fxctx/fxctx.go

// Copyright 2018 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.
package fxctx

import (
	"context"
	"encoding/json"
	"fmt"
)

// ContextKey is used to store values in Context objects.
type ContextKey string

// Set associates a JSON encodable value with a key in a Context object.
func Set(ctx context.Context, key ContextKey, value interface{}) (context.Context, error) {
	data, err := json.Marshal(value)
	if err != nil {
		return nil, err
	}

	return context.WithValue(ctx, key, data), nil
}

// Get reads a value from a Context into `out`.  Returns true iff the given key is present
// in the Context, even if the key cannot be read into `out`.
func Get(ctx context.Context, key ContextKey, out interface{}) error {
	value := ctx.Value(key)
	if value == nil {
		return fmt.Errorf("not found %v", key)
	}

	bytes, ok := value.([]byte)
	if !ok {
		return fmt.Errorf("expected a []byte value for %v but got %v", key, value)
	}

	return json.Unmarshal(bytes, out)
}