Rxjava操作符1：withLatestFrom

WithLatestFrom.swift:
Merges two observable sequences into one observable sequence by combining each element from self with the latest element from the second source, if any.

withLatestFrom將兩個可觀察序列合併爲一個可觀察序列，當第二個源有新數據的時候，再發送第一個源，會將第二個源的新數據訂閱到。
說白了就是：第一個源數據來的時候，合併第二個源最新的數據，測試代碼：

Observable.interval(1, TimeUnit.SECONDS)
                .withLatestFrom(test, new BiFunction<Long, Integer, Integer>() {
                    @Override
                    public Integer apply(Long aLong, Integer integer) throws Exception {
                        return integer;
                    }
                })
                // .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Consumer<Integer>() {
                    @Override
                    public void accept(Integer o) throws Exception {
                        Log.e("RxTest", "accept " + o);
                    }
                });
        test.onNext(1);

輸出：
accept 1
accept 1
accept 1
accept 1
accept 1
……