自由落體方程式

前文章"在畫布(Canvas)上繪畫圖形"以Math.sin()作為例子示範如何在畫布(Canvas)上繪畫圖形. 本文修改一點點, 在畫布上繪畫自由落體方程式(參考: Wikipedia - Equations for a falling body).



本實例中, 我們使用公式 d = (gt^2)/2 計算一個自由落體在某一瞬間的下降距離.

修改"在畫布(Canvas)上繪畫圖形"中的MyCanvas.java

package com.AndroidCanvasDraw;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

public class MyCanvas extends View {
 
 private Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
 int canvasVerticalCenter;
 
 int initX, initY;
 float initV;
 final float constG = 9.8f;

 public MyCanvas(Context context) {
  super(context);
  // TODO Auto-generated constructor stub
  init();
 }

 public MyCanvas(Context context, AttributeSet attrs) {
  super(context, attrs);
  // TODO Auto-generated constructor stub
  init();
 }

 public MyCanvas(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  // TODO Auto-generated constructor stub
  init();
 }
 
 private void init(){
  paint.setStyle(Paint.Style.FILL);
  paint.setStrokeWidth(3);
  paint.setColor(Color.WHITE); 
  
  initX = 0;
  initY = 300;
  initV = 0;
 }

 @Override
 protected void onDraw(Canvas canvas) {
  // TODO Auto-generated method stub
  prepareProject();
  
  for(int t = 0; t < getWidth(); t++){
   //d = g*t*t/2
   float scaledDuration = (float)t/20;
   float d = constG * scaledDuration * scaledDuration /2;
   float h = initY - d;
    
   if (h >= 0f){
    project(t, (int)h, canvas);
   }else{
    break;
   }
  }
 }
 
 void prepareProject(){
  canvasVerticalCenter = getHeight();
 }
 
 void project(int x, int y, Canvas c){
  c.drawPoint(x, canvasVerticalCenter-y, paint);
 }

 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  // TODO Auto-generated method stub
  setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec),
    MeasureSpec.getSize(heightMeasureSpec));
 }

}