Question

我希望OnPaint函数中的graphics.DrawImage（image，0,0）能够在使用下面的程序时显示从文件中读取的图像。而是为应用程序获取白色画布。我做错了什么？

我在Windows 10上使用visual studio 2017社区。谢谢！

/* https://msdn.microsoft.com/en-us/library/vs/alm/ms533895(v=vs.85).aspx */

#define UNICODE
#include <windows.h>
#include <objidl.h>
#include <gdiplus.h>
using namespace Gdiplus;
#pragma comment (lib,"Gdiplus.Lib")
#pragma comment (lib,"User32.Lib")
#pragma comment (lib,"Gdi32.Lib")

VOID OnPaint(HDC hdc, Image * image)
{
   Graphics graphics(hdc);
   graphics.DrawImage(image,0,0);
}

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

INT WINAPI WinMain(HINSTANCE hInstance, HINSTANCE, PSTR, INT iCmdShow)
{
   HWND                hWnd;
   MSG                 msg;
   WNDCLASS            wndClass;
   GdiplusStartupInput gdiplusStartupInput;
   ULONG_PTR           gdiplusToken;

   // Initialize GDI+.
   GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);

   wndClass.style          = CS_HREDRAW | CS_VREDRAW;
   wndClass.lpfnWndProc    = WndProc;
   wndClass.cbClsExtra     = 0;
   wndClass.cbWndExtra     = 0;
   wndClass.hInstance      = hInstance;
   wndClass.hIcon          = LoadIcon(NULL, IDI_APPLICATION);
   wndClass.hCursor        = LoadCursor(NULL, IDC_ARROW);
   wndClass.hbrBackground  = (HBRUSH)GetStockObject(WHITE_BRUSH);
   wndClass.lpszMenuName   = NULL;
   wndClass.lpszClassName  = TEXT("GettingStarted");

   RegisterClass(&wndClass);

   hWnd = CreateWindow(
      TEXT("GettingStarted"),   // window class name
      TEXT("Getting Started"),  // window caption
      WS_OVERLAPPEDWINDOW,      // window style
      CW_USEDEFAULT,            // initial x position
      CW_USEDEFAULT,            // initial y position
      CW_USEDEFAULT,            // initial x size
      CW_USEDEFAULT,            // initial y size
      NULL,                     // parent window handle
      NULL,                     // window menu handle
      hInstance,                // program instance handle
      NULL);                    // creation parameters

   ShowWindow(hWnd, iCmdShow);
   UpdateWindow(hWnd);

   while(GetMessage(&msg, NULL, 0, 0))
   {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
   }

   GdiplusShutdown(gdiplusToken);
   return msg.wParam;
}  // WinMain

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, 
   WPARAM wParam, LPARAM lParam)
{
   HDC          hdc;   
   PAINTSTRUCT  ps;
   Image *image = NULL;
   switch(message)
   {
   case WM_CREATE:
       //create image
     MessageBox(NULL, L"small.png", L"File Path", MB_OK);
       image = new Image(L"small.png");
       if (image)
       return 0;
       else
       return -1;
   case WM_PAINT:
      hdc = BeginPaint(hWnd, &ps);
      OnPaint(hdc,image);
      EndPaint(hWnd, &ps);
      return 0;
   case WM_DESTROY:
      PostQuitMessage(0);
      return 0;
   default:
      return DefWindowProc(hWnd, message, wParam, lParam);
   }
} // WndProc

Answer 1

Hans Passant指出了这个错误 - image中的WndProc参数重置为NULL，graphics.DrawImage(image,0,0)无法显示任何内容。

gdi plus图片未显示

1 个答案: