如果忽略了when，那么这个值就默认为always，也就是说，每当ContrainedExpression的一个依赖关系发生改变，该约束都将重新计算。
If when is omitted, then the value defaults to always, which implies that every time a dependency of ContrainedExpression changes, the constraint will be reevaluated.
如果有资源被再次请求，并且依赖性没有受到任何修改的话，那么缓存的资源表示会直接被返回，系统无须再重新对其进行计算。
If any resource is re-requested and dependencies have not changed then the cached representation is returned instead of being recomputed.
如果有资源被再次请求，并且依赖性没有受到任何修改的话，那么缓存的资源表示会直接被返回，系统无须再重新对其进行计算。
If any resource is re-requested and dependencies have not changed then the cached representation is returned instead of being recomputed.
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