specified by the client.
// Each key gets a corresponding entry in |ContextUpdate.values| when it is
// generated in response to this query.
vector<ContextQueryEntry> selector;
struct ContextQueryEntry {
string key;
ContextSelector value;
struct ContextSelector {
// A partial metadata struct. Any metadata value given in |meta| restricts
// the set of values in the eventual ContextUpdate to include only those
// whose metadata match values given.
// A null |meta| will match any value of the given |type|. Use this sparingly
// as a full update could become quite large.
// TODO(thatguy): Find users of this and eliminate them if it becomes a
// scaling problem.
// For example, to select all context values in a given story, one would
// specify:
// meta.value["story"]["id"] = "..."
ContextMetadata? meta;
// A string specifying the type of context object to return.
ContextValueType type;
interface ContextListener {
// Receives a full snapshot of context state every time context is updated
// in such a way that results in a changed set of values for the ContextQuery
// used in the subscription for this listener.
// TODO(thatguy): A broad ContextQuery will result in a large
// |ContextUpdate|, which may exceed the FIDL max message size. In this case
// updates would not be propagated. If this becomes an issue, allowing
// clients to opt for delta-updates instead of state snapshots would suffice,
// albeit also add some complexity to client implementations.
1: OnContextUpdate(ContextUpdate update);
struct ContextUpdate {
// A list of ContextValue for each key in |ContextQuery.selector|. An empty
// list means no values matched the given ContextSelector.
vector<ContextUpdateEntry> values;
struct ContextUpdateEntry {
string key;
vector<ContextValue> value;