Updating Interface Text | CodeSignal Learn

Updating Interface Text | CodeSignal Learn

Introduction

Welcome back to Interactive Controls and Layouts . We have reached Lesson 4, which is a significant milestone in the course: our window already displays controls, accepts typed input, and responds to button clicks. Now, we will connect those components so the interface can transfer text from one control to another. In this lesson, we will read the current text from an edit control and place that text into a static label when the button is clicked. This is a small feature, but it teaches a very important concept in GUI programming: interactive data flow inside the window procedure.

Entering The Setup Phase

The main logic still resides inside the window procedure. We begin the same way as before by checking which message arrived, then preparing the interface during WM_CREATE. C++LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) { switch (uMsg) { case WM_CREATE: { HINSTANCE hInst = (HINSTANCE)GetWindowLongPtr(hwnd, GWLP_HINSTANCE); The function signature is the standard form for a WinAPI WindowProc. hwnd identifies the main window, and uMsg indicates which event Windows is reporting. When that message is WM_CREATE, the window is being set up for the first time. We also read the instance handle with GetWindowLongPtr because the child controls created in this branch require that GWLP_HINSTANCE value.

Building The Three Controls

Once we have the instance handle hInst, we create the label, the edit box, and the button. The layout is familiar, but several details now support the text update behavior we want. C++ CreateWindowW(L"STATIC", L"Waiting for input...", WS_VISIBLE | WS_CHILD, 20, 20, 200, 20, hwnd, (HMENU)ID_LABEL_WELCOME, hInst, NULL); CreateWindowW(L"EDIT", L"", WS_CHILD | WS_VISIBLE | WS_BORDER | ES_AUTOHSCROLL, 20, 50, 200, 25, hwnd, (HMENU)ID_EDIT_INPUT, hInst, NULL); CreateWindowW(L"BUTTON", L"Update Label", WS_VISIBLE | WS_CHILD | BS_DEFPUSHBUTTON, 20, 100, 120, 30, hwnd, (HMENU)ID_BUTTON_ACTION, hInst, NULL); return 0;} The starting text L"Waiting for input..." makes the label appear active even before any typing occurs. The EDIT control begins empty, providing a blank space to type. The button text, L"Update Label", clearly describes the action. Most importantly, each control receives its own ID through HMENU, so the parent window can later locate the specific controls it needs using GetDlgItem .

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Listening For The Right Command

Now, we move to the interaction step. In the previous lesson, we learned that WM_COMMAND carries both the control ID and the notification code. Here, the code checks those values directly inside one condition. case WM_COMMAND: { if (LOWORD(wParam) == ID_BUTTON_ACTION && HIWORD(wParam) == BN_CLICKED) { // Obtain handles to our child controls via their unique IDs HWND hEdit = GetDlgItem(hwnd, ID_EDIT_INPUT); HWND hLabel = GetDlgItem(hwnd, ID_LABEL_WELCOME); This if statement is very precise. LOWORD(wParam) == ID_BUTTON_ACTION confirms that the message originated from our button, and HIWORD(wParam) == BN_CLICKED confirms that the event was an actual click. Once both tests pass, we use GetDlgItem to retrieve the child window handles. The function GetDlgItem translates a control ID back into a usable HWND. It takes the handle of the parent window (hwnd) and the unique ID of the child (the HMENU value we passed to CreateWindowW). Although "Dlg" stands for "Dialog," this function works with any parent-child window relationship. This is a best practice in WinAPI development because it allows you to retrieve a handle exactly when you need it, rather than cluttering your application with global variables to store every button and label handle.

Reading Text Into A Buffer

With the edit control handle ready, we need a place to store the text we read. In WinAPI, this usually involves preparing a character buffer and passing it to a text retrieval function. // Allocate a buffer to store the retrieved text wchar_t textBuffer[256]; // Get the text from the Edit control GetWindowTextW(hEdit, textBuffer, 256); wchar_t textBuffer[256] creates room for up to 256 wide characters. The use of wchar_t matches the Unicode version of the API, which is why the function name GetWindowTextW ends in W . Then, GetWindowTextW copies the current edit box text into that buffer. If the edit box is empty, the buffer will simply hold an empty string. If it contains text, the buffer acts as a bridge between input and output.

Updating The Label On Screen

Once the text has been copied into the buffer, the final step is simple: send that same text to the static control. This is where the interface visibly changes. // Set the text of the Static label to the content of the buffer SetWindowTextW(hLabel, textBuffer);} return 0;} SetWindowTextW replaces the label's current text with whatever is stored in textBuffer . Even though the label is a STATIC control, its displayed text can still be changed at runtime. Thus, if we type Hello into the edit box and click the button, the label changes from L"Waiting for input..." to Hello . That immediate feedback makes the window feel interactive.

C++

        case WM_COMMAND: {
            if (LOWORD(wParam) == ID_BUTTON_ACTION && HIWORD(wParam) == BN_CLICKED) {
                // Obtain handles to our child controls via their unique IDs
                HWND hEdit = GetDlgItem(hwnd, ID_EDIT_INPUT);
                HWND hLabel = GetDlgItem(hwnd, ID_LABEL_WELCOME);

C++

                // Allocate a buffer to store the retrieved text
                wchar_t textBuffer[256];

                // Get the text from the Edit control
                GetWindowTextW(hEdit, textBuffer, 256);