如何创建一个允许非消费和消费功能的构建器

Question

I want to create a builder for a struct representing an analytics event. once the event is "sent" the builder should be consumed.

I read this primer:

https://doc.rust-lang.org/1.0.0/style/ownership/builders.html

so to allow simple chaining without reassignment (which might be required since there are many execution branches), my builder looks like this (simplified)

pub struct AnalyticsEvent {

}

impl AnalyticsEvent {
  pub fn new() -> Self {
    AnalyticsEvent { }
  }
 pub fn add_property(&mut self) -> &mut AnalyticsEvent {
        self
    }

// now I want the "send" to consume the event
pub fn send(self) {
/// ....
    }
}

link to playground

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=fbe9e261a419608f2c592799f22ba9f9

Answer 1

You can't mix both in the same chain, since your non-consuming methods only return &mut T there's no way for self -consuming methods to be called on them.

So either keep the builder interface as-is and replace

    event.add_property().send();

by

    event.add_property();
    event.send();

or change the builder interface to always work on owned objects, requiring the odd reassignment in some cases.

In the latter case you could mitigate the issue by adding convenience methods eg a map which would enclose the conditional situation:

    event.add_property()
         .add_property()
         .map(|e| if condition {
             e.add_property()
         } else {
             e
         })
         .add_property()
         .send();