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GDI Topics: Using a Trajectory

Introduction

A trajectory is a path along which an object travels. In one example, an object could go from one point to another without coming back. In another example, an object could go back and forth from one object to another. The study or implementation of trajectories is common in graphics applications, in games, or other simulation types.

Practical Learning: Introducing Trajectories

To start this exercise, create an MFC Application named Trajectory1
Create it as a Single Document type

To fill the window's content with a black background, access the source file of the view and change its OnDraw event as follows:

void CTrajectory1View::OnDraw(CDC* pDC)
{
	CTrajectory1Doc* pDoc = GetDocument();
	ASSERT_VALID(pDoc);
	if (!pDoc)
		return;

	// TODO: add draw code for native data here
	CBrush brsBlack;
	CRect rctClient;

	brsBlack.CreateSolidBrush(RGB(0, 0, 0));

	GetWindowRect(&rctClient);

	CBrush *pOldBrush = pDC->SelectObject(&brsBlack);
	pDC->Rectangle(0, 0, rctClient.Width(), rctClient.Height());

	pDC->SelectObject(pOldBrush);
}

Execute the application to see the result

Moving a Shape

The primary implementation of using a trajectory consist of moving an object or a shape along a path. To do this, you can draw the shape in response to a timer.

Practical Learning: Moving a Shape

Using the Resource Symbols dialog box, create an ID named IDT_MOVE

Generate the OnInitialUpdate event for the view class and use it to initialize the timer as follows:

void CTrajectory1View::OnInitialUpdate()
{
	CView::OnInitialUpdate();

	SetTimer(IDT_MOVE, 10, NULL);
	// TODO: Add your specialized code here and/or call the base class
}

In the view header file, declare the following variables:

// Trajectory1View.h : interface of the CTrajectory1View class
//


#pragma once


class CTrajectory1View : public CView
{
protected: // create from serialization only
	CTrajectory1View();
	DECLARE_DYNCREATE(CTrajectory1View)

// Attributes
public:
	CTrajectory1Doc* GetDocument() const;

// Operations
public:
	static const int ShapeWidth  = 45;
	static const int ShapeHeight = 15;
	int x;
	int y;
	bool MovingRight;

// Overrides
public:
	virtual void OnDraw(CDC* pDC);  // overridden to draw this view
	virtual BOOL PreCreateWindow(CREATESTRUCT& cs);
protected:
	virtual BOOL OnPreparePrinting(CPrintInfo* pInfo);
	virtual void OnBeginPrinting(CDC* pDC, CPrintInfo* pInfo);
	virtual void OnEndPrinting(CDC* pDC, CPrintInfo* pInfo);

// Implementation
public:
	virtual ~CTrajectory1View();
#ifdef _DEBUG
	virtual void AssertValid() const;
	virtual void Dump(CDumpContext& dc) const;
#endif

protected:

// Generated message map functions
protected:
	DECLARE_MESSAGE_MAP()
};

#ifndef _DEBUG  // debug version in Trajectory1View.cpp
inline CTrajectory1Doc* CTrajectory1View::GetDocument() const
   { return reinterpret_cast<CTrajectory1Doc*>(m_pDocument); }
#endif

In the constructor of the view class, initialize the variables as follows:

CTrajectory1View::CTrajectory1View()
{
	// TODO: add construction code here
	x = 0;
	y = 200;
	MovingRight = false;
}

Change the OnDraw event as follows:

void CTrajectory1View::OnDraw(CDC* pDC)
{
	CTrajectory1Doc* pDoc = GetDocument();
	ASSERT_VALID(pDoc);
	if (!pDoc)
		return;

	// TODO: add draw code for native data here
	CPen   penYellow;
	CBrush brsBlack, brsFuchsia;
	CRect rctClient;

	brsBlack.CreateSolidBrush(RGB(0, 0, 0));
	brsFuchsia.CreateSolidBrush(RGB(255, 0, 255));
	penYellow.CreatePen(PS_SOLID, 1, RGB(255, 255, 0));

