How to get IME contents in keyPressed callback?

I’m trying to use UltraLight (a embedded HTML renderer) inside JUCE, where keyboard events need to be injected. I wrote following code to handle text input:

class UltraLightComponent: public juce::Component
{
    // ......
    bool keyPressed( const juce::KeyPress& key ) override;
    // ......
};

bool UltraLightComponent::keyPressed( const juce::KeyPress& k )
{
    // ......

    // inject text input
    char32_t text_char = (char32_t) k.getTextCharacter();
    if ( text_char != 0 )
    {
        DBG( "text character: " + juce::String::toHexString( text_char ) );
        ultralight::KeyEvent text_key;
        text_key.type = ultralight::KeyEvent::kType_Char;
        text_key.text = ultralight::String32{ &text_char, 1 };
        guts->view->FireKeyEvent( text_key );
    }
    // ......
}

It works fine when I type in English characters from keyboard directly. But when I type in Chinese characters using input method (actually the one from Microsoft), getTextCharacter() always return zero. So is there any extra things I should do to make JUCE components handle IME?