harmony 鸿蒙Lightweight Data Store Development

2022-08-09
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Lightweight Data Store Development

When to Use

The lightweight data store is ideal for storing lightweight and frequently used data, but not for storing a large amount of data or data with frequent changes. The application data is persistently stored on a device in the form of files. Note that the instance accessed by an application contains all data of the file. The data is always loaded to the memory of the device until the application removes it from the memory. The application can perform data operations using the Preferences APIs.

Available APIs

The lightweight data store provides applications with data processing capability and allows applications to perform lightweight data storage and query. Data is stored in key-value pairs. Keys are of the string type, and values can be of the string, Boolean, integer, long integer, float, double, or string array type.

Creating a Preferences Instance

Create a Preferences instance for data operations. A Preferences instance is created after data is read from a specified file and loaded to the instance.

Table 1 API for creating a Preferences instance

Class	Method	Description
PreferencesHelper	static std::shared_ptr GetPreferences(const std::string &path, int &errCode);	Creates a Preferences instance. path: storage path of the application data. errCode: error code. Return value: Preferences instance created.

Writing Data

Call the put() method to add or modify data in a Preferences instance.

Table 2 APIs for writing data

Class	Method	Description
Preferences	int PutInt(const std::string &key, int value);	key: key of the data to write. It cannot be empty. value: value of the data to write. Return value: Returns 0 if the operation is successful; returns the error code otherwise.
Preferences	int PutString(const std::string &key, const std::string &value);	key: key of the data to write. It cannot be empty. value: value of the data to write. Return value: Returns 0 if the operation is successful; returns the error code otherwise.
Preferences	int PutBool(const std::string &key, bool value);	key: key of the data to write. It cannot be empty. value: value of the data to write. Return value: Returns 0 if the operation is successful; returns the error code otherwise.
Preferences	int PutLong(const std::string &key, int64_t value);	key: key of the data to write. It cannot be empty. value: value of the data to write. Return value: Returns 0 if the operation is successful; returns the error code otherwise.
Preferences	int PutFloat(const std::string &key, float value);	key: key of the data to write. It cannot be empty. value: value of the data to write. Return value: Returns 0 if the operation is successful; returns the error code otherwise.
Preferences	int PutDouble(const std::string &key, double value);	key: key of the data to write. It cannot be empty. value: value of the data to write. Return value: Returns 0 if the operation is successful; returns the error code otherwise.
Preferences	int PutStringSet(const std::string &key, const std::set<std::string> &value);	key: key of the data to write. It cannot be empty. value: value of the data to write. Return value: Returns 0 if the operation is successful; returns the error code otherwise.

Reading Data

Call the get() method to read data from a Preferences instance.

Table 3 APIs for reading data

Class	Method	Description
Preferences	int GetInt(const std::string &key, const int defValue = 0);	key: key of the data to read. It cannot be empty. defValue: default value to return if the operation fails or the value does not exist. Return value: value obtained.
Preferences	std::string GetString(const std::string &key, const std::string &defValue = {});	key: key of the data to read. It cannot be empty. defValue: default value to return if the operation fails or the value does not exist. Return value: value obtained.
Preferences	bool GetBool(const std::string &key, const bool defValue = false);	key: key of the data to read. It cannot be empty. defValue: default value to return if the operation fails or the value does not exist. Return value: value obtained.
Preferences	float GetFloat(const std::string &key, const float defValue = 0);	key: key of the data to read. It cannot be empty. defValue: default value to return if the operation fails or the value does not exist. Return value: value obtained.
Preferences	double GetDouble(const std::string &key, const double defValue = 0);	key: key of the data to read. It cannot be empty. defValue: default value to return if the operation fails or the value does not exist. Return value: value obtained.
Preferences	int64_t GetLong(const std::string &key, const int64_t defValue = 0);	key: key of the data to read. It cannot be empty. defValue: default value to return if the operation fails or the value does not exist. Return value: value obtained.
Preferences	std::set<std::string> GetStringSet(const std::string &key, const std::set<std::string> &defValue = {});	key: key of the data to read. It cannot be empty. defValue: default value to return if the operation fails or the value does not exist. Return value: value obtained.

Storing Data Persistently

Call the Flush() or FlushSync() method to write the cached data back to its text file for persistent storage.

Table 4 APIs for data persistence

Class	Method	Description
Preferences	void Flush();	Writes data in the Preferences instance back to its file through an asynchronous thread.
Preferences	int FlushSync();	Writes data in the Preferences instance back to its file through a synchronous thread.

Observing Data Changes

Specify PreferencesObserver as the callback to subscribe to data changes. When the value of the subscribed key is changed and the flush() method is executed, PreferencesObserver will be invoked.

