| // Copyright 2009 The Go 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 sync |
| |
| // Once is an object that will perform exactly one action. |
| type Once struct { |
| m Mutex |
| done bool |
| } |
| |
| // Do calls the function f if and only if the method is being called for the |
| // first time with this receiver. In other words, given |
| // var once Once |
| // if Do(f) is called multiple times, only the first call will invoke f, |
| // even if f has a different value in each invocation. A new instance of |
| // Once is required for each function to execute. |
| // |
| // Do is intended for initialization that must be run exactly once. Since f |
| // is niladic, it may be necessary to use a function literal to capture the |
| // arguments to a function to be invoked by Do: |
| // config.once.Do(func() { config.init(filename) }) |
| // |
| // Because no call to Do returns until the one call to f returns, if f causes |
| // Do to be called, it will deadlock. |
| // |
| func (o *Once) Do(f func()) { |
| o.m.Lock() |
| defer o.m.Unlock() |
| if !o.done { |
| o.done = true |
| f() |
| } |
| } |