1 // Copyright 2017, OpenCensus Authors 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); 4 // you may not use this file except in compliance with the License. 5 // You may obtain a copy of the License at 6 // 7 // http://www.apache.org/licenses/LICENSE-2.0 8 // 9 // Unless required by applicable law or agreed to in writing, software 10 // distributed under the License is distributed on an "AS IS" BASIS, 11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 // See the License for the specific language governing permissions and 13 // limitations under the License. 14 // 15 16 package tag 17 18 import ( 19 "context" 20 ) 21 22 // FromContext returns the tag map stored in the context. 23 func FromContext(ctx context.Context) *Map { 24 // The returned tag map shouldn't be mutated. 25 ts := ctx.Value(mapCtxKey) 26 if ts == nil { 27 return nil 28 } 29 return ts.(*Map) 30 } 31 32 // NewContext creates a new context with the given tag map. 33 // To propagate a tag map to downstream methods and downstream RPCs, add a tag map 34 // to the current context. NewContext will return a copy of the current context, 35 // and put the tag map into the returned one. 36 // If there is already a tag map in the current context, it will be replaced with m. 37 func NewContext(ctx context.Context, m *Map) context.Context { 38 return context.WithValue(ctx, mapCtxKey, m) 39 } 40 41 type ctxKey struct{} 42 43 var mapCtxKey = ctxKey{} 44