1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
|
// Copyright 2016 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package android
import (
"sync"
"sync/atomic"
)
type OncePer struct {
values atomic.Value
valuesLock sync.Mutex
}
type valueMap map[interface{}]interface{}
// Once computes a value the first time it is called with a given key per OncePer, and returns the
// value without recomputing when called with the same key. key must be hashable.
func (once *OncePer) Once(key interface{}, value func() interface{}) interface{} {
// Atomically load the map without locking. If this is the first call Load() will return nil
// and the type assertion will fail, leaving a nil map in m, but that's OK since m is only used
// for reads.
m, _ := once.values.Load().(valueMap)
if v, ok := m[key]; ok {
return v
}
once.valuesLock.Lock()
defer once.valuesLock.Unlock()
// Check again with the lock held
m, _ = once.values.Load().(valueMap)
if v, ok := m[key]; ok {
return v
}
// Copy the existing map
newMap := make(valueMap, len(m))
for k, v := range m {
newMap[k] = v
}
v := value()
newMap[key] = v
once.values.Store(newMap)
return v
}
func (once *OncePer) OnceStringSlice(key interface{}, value func() []string) []string {
return once.Once(key, func() interface{} { return value() }).([]string)
}
func (once *OncePer) Once2StringSlice(key interface{}, value func() ([]string, []string)) ([]string, []string) {
type twoStringSlice [2][]string
s := once.Once(key, func() interface{} {
var s twoStringSlice
s[0], s[1] = value()
return s
}).(twoStringSlice)
return s[0], s[1]
}
|
