缩放使用了Matrix, 详见 http://baiwanlu.com/?p=653

package mutl.touch.sample;

import android.app.Activity;
import android.graphics.Matrix;
import android.graphics.PointF;
import android.os.Bundle;
import android.util.FloatMath;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.ImageView;

public class Main extends Activity {
	private static final int NONE = 0;
	private static final int DRAG = 1;
	private static final int ZOOM = 2;
	
	private int mode = NONE;
	private float oldDist;
	private Matrix matrix = new Matrix();
	private Matrix savedMatrix = new Matrix();
	private PointF start = new PointF();
	private PointF mid = new PointF();

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		ImageView view = (ImageView) findViewById(R.id.image_view);
		view.setOnTouchListener(new OnTouchListener() {
			@Override
			public boolean onTouch(View v, MotionEvent event) {
				ImageView view = (ImageView) v;
				switch (event.getAction() & MotionEvent.ACTION_MASK) {
				case MotionEvent.ACTION_DOWN:
					savedMatrix.set(matrix);
					//设置起点
					start.set(event.getX(), event.getY());
					mode = DRAG;
					break;
				case MotionEvent.ACTION_UP:
				case MotionEvent.ACTION_POINTER_UP:
					mode = NONE;
					break;
				case MotionEvent.ACTION_POINTER_DOWN:
					oldDist = spacing(event);
					//当有两个触点距离大于10f 判断为缩放
					if (oldDist > 10f) {
						savedMatrix.set(matrix);
						midPoint(mid, event);
						mode = ZOOM;
					}
					break;
				case MotionEvent.ACTION_MOVE:
					if (mode == DRAG) {
						matrix.set(savedMatrix);
						//平移
						matrix.postTranslate(event.getX() - start.x, event.getY()
								- start.y);
					} else if (mode == ZOOM) {
						float newDist = spacing(event);
						if (newDist > 10f) {
							matrix.set(savedMatrix);
							float scale = newDist / oldDist;
							//以两个触点的中间点为缩放中心
							matrix.postScale(scale, scale, mid.x, mid.y);
						}
					}
					break;
				}

				view.setImageMatrix(matrix);
				return true;
			}
			
			//取得两个触点的距离
			private float spacing(MotionEvent event) {
				float x = event.getX(0) - event.getX(1);
				float y = event.getY(0) - event.getY(1);
				return FloatMath.sqrt(x * x + y * y);
			}

			//取得两个触点的中间点
			private void midPoint(PointF point, MotionEvent event) {
				float x = event.getX(0) + event.getX(1);
				float y = event.getY(0) + event.getY(1);
				point.set(x / 2, y / 2);
			}});
	}
}

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