	GetWindowRect(&rctClient);

	CBrush *pOldBrush = pDC->SelectObject(&brsBlack);
	pDC->Rectangle(0, 0, rctClient.Width(), rctClient.Height());

	pOldBrush = pDC->SelectObject(&brsFuchsia);
	CPen *pOldPen = pDC->SelectObject(&penYellow);
	pDC->RoundRect(this->x, this->y, this->x + ShapeWidth,
		       this->y + ShapeHeight, 5, 5);

	pDC->SelectObject(pOldPen);
	pDC->SelectObject(pOldBrush);
}

Generate the event of the WM_TIMER message for the view class and implement it as follows:

void CTrajectory1View::OnTimer(UINT_PTR nIDEvent)
{
	// TODO: Add your message handler code here and/or call default
	CRect rctClient;

	GetWindowRect(&rctClient);

	if( this->x < 0 )
	{
		this->x = 0;
		this->MovingRight = true;
	}

	if( this->x > rctClient.Width() - ShapeWidth )
	{
		this->x = rctClient.Width() - ShapeWidth;
		this->MovingRight = FALSE;
	}

	if( x < 0 )
	{
		x = 0;
		MovingRight = TRUE;
	}

	if( x > rctClient.Width() - ShapeWidth )
	{
		x = rctClient.Width() - ShapeWidth;
		MovingRight = FALSE;
	}

	Invalidate();

	if( MovingRight == TRUE )
		x++;
	else
		x--;

	CView::OnTimer(nIDEvent);
}

Generate the event of the WM_ERASEBKGND message of the view class and change its return value as follows:

BOOL CTrajectory1View::OnEraseBkgnd(CDC* pDC)
{
	// TODO: Add your message handler code here and/or call default

	return TRUE; // CView::OnEraseBkgnd(pDC);
}

Execute the application to see the result:

Close it and return to MSVC

Creating a Shape Class

Object oriented programming consists of creating objects that each can perform a specific tucancode.net and take of assignments related to it. In trajectory-related programming, you can create a class for each object or shape. This can make it easy to recognize and use the object. If you happen to use different objects, you can then customize the behavior of each.

Practical Learning: Creating a Shape Class

To create a new class, on the main menu, click Project -> Add Class...
In the Templates list, click C++ Class and click Add
Set the Class Name to CShape and press Enter

Change the header file as follows:

#pragma once

class CShape
{
public:
	CShape(void);
public:
	~CShape(void);

	static const int Width  = 45;
	static const int Height = 15;
	int x;
	int y;
	bool MovingRight;

	void Draw(CDC* pDC);
	void Move(CRect rctClient);
};

Change the source file as follows:

#include "StdAfx.h"
#include "Shape.h"

CShape::CShape(void)
	: x(0),
	  y(150),
	  MovingRight(false)
{
}

CShape::~CShape(void)
{
}

void CShape::Draw(CDC* pDC)
{
	CPen   penYellow;
	CBrush brsFuchsia;

	brsFuchsia.CreateSolidBrush(RGB(255, 0, 255));
	penYellow.CreatePen(PS_SOLID, 1, RGB(255, 255, 0));

	CBrush *pOldBrush = pDC->SelectObject(&brsFuchsia);
	CPen *pOldPen = pDC->SelectObject(&penYellow);
	pDC->RoundRect(this->x,
		       this->y, 
		       this->x + CShape::Width,
		       this->y + CShape::Height,
		       5,
		       5);

	pDC->SelectObject(pOldBrush);
}

void CShape::Move(CRect rctClient)
{
	if( this->x < 0 )
	{
		this->x = 0;
		this->MovingRight = true;
	}

	if( this->x > rctClient.Width() - CShape::Width )
	{
		this->x = rctClient.Width() - CShape::Width;
		this->MovingRight = FALSE;
	}
}

