学步园

首先是布局文件，引入图片

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <ImageView 
        android:layout_width="fill_parent"
    	android:layout_height="fill_parent"
    	android:id="@+id/imageView"
    	android:src="@drawable/vege"
    	android:contentDescription="@string/app_name"
    	android:scaleType="matrix"
        />
</LinearLayout>

activity

package cn.wonders.drag;

import android.support.v7.app.ActionBarActivity;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.ImageView;
import android.graphics.Matrix;
import android.graphics.PointF;
import android.os.Bundle;

public class MainActivity extends ActionBarActivity {
	private ImageView imageView;
	
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);

		imageView = (ImageView) findViewById(R.id.imageView);
		imageView.setOnTouchListener(new TonchListener());
	}

	private final class TonchListener implements OnTouchListener {
		private PointF startPoint = new PointF();
		private Matrix matrix = new Matrix();
		//照片当前的移动位子
		private Matrix currentMatrix = new Matrix();
		//模式
		private int mode = 0;
		//拖拉
		private final static int GRAG = 1;
		//多点触摸
		private final static int ZOOM = 2;
		//开始时2根手指的距离
		private float startDis;
		//中间点
		private PointF midPoint;
		
		@Override
		public boolean onTouch(View v, MotionEvent event) {
			//取后8位
			switch (event.getAction() & MotionEvent.ACTION_MASK) {
			case MotionEvent.ACTION_DOWN:
				//第一次按下时记录坐标
				mode = GRAG;
				currentMatrix.set(imageView.getImageMatrix());
				startPoint.set(event.getX(), event.getY());
				break;
				
			case MotionEvent.ACTION_MOVE:
				if(mode == GRAG) {
					float dx = event.getX() - startPoint.x;
					float dy = event.getY() - startPoint.y;
					//实现位子的移动,在上一次的基础上移动
					matrix.set(currentMatrix);
					matrix.postTranslate(dx, dy);
				}else if (mode == ZOOM) {//缩放
					float endDis = distance(event);
					//得到缩放倍数
					if(endDis > 10f) {
						float scale = endDis/startDis;
						//参数：x和y轴的放大倍数，以及中心点
						matrix.postScale(scale, scale, midPoint.x, midPoint.y);
					}
				}
				break;
				
			case MotionEvent.ACTION_UP:
				
				break;
				//在此前已经有手指按下，屏幕上至少有2根手指
			case MotionEvent.ACTION_POINTER_DOWN:
				mode = ZOOM;
				startDis = distance(event);
				//距离大于10个像素才处理，防止有手指分叉或有茧；
				if(startDis > 10f) {
					midPoint = mid(event);
					//记录当前的放大倍数
					currentMatrix.set(imageView.getImageMatrix());
				}
				break;
			case MotionEvent.ACTION_POINTER_UP:
				mode = 0;
				break;
			default:
				break;
			} 
			imageView.setImageMatrix(matrix);
			//消费掉事件
			return true;
		}
	}
	
	//返回2个手指间的距离
	private static float distance(MotionEvent event) {
		float dx = event.getX(1) - event.getX(0);
		float dy = event.getY(1) - event.getY(0);
		return (float) Math.sqrt(dx*dx+dy*dy);
	}

	//计算中心点,为2根手指的中心点
	private static PointF mid(MotionEvent event) {
		float dx = event.getX(1) + event.getX(0);
		float dy = event.getY(1) + event.getY(0);
		return new PointF(dx/2, dy/2);
	}
}