runTransaction和set()之间的区别

本文介绍了runTransaction和set()之间的区别的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

在查看原始资源之后，我正在看这个用于Flutter的Firebase API的演示程序(达特朗).我并没有想过 runTransaction 和 set()之间的区别以及作者为什么一个使用另一个.

I am looking at this demo program for Firebase API for Flutter (dartlang) and after looking at the original source. I am not grokking the difference between runTransaction and set() and why the authors used one over the other.

Future<Null> _increment() async {
    // Increment counter in transaction.
    final TransactionResult transactionResult =
        await _counterRef.runTransaction((MutableData mutableData) async {
      mutableData.value = (mutableData.value ?? 0) + 1;
      return mutableData;
    });

    if (transactionResult.committed) {
      _messagesRef.push().set(<String, String>{
        _kTestKey: '$_kTestValue ${transactionResult.dataSnapshot.value}'
      });
    } else {
      print('Transaction not committed.');
      if (transactionResult.error != null) {
        print(transactionResult.error.message);
      }
    }
}

每个源 runTransaction 中的注释为:

对于 set 方法，注释应为:

有人ELI5有什么区别吗?为什么作者选择两种不同的方法写入数据库?

Can someone ELI5 what is the difference and why the authors chose two different methods to write to the db?

完整的演示代码位于此处

推荐答案

我无法告诉您为什么作者选择了另一个，但是事务和常规写入之间的区别是很明显的.

I can't tell you why the authors picked one over the other, but the difference between a transaction and a regular write is quite clear.

常规写入正是该名称所隐含的含义:它将值写入您指定的位置.

A regular write is precisely what the name implies: it writes the value to the location that you specify.

在Firebase数据库中，您可以使用事务根据该位置的当前值将新值写入该位置.因此，它结合了读取和写入操作.Firebase事务与众不同之处在于它们是比较设置操作.

In the Firebase Database you use a transaction to write a new value to a location, based on the current value in that location. So it combines a read and a write operation. What's somewhat unusual about Firebase transactions is that they are compare-and-set operations.

有关这意味着什么的更多详细信息，我建议阅读以下内容:

For more details on what that means, I recommend reading these:

交易中的数据为空
交易updateFunction参数为空
firebase交易的奇怪行为

该链接还显示了事务与您自己读取和写入事务有何不同:如果事务自读取以来未修改当前值，则事务将仅执行写入操作.

That link also shows how a transaction is different from reading and writing it yourself: a transaction will only perform the write operation if the current value was not modified since it was read.

这篇关于runTransaction和set()之间的区别的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！

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