一个简单的画图程序

    Android里面画图用到的一些相关的东西有：View，Canvas，Paint，Bitmap。

    一，View是画图的目的地，也是直接展现给用户的接口；

    二，Canvas是画笔（与J2ME里的Canvas不一样，跟像J2ME里的Graphics），提供了drawText, drawRect, drawPicture等各种画图方法。既然有“画笔”，就必然还有“画布”。Canvas这个单词本身有画布的意思，但是此处被赋予了“画笔”的作用，那么Android里的“画布”究竟是什么呢？答案是：Bitmap。Bitmap直译是“位图”的意思，在Android中它除了作为图像，还被用作“画布”。因为图像（Image）有Immutable（不可变的）和Mutable（可变的）之分。创建自图形文件的Bitmap是imutable，只给定宽高以及其他一些参数创建的Bitmap则是mutable。画布由于其内容会随着画笔的动作而更改，所以画布都是使用mutable Bitmap。

//Imutalbe bitmap
Bitmap temp = BitmapFactory.decodeFile("/sdcard/zgr.jpg");
Bitmap immutalbe = Bitmap.createBitmap(temp);
		
//Mutable
Bitmap mutalbe = Bitmap.createBitmap(240, 320, Bitmap.Config.RGB_565);

 

    三，Paint可以理解为画笔属性的设置。比如颜色，消除锯齿等等。

    四，Bitmap在前面已经有了一些介绍，关于创建mutable Bitmap所用到的Bitmap.Config里的设定的详细介绍再讨论。

 

以下写了一个例子，功能是在触摸屏幕时在触摸处画点。

package com.cim;

import android.app.Activity;
import android.content.Context;
import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnTouchListener;

public class Test extends Activity {
	MediaPlayer player = null;
	
	OnTouchListener touchListener = new OnTouchListener(){
		public boolean onTouch(View v, MotionEvent e) {
			if(e.getAction() == MotionEvent.ACTION_DOWN){
				player.start();
				System.out.println("OnTouch!!");
			}
			
			return false;
		}
		
	};
	
	private OnClickListener clickListener = new OnClickListener(){

		@Override
		public void onClick(View v) {
			if(player != null)
				player.start();
			
			System.out.println("OnClike!!");
		}
		
	};
	
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.main);
        MyView view = new MyView(this);
        setContentView(view);
        view.requestFocus();
        
        player = MediaPlayer.create(this, R.raw.hello);
//        View button = findViewById(R.id.Button);
//        button.setOnClickListener(clickListener);
//        button.setOnTouchListener(touchListener);
    }
}

class MyView extends View{
	private Paint mPaint;
	
	private Bitmap mBitmap;
	
	private Bitmap srcBitmap;

	public MyView(Context context) {
		super(context);
		
		mPaint = new Paint();
		mPaint.setAntiAlias(true);
		mPaint.setARGB(0, 0xff, 0, 0);
		mBitmap = Bitmap.createBitmap(320, 480, Bitmap.Config.RGB_565);

	}
	
	@Override
	protected void onDraw(Canvas canvas) {
		super.onDraw(canvas);
		
		if(mBitmap != null)
			canvas.drawBitmap(mBitmap, 0, 0, null);

	}
	

	@Override
	public boolean onTouchEvent(MotionEvent event) {
		int N = event.getHistorySize();
		float x = 0;
		float y = 0;
		float press = event.getPressure();
		x = /*event.getXPrecision() * */event.getX();
		y = /*event.getYPrecision() * */event.getY();
		System.out.println(">>>VIEW: N = " + N);
		System.out.println(">>>VIEW: X = " + event.getX() + " Y = " + event.getY());
		System.out.println(">>>VIEW: Press = " + event.getPressure() + " Size = " + event.getSize());
		System.out.println(">>>VIEW: XPre = " + event.getXPrecision() + " YPre = " + event.getYPrecision());
		System.out.println(">>>VIEW: X * XPre = " + x 
				+ " Y * YPre = " + y);
		
		for(int i = 0; i < N; i++){
			System.out.println(">>>VIEW: i = " + i 
					+ " Xhis = " + event.getHistoricalX(i)
					+ " Yhis = " + event.getHistoricalY(i));
			System.out.println(">>>VIEW: Pressure: " + event.getHistoricalPressure(i)
					+ " size: " + event.getHistoricalSize(i));
		}
		
		Canvas canvas = new Canvas();
		canvas.setBitmap(mBitmap);
		//mPaint.setARGB((int) (255 * press), 0xff, 0, 0);
		mPaint.setARGB(100, 0xff, 0, 0);
		canvas.drawCircle(x, y, 4, mPaint);
		mPaint.setARGB(255, 0, 0xff, 0);
		canvas.drawCircle(x + 4, y, 4, mPaint);
		mPaint.setARGB(200, 0, 0, 0xff);
		canvas.drawCircle(x, y + 4, 4, mPaint);
		invalidate((int)(x - 4), (int)(y - 4), (int)(x + 8), (int)(y + 8));
		
		return true;
	}
	
	
}

 

需要注意：每次调用触屏事件进行画点动作时，都需要调用invaliddate(rect)方法来出发OnDraw(canvas)函数进行重画。因为OnTouchEvent()函数只是将点画到了mBitmap上，而非与用户交互的View上；要给用户显示实时的效果，必须激发OnDraw()函数才行。