Access the header file of the view class and change it as follows:

// Trajectory2View.h : interface of the CTrajectory1View class
//
#pragma once

#include "Shape.h"

class CTrajectory1View : public CView
{
protected: // create from serialization only
	CTrajectory1View();
	DECLARE_DYNCREATE(CTrajectory1View)

// Attributes
public:
	CTrajectory1Doc* GetDocument() const;

// Operations
public:
	CShape *shape;

// Overrides
public:
	. . .
};

#ifndef _DEBUG  // debug version in Trajectory2View.cpp
inline CTrajectory1Doc* CTrajectory1View::GetDocument() const
   { return reinterpret_cast<CTrajectory1Doc*>(m_pDocument); }
#endif

Access the source file of the view class and change it as follows:

// Trajectory2View.cpp : implementation of the CTrajectory1View class
//

#include "stdafx.h"
#include "Trajectory2.h"

#include "Trajectory2Doc.h"
#include "Trajectory2View.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#endif

// CTrajectory1View

IMPLEMENT_DYNCREATE(CTrajectory1View, CView)

BEGIN_MESSAGE_MAP(CTrajectory1View, CView)
	// Standard printing commands
	ON_COMMAND(ID_FILE_PRINT, &CView::OnFilePrint)
	ON_COMMAND(ID_FILE_PRINT_DIRECT, &CView::OnFilePrint)
	ON_COMMAND(ID_FILE_PRINT_PREVIEW, &CView::OnFilePrintPreview)
	ON_WM_TIMER()
	ON_WM_ERASEBKGND()
END_MESSAGE_MAP()

// CTrajectory1View construction/destruction

CTrajectory1View::CTrajectory1View()
{
	// TODO: add construction code here
	shape = new CShape;
}

CTrajectory1View::~CTrajectory1View()
{
	delete shape;
}

BOOL CTrajectory1View::PreCreateWindow(CREATESTRUCT& cs)
{
	// TODO: Modify the Window class or styles here by modifying
	//  the CREATESTRUCT cs

	return CView::PreCreateWindow(cs);
}

// CTrajectory1View drawing

void CTrajectory1View::OnDraw(CDC* pDC)
{
	CTrajectory1Doc* pDoc = GetDocument();
	ASSERT_VALID(pDoc);
	if (!pDoc)
		return;

	// TODO: add draw code for native data here
	CPen   penYellow;
	CBrush brsBlack, brsFuchsia;
	CRect rctClient;

	brsBlack.CreateSolidBrush(RGB(0, 0, 0));
	brsFuchsia.CreateSolidBrush(RGB(255, 0, 255));
	penYellow.CreatePen(PS_SOLID, 1, RGB(255, 255, 0));

	GetWindowRect(&rctClient);

	CBrush *pOldBrush = pDC->SelectObject(&brsBlack);
	pDC->Rectangle(0, 0, rctClient.Width(), rctClient.Height());

	shape->Draw(pDC);

	pDC->SelectObject(pOldBrush);
}

. . .

// CTrajectory1View message handlers

void CTrajectory1View::OnTimer(UINT_PTR nIDEvent)
{
	// TODO: Add your message handler code here and/or call default
	CRect rctClient;

	GetWindowRect(&rctClient);
	shape->Move(rctClient);

	Invalidate();

	if( shape->MovingRight == TRUE )
		shape->x++;
	else
		shape->x--;

	CView::OnTimer(nIDEvent);
}

void CTrajectory1View::OnInitialUpdate()
{
	CView::OnInitialUpdate();

	SetTimer(IDT_MOVE, 10, NULL);
	// TODO: Add your specialized code here and/or call the base class
}

BOOL CTrajectory1View::OnEraseBkgnd(CDC* pDC)
{
	// TODO: Add your message handler code here and/or call default

	return TRUE; // CView::OnEraseBkgnd(pDC);
}

Execute the application and notice that it produces the same result

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