Table 5 APIs for observing data changes

Class	Method	Description
Preferences	void RegisterObserver(std::shared_ptr preferencesObserver);	Subscribes to data changes. preferencesObserver: callback invoked to return the data changes.
Preferences	void UnRegisterObserver(std::shared_ptr preferencesObserver);	Unsubscribes from data changes. preferencesObserver: callback used to report data changes.

Deleting Data

Use the following APIs to delete a Preferences instance or data file.

Table 6 APIs for deleting data

Class	Method	Description
PreferencesHelper	int DeletePreferences(const std::string &path);	Deletes a Preferences instance from the memory and deletes its file from the device. path: storage path of the application data. Return value: Returns 0 if the operation is successful; returns the error code otherwise.
PreferencesHelper	int RemovePreferencesFromCache(const std::string &path);	Deletes a Preferences instance from the memory. path: storage path of the application data. Return value: Returns 0 if the operation is successful; returns the error code otherwise.

How to Develop

Import the Preferences module and related header files to the development environment.

Header file path: //distributeddatamgr_appdatamgr/interfaces/innerkits/native_preferences/include

Create a Preferences instance.

Read the specified file and load its data to the Preferences instance for data operations.

int errCode = E_OK;
Preferences pref = PreferencesHelper::GetPreferences(PREF_TEST_PATH + "test.xml", errCode); // PREF_TEST_PATH must be the application sandbox path.
EXPECT_EQ(errCode, E_OK);

Write data.

Use the put() method of the Preferences class to write data to the cached Preferences instance.
```
pref->PutString("test", "remove");
```

Read data.

Use the get() method of the Preferences class to read data.

std::string ret = pref->GetString("test", "defaultValue");
EXPECT_EQ(ret, "remove");

Store data persistently.

Use the Flush() or FlushSync() method to flush data in the Preferences instance to its file.

```C++
int err = pref->FlushSync();
EXPECT_EQ(ret, E_OK);
```

Subscribe to data changes.

Specify PreferencesObserver as the callback to subscribe to data changes for an application. When the value of the subscribed key is changed and the flush() or flushSync() method is executed, PreferencesObserver will be invoked. Unregister the PreferencesObserver when it is no longer required.

Customize a class to implement the PreferencesObserver:

class PreferencesObserverCounter : public PreferencesObserver {
public:
virtual ~PreferencesObserverCounter();
void OnChange(Preferences &preferences, const std::string &key) override;


    std::atomic_int notifyTimes;
    static const std::vector<std::string> NOTIFY_KEYS_VECTOR;
};


PreferencesObserverCounter::~PreferencesObserverCounter() {}


void PreferencesObserverCounter::OnChange(Preferences &preferences, const std::string &key)
{
    for (auto it = NOTIFY_KEYS_VECTOR.cbegin(); it != NOTIFY_KEYS_VECTOR.cend(); it++) {
        if (key.compare(*it)) {
            notifyTimes++;
            break;
        }
    }
}


const std::vector<std::string> PreferencesObserverCounter::NOTIFY_KEYS_VECTOR = { PreferencesTest::KEY_TEST_INT_ELEMENT,
    PreferencesTest::KEY_TEST_LONG_ELEMENT, PreferencesTest::KEY_TEST_FLOAT_ELEMENT,
    PreferencesTest::KEY_TEST_BOOL_ELEMENT, PreferencesTest::KEY_TEST_STRING_ELEMENT };

Subscribe to data changes and invoke the callback:

std::shared_ptr<PreferencesObserver> counter =
    std::make_shared<PreferencesObserverCounter>();
pref->RegisterObserver(counter); // Register a callback to return data changes.


pref->PutString(PreferencesTest::KEY_TEST_STRING_ELEMENT, "test");
pref->Flush(); // Trigger the onChanged callback of the counter.
EXPECT_EQ(static_cast<PreferencesObserverCounter *>(counter.get())->notifyTimes, 1);


/* same value */
pref->PutInt(PreferencesTest::KEY_TEST_INT_ELEMENT, 2);
pref->PutString(PreferencesTest::KEY_TEST_STRING_ELEMENT, "test");
pref->Flush();
EXPECT_EQ(static_cast<PreferencesObserverCounter *>(counter.get())->notifyTimes, 2);


pref->UnRegisterObserver(counter); // Unregister the callback for data changes.

Delete the specified file.

Delete the Preferences singleton of the specified file from the memory, and delete the specified file, its backup file, and damaged files. After the specified files are deleted, the application cannot use that instance to perform any data operation. Otherwise, data inconsistency will occur. The deleted data and files cannot be restored.
```
pref = nullptr;
int ret = PreferencesHelper::DeletePreferences("/data/test/test");
EXPECT_EQ(ret, E_OK);
```