harmony 鸿蒙Working with Latin1/UTF16 Strings Using JSVM-API

2025-06-12
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Working with Latin1/UTF16 Strings Using JSVM-API

Introduction

This topic walks you through on how to create and use external strings using JSVM-API.

Basic Concepts

By using JSVM-API, you can create JS strings directly in the memory allocated for Latin1/UTF16 strings. This allows you to manipulate these strings like normal JS strings.

Available APIs

API	Description
OH_JSVM_CreateExternalStringLatin1	Creates an external JS string from a C string encoded in ISO-8859-1 format.
OH_JSVM_CreateExternalStringUtf16	Creates an external JS string from a C string encoded in UTF16-LE format.

Example

If you are just starting out with JSVM-API, see JSVM-API Development Process. The following demonstrates only the C++ code involved in manipulating external strings.

Determining a Number Object

CPP code:

#include <cstring>
#include <string>
static char stringLatin1[] = "hello";
static char16_t stringUTF16[] = u"world";

static JSVM_Value testExternalString(JSVM_Env env, JSVM_CallbackInfo info) {
    JSVM_VM vm;
    OH_JSVM_GetVM(env, &vm);

    JSVM_HandleScope handleScope;
    OH_JSVM_OpenHandleScope(env, &handleScope);
    JSVM_Value jsStrLatin1 = nullptr;
    bool copied = true;
    char buf[10];
    OH_JSVM_CreateExternalStringLatin1(env, stringLatin1, strlen(stringLatin1), nullptr, nullptr,
                                       &jsStrLatin1, &copied);
    OH_JSVM_GetValueStringUtf8(env, jsStrLatin1, buf, 10, nullptr);
    OH_LOG_INFO(LOG_APP, "created latin1 string is : %{public}s\n", buf);
    // If the value of copied is true, the external string fails to be created. Otherwise, the external string is created successfully.
    OH_LOG_INFO(LOG_APP, "create external string failed : %{public}d\n", copied);
    copied = true;
    JSVM_Value jsStrUTF16 = nullptr;
    OH_JSVM_CreateExternalStringUtf16(env, stringUTF16, std::char_traits<char16_t>::length(stringUTF16),
                                      nullptr, nullptr, &jsStrUTF16, &copied);
    OH_JSVM_GetValueStringUtf8(env, jsStrUTF16, buf, 10, nullptr);
    OH_LOG_INFO(LOG_APP, "created utf16 string is : %{public}s\n", buf);
    // If the value of copied is true, the external string fails to be created. Otherwise, the external string is created successfully.
    OH_LOG_INFO(LOG_APP, "create external string failed : %{public}d\n", copied);
    OH_JSVM_CloseHandleScope(env, handleScope);

    return nullptr;
}

static JSVM_CallbackStruct param[] = {
    {.data = nullptr, .callback = testExternalString},
};

static JSVM_CallbackStruct *method = param;

// Alias for the wrapperObject method to be called from JS.
static JSVM_PropertyDescriptor descriptor[] = {
    {"testExternalString", nullptr, method++, nullptr, nullptr, nullptr, JSVM_DEFAULT},
};

// Call the C++ code from JS.
const char *srcCallNative = R"JS(testExternalString();)JS";

Expected Result

created latin1 string is : hello
create external string failed: 0
created utf16 string is : world
create external string failed: 0

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