重试后退，直到成功或达到最大尝试次数

以下代码将尝试执行 loadFromHttp() 最多 5 次（maxAttempts），每次尝试都会延迟多少秒。如果超过 maxAttempts，Observable 就会放弃。

    // assume loadFromHttp() returns a Promise, which might fail.
    Rx.Observable.from(loadFromHttp())
    .retryWhen((attempts) => {
        let maxAttempts = 5;

        Rx.Observable.range(1, maxAttempts+1).zip(attempts, (i, attempt) => [i, attempt])
        .flatMap(([i, attempt]) => {
            if (i <= maxAttempts) {
                console.log(`Retrying in ${i} second(s)`);
                return Rx.Observable.timer(i * 1000);
            } else {
                throw attempt;
            }
        })
    })