Text Input And Clipboard Handling

Last Updated: Nov 26th, 2023

Getting text input from the keyboard is a common task in games. Here we'll be getting text using SDL 2's new text input and clipboard handling.

            //Main loop flag
            bool quit = false;

            //Event handler
            SDL_Event e;

            //Set text color as black
            SDL_Color textColor = { 0, 0, 0, 0xFF };

            //The current input text.
            std::string inputText = "Some Text";
            gInputTextTexture.loadFromRenderedText( inputText.c_str(), textColor );

            //Enable text input
            SDL_StartTextInput();

Before we go into the main loop we declare a string to hold our text and render it to a texture. We then call SDL_StartTextInput so the SDL text input functionality is enabled.

            //While application is running
            while( !quit )
            {
                //The rerender text flag
                bool renderText = false;

                //Handle events on queue
                while( SDL_PollEvent( &e ) != 0 )
                {

We only want to update the input text texture when we need to so we have a flag that keeps track of whether we need to update the texture.

                    //Special key input
                    else if( e.type == SDL_KEYDOWN )
                    {
                        //Handle backspace
                        if( e.key.keysym.sym == SDLK_BACKSPACE && inputText.length() > 0 )
                        {
                            //lop off character
                            inputText.pop_back();
                            renderText = true;
                        }
                        //Handle copy
                        else if( e.key.keysym.sym == SDLK_c && SDL_GetModState() & KMOD_CTRL )
                        {
                            SDL_SetClipboardText( inputText.c_str() );
                        }
                        //Handle paste
                        else if( e.key.keysym.sym == SDLK_v && SDL_GetModState() & KMOD_CTRL )
                        {
                            //Copy text from temporary buffer
                            char* tempText = SDL_GetClipboardText();
                            inputText = tempText;
                            SDL_free( tempText );

                            renderText = true;
                        }
                    }

There are a couple special key presses we want to handle. When the user presses backspace we want to remove the last character from the string.

When the user is holding control and presses c, we want to copy the current text to the clip board using SDL_SetClipboardText. You can check if the ctrl key is being held using SDL_GetModState.

When the user does ctrl + v, we want to get the text from the clip board using SDL_GetClipboardText. This function returns a newly allocated string, so we should get this string, copy it to our input text, then free it once we're done with it.

Also notice that whenever we alter the contents of the string we set the text update flag.

                    //Special text input event
                    else if( e.type == SDL_TEXTINPUT )
                    {
                        //Not copy or pasting
                        if( !( SDL_GetModState() & KMOD_CTRL && ( e.text.text[ 0 ] == 'c' || e.text.text[ 0 ] == 'C' || e.text.text[ 0 ] == 'v' || e.text.text[ 0 ] == 'V' ) ) )
                        {
                            //Append character
                            inputText += e.text.text;
                            renderText = true;
                        }
                    }
                }

With text input enabled, your key presses will also generate SDL_TextInputEvents which simplifies things like shift key and caps lock. Here we first want to check that we're not getting a ctrl and c/v event because we want to ignore those since they are already handled as keydown events. If it isn't a copy or paste event, we append the character to our input string.

                //Rerender text if needed
                if( renderText )
                {
                    //Text is not empty
                    if( inputText != "" )
                    {
                        //Render new text
                        gInputTextTexture.loadFromRenderedText( inputText.c_str(), textColor );
                    }
                    //Text is empty
                    else
                    {
                        //Render space texture
                        gInputTextTexture.loadFromRenderedText( " ", textColor );
                    }
                }

If the text render update flag has been set, we rerender the texture. One little hack we have here is if we have an empty string, we render a space because SDL_ttf will not render an empty string.

                //Clear screen
                SDL_SetRenderDrawColor( gRenderer, 0xFF, 0xFF, 0xFF, 0xFF );
                SDL_RenderClear( gRenderer );

                //Render text textures
                gPromptTextTexture.render( ( SCREEN_WIDTH - gPromptTextTexture.getWidth() ) / 2, 0 );
                gInputTextTexture.render( ( SCREEN_WIDTH - gInputTextTexture.getWidth() ) / 2, gPromptTextTexture.getHeight() );

                //Update screen
                SDL_RenderPresent( gRenderer );
            }

At the end of the main loop we render the prompt text and the input text.

            //Disable text input
            SDL_StopTextInput();

Once we're done with text input we disable it since enabling text input introduces some overhead.

Download the media and source code for this demo here.

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