View.OnTouchListener，MotionEvent.ACTION_POINTER_DOWN、MotionEvent.ACTION_MOVE

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package com.tongzhen.magnify_test;

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

public class MainActivity extends Activity implements View.OnTouchListener {

    private ImageView img;
    // 縮放控制
    private Matrix savedMatrix = new Matrix(); // 保存按下时的状态
    private Matrix matrix = new Matrix();      // 每次和按下的状态进行比较

    // 不同状态的表示：
    private static final int NONE = 0;
    private static final int DRAG = 1;
    private static final int ZOOM = 2;
    private int mode = NONE;

    // 定义第一个按下的点，两只接触点的中点，以及两指按下的距离：
    private PointF startPoint = new PointF();
    private PointF midPoint = new PointF();
    private float oriDis = 1f;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        img = (ImageView) this.findViewById(R.id.main_imgZooming);
        img.setOnTouchListener(this);
    }

    // 计算两个触摸点之间的距离
    private float distance(MotionEvent event) {
        float x = event.getX(0) - event.getX(1);
        float y = event.getY(0) - event.getY(1);
        return Float.valueOf(String.valueOf(Math.sqrt(x * x + y * y))) ;
    }

    // 计算两个触摸点的中点
    private PointF middle(MotionEvent event) {
        float x = event.getX(0) + event.getX(1);
        float y = event.getY(0) + event.getY(1);
        return new PointF(x / 2, y / 2);
    }
    
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        if (v == img) {
            ImageView view = (ImageView) