RxJava的`Completable.andThen`没有串行执行?

问题描述

我有一个用例，其中我在Completable中初始化一些全局变量，然后在链的下一步(使用 andThen 运算符)中使用这些变量.

以下示例详细说明了我的用例

假设您有一个类 User

 类User {字符串名称；} 

我有一个这样的Observable，

 私有用户mUser；//这是一个全局变量公共Observable< String>stringObservable(){返回Completable.fromAction(()-> {mUser = new User();mUser.name =名称";}).then(Observable.just(mUser.name));} 

首先，我要在 Completable.fromAction 中进行一些初始化，并且我希望 andThen 运算符仅在完成 Completable.fromAction 之后才能启动

这意味着我希望 andThen 运算符启动时会初始化 mUser .

以下是我对此观察性的订阅

  stringObservable().subscribe(s-> Log.d(TAG，"success:" + s)，投掷->Log.e(TAG，"error:" + throwable.getMessage())); 

但是当我运行此代码时，出现错误

 尝试从空对象引用中的字段'java.lang.String User.name'中读取 

这意味着 mUser 为null，然后Then 在执行 Completable.fromAction 中的代码之前启动.这里发生了什么事?

根据 andThen

的文档

解决方案

问题不在于 andThen ，而是内部的语句 Observable.just(mUser.name) andThen .尽管 just 运算符仅在 Completable.fromAction 之后才会发出，但它会尝试立即创建可观察对象.

这里的问题是，尝试使用just创建 Observable 时， mUser 为空.

解决方案:您需要推迟String Observable的创建，直到发生订阅为止，直到 andThen 的上游开始发出.

而不是 andThen(Observable.just(mUser.name));

使用

  andThen(Observable.defer(()-> Observable.just(mUser.name))); 

或

  andThen(Observable.fromCallable(()-> mUser.name)); 

I have a usecase where I initiallize some global variables in a Completable , and in the next step in the chain (using andThen operator) I make use of those variables.

Following sample explains my usecase in detail

Say you have a class User

        class User {
            String name;
        }

and I have an Observable like this ,

        private User mUser; // this is a global variable

        public Observable<String> stringObservable() {
            return Completable.fromAction(() -> {
                mUser = new User();
                mUser.name = "Name";
            }).andThen(Observable.just(mUser.name));
        }

First I'm doing some initiallizations in my Completable.fromAction and I expect andThen operator to start only after completing the Completable.fromAction.

Which means I expect mUser to be initallized when the andThen operator starts.

Following is my subscription to this observable

             stringObservable()
            .subscribe(s -> Log.d(TAG, "success: " + s),
                    throwable -> Log.e(TAG, "error: " + throwable.getMessage()));

But when I run this code , I get an error

          Attempt to read from field 'java.lang.String User.name' on a null object reference

which means mUser is null , andThen started before executing the code in Completable.fromAction. Whats happening here?

According to documentation of andThen

解决方案

The issue is not with andThen but with the statement Observable.just(mUser.name) inside andThen . The just operator will try to create the observable immediately though it will emit only after Completable.fromAction.

Problem here is , while trying to create the Observable using just , the mUser is null.

Solution : You need to defer the creation of the String Observable till a subscription happens , till the upstream of andThen starts emission.

Instead of andThen(Observable.just(mUser.name));

use

 andThen(Observable.defer(() -> Observable.just(mUser.name)));

Or

 andThen(Observable.fromCallable(() -> mUser.name));

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