Plugin System – an alternative to GetProcAddress and interfaces

// Host.cpp - a plugin-based image viewer
// In this version the host manually binds to all plugins through their GetParser function
//
// Part of the Plugin System tutorial
//
///////////////////////////////////////////////////////////////////////////////

#include <windows.h>
#include <Shlwapi.h>
#include <stdio.h>
#include "ImageParser.h"

// a prototype for the GetParser function
typedef IImageParser *(*TGetParser)( void );

// the currently displayed bitmap
static HBITMAP g_Bitmap=NULL;

// a global list with all parsers
const int MAX_PARSERS=100;
IImageParser *g_Parsers[MAX_PARSERS];
int g_NumParsers=0;

// Adds the parser to the g_Parsers list
void AddParser( IImageParser *parser )
{
	g_Parsers[g_NumParsers++]=parser;
}

// Loads all plugins in the same folder as the EXE
void LoadPlugins( void )
{
	char path[_MAX_PATH];
	GetModuleFileName(NULL,path,_MAX_PATH);
	*PathFindFileName(path)=0;
	char spec[_MAX_PATH];
	sprintf(spec,"%s*.imp",path);
	WIN32_FIND_DATA data;
	// find all DLLs
	HANDLE h=FindFirstFile(spec,&data);
	if (h!=INVALID_HANDLE_VALUE) {
		do {
			char fname[_MAX_PATH];
			sprintf(fname,"%s%s",path,data.cFileName);
			HMODULE hModule=LoadLibrary(fname);
			if (hModule) {
				// get the parser pointer and add it to the parsers list
				TGetParser getparser=(TGetParser)GetProcAddress(hModule,"GetParser");
				if (getparser)
					AddParser(getparser());
			}
		} while (FindNextFile(h,&data));
		FindClose(h);
	}
}

// Loads an image file
void LoadImage( const char *fname )
{
	// find the parser for that type and load the image
	char ext[_MAX_EXT];
	_splitpath(fname,NULL,NULL,NULL,ext);
	for (int i=0;i<g_NumParsers;i++)
		if (g_Parsers[i]->SupportsType(ext)) {
			g_Bitmap=g_Parsers[i]->ParseFile(fname);
			break;
		}
}

LRESULT CALLBACK WindowProc( HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam )
{
	if (uMsg==WM_DESTROY)
		PostQuitMessage(0);

	if (uMsg==WM_PAINT) {
		// paints the window
		// if a bitmap is loaded - draw the bitmap
		// otherwise draw a text prompt
		PAINTSTRUCT ps;
		HDC hdc=BeginPaint(hwnd,&ps);
		if (g_Bitmap) {
			HDC hsrc=CreateCompatibleDC(hdc);
			SelectObject(hsrc,g_Bitmap);
			BITMAP bmp;
			GetObject(g_Bitmap,sizeof(bmp),&bmp);
			BitBlt(hdc,0,0,bmp.bmWidth,bmp.bmHeight,hsrc,0,0,SRCCOPY);
			DeleteDC(hsrc);
		}
		else {
			SelectObject(hdc,GetStockObject(DEFAULT_GUI_FONT));
			SetBkMode(hdc,TRANSPARENT);
			SetTextColor(hdc,0xFFFFFF);
			const char *s="Drop an image file here";
			TextOut(hdc,0,0,s,strlen(s));
		}
		EndPaint(hwnd,&ps);
		return 0;
	}

	if (uMsg==WM_DROPFILES) {
		// destroy the previous image
		if (g_Bitmap) DeleteObject(g_Bitmap);
		g_Bitmap=NULL;

		// get the file name
		HDROP hDrop=(HDROP)wParam;
		char fname[_MAX_PATH];
		DragQueryFile(hDrop,0,fname,_MAX_PATH);
		DragFinish(hDrop);

		// load the image file
		LoadImage(fname);
		InvalidateRect(hwnd,NULL,TRUE);
		return 0;
	}

	return DefWindowProc(hwnd,uMsg,wParam,lParam);
}

int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow )
{
	// Create main window
	WNDCLASS wc;
	memset(&wc,0,sizeof(wc));
	wc.style=CS_VREDRAW|CS_HREDRAW;
	wc.lpfnWndProc=WindowProc;
	wc.hInstance=hInstance;
	wc.hCursor=LoadCursor(NULL,IDC_CROSS);
	wc.hbrBackground=(HBRUSH)GetStockObject(BLACK_BRUSH);
	wc.lpszClassName="ImageViewer";
	RegisterClass(&wc);
	HWND main=CreateWindowEx(WS_EX_APPWINDOW,"ImageViewer","ImageViewer",WS_OVERLAPPEDWINDOW|WS_VISIBLE|WS_CLIPCHILDREN,
		CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,NULL,NULL,hInstance,NULL);
	DragAcceptFiles(main,TRUE);
	LoadPlugins();

	// Process messages
	while (1) {
		MSG msg;
		if (!GetMessage(&msg,0,0,0)) break;
		TranslateMessage(&msg);
		DispatchMessage(&msg);
	}
	return 0;
}