多点触摸的操作与图片放大缩小

import android.app.Activity;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.widget.FrameLayout;
import android.widget.FrameLayout.LayoutParams;
import android.widget.ImageView;

public class MainActivity extends Activity {
    
    private ImageView iv;
    private FrameLayout root;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        root = (FrameLayout) findViewById(R.id.container);
        iv = (ImageView) findViewById(R.id.iv);

        root.setOnTouchListener(new View.OnTouchListener() {

            float currentDistance;
            float lastDistance = -1; // 记录最有一次所记录的值

            @Override
            public boolean onTouch(View v, MotionEvent event) {

                switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN: // 触摸按下
                    System.out.println("action down");
                    break;
                case MotionEvent.ACTION_MOVE: // 触摸移动
                    // System.out.println("action move");
                    System.out.println("触摸点个数：" + event.getPointerCount());

                    if (event.getPointerCount() >= 2) { // 判断大于等于两个触摸点

                        float offsetX = event.getX(0) - event.getX(1);
                        float offsetY = event.getY(0) - event.getY(1);
                        // 计算出两个点之间的距离
                        currentDistance = (float) Math.sqrt(offsetX * offsetX
                                + offsetY * offsetY);

                        if (lastDistance < 0) { // 初始值
                            lastDistance = currentDistance;
                        } else {
                            // 当前距离比上次距离>5个像素，可以识别为放大。
                            if (currentDistance - lastDistance > 5) { 
                                System.out.println("放大");

                                FrameLayout.LayoutParams lp = (LayoutParams) iv
                                        .getLayoutParams();
                                lp.width = (int) (1.1f * iv.getWidth());
                                lp.height = (int) (1.1f * iv.getHeight());

                                iv.setLayoutParams(lp);

                                lastDistance = currentDistance;
                            } else if (lastDistance - currentDistance > 5) { //
                                System.out.println("缩小");

                                FrameLayout.LayoutParams lp = (LayoutParams) iv
                                        .getLayoutParams();
                                lp.width = (int) (0.9f * iv.getWidth());
                                lp.height = (int) (0.9f * iv.getHeight());

                                iv.setLayoutParams(lp);

                                lastDistance = currentDistance;
                            }
                        }
                    }
                    break;
                case MotionEvent.ACTION_UP: // 触摸弹起
                    // System.out.println("action up");
                    break;
                }

                return true;
            }
        });
    }

}

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.jikexueyuan.multouch.MainActivity"
    tools:ignore="MergeRootFrame" >

    <ImageView
        android:id="@+id/iv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_launcher" />

</FrameLayout>