harmony 鸿蒙Preferences Overview

2022-08-09
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Preferences Overview

Preferences are used to implement quick access and persistence of the data in the key-value structure.

After an application obtains a Preferences instance, the data in the instance will be cached in the memory for faster access.

The cached data can also be written to a text file for persistent storage. Since file read and write consume system resources, you are advised to minimize the frequency of reading and writing files.

You do not need to care about the implementation of the database locking mechanism.

Basic Concepts

Key-value structure

A type of data structure. The Key is the unique identifier for a piece of data, and the Value is the specific data being identified.

Non-relational database

A database not in compliance with the atomicity, consistency, isolation, and durability (ACID) properties of relational data transactions. The data in a non-relational database is independent. The database that organizes data in the key-value structure is a non-relational database.

Working Principles

An application can load data from a Preferences persistent file to a Preferences instance. The system stores the Preferences instance in the memory through a static container. Each file of an application or process has only one Preferences instance in the memory, till the application removes the instance from the memory or deletes the Preferences persistent file.
When obtaining a Preferences instance, the application can read data from or write data to the instance. The data in the Preferences instance can be flushed to its Preferences persistent file by calling the flush() method.

Figure 1 Working mechanism

Constraints

Preferences instances are loaded to the memory. To minimize non-memory overhead, the number of data records stored in a Preferences instance cannot exceed 10,000. Delete the instances that are no longer used in a timely manner.
The Key in key-value pairs is of the string type. It cannot be empty or exceed 80 bytes.
The Value of the string type in key-value pairs can be empty, but cannot exceed 8192 bytes if not empty.