PromiseKit：如何在 ObjC 中进行重试/轮询？

Posted 2023-02-24

【中文标题】PromiseKit：如何在 ObjC 中进行重试/轮询？

【英文标题】：PromiseKit: How to do retry / polling in ObjC?

【发布时间】：2019-03-29 09:27:21

【问题描述】：

PromiseKit "Common Patterns" 文档在 Swift 中。

如何在 Objective C 中编写重试/轮询代码？

重试/轮询

func attempt<T>(maximumRetryCount: Int = 3, delayBeforeRetry: DispatchTimeInterval = .seconds(2), _ body: @escaping () -> Promise<T>) -> Promise<T> 
    var attempts = 0
    func attempt() -> Promise<T> 
        attempts += 1
        return body().recover  error -> Promise<T> in
            guard attempts < maximumRetryCount else  throw error 
            return after(delayBeforeRetry).then(on: nil, attempt)
        
    
    return attempt()


attempt(maximumRetryCount: 3) 
    flakeyTask(parameters: foo)
.then 
    //…
.catch  _ in
    // we attempted three times but still failed

【参考方案1】：

由 PromiseKit 开发人员在Github 上回答：

- (AnyPromise *) attempt:(NSUInteger) maximumRetryCount delayBeforeRetry:(NSTimeInterval) delay provider:(AnyPromise* (^)()) provide 
    __block NSUInteger attempts = 0;
    AnyPromise *(^attempt)() = ^
        attempts++;
        return provide().catch(^(id error) 
            if (attempts >= maximumRetryCount) @throw error;
            return PMKAfter(delay).then(attempt);
        );
    ;
    return attempt();

那么，如果你有这样的方法：

- (AnyPromise *) myPromise 
    return [AnyPromise promiseWithResolverBlock:^(PMKResolver resolve) 
        ...
        resolve(nil) / resolve (error); 
    ];

调用它

[self attempt:3 delayBeforeRetry:2 provider:^ return [self myPromise]; ]