我一直在阅读这篇来自MSDN的异步文章,但我无法理解给定示例中Lazy<T>
的用途。
public class AsyncCache<TKey, TValue>
{
private readonly Func<TKey, Task<TValue>> _valueFactory;
private readonly ConcurrentDictionary<TKey, Lazy<Task<TValue>>> _map;
public AsyncCache(Func<TKey, Task<TValue>> valueFactory)
{
if (valueFactory == null) throw new ArgumentNullException("loader");
_valueFactory = valueFactory;
_map = new ConcurrentDictionary<TKey, Lazy<Task<TValue>>>();
}
public Task<TValue> this[TKey key]
{
get
{
if (key == null) throw new ArgumentNullException("key");
return _map.GetOrAdd(key, toAdd =>
new Lazy<Task<TValue>>(() => _valueFactory(toAdd))).Value;
}
}
}
据我所知,当您调用
Lazy<T>
的.Value
时,它会调用构造函数。从示例中可以看出,它会立即调用,那为什么要添加Lazy<T>
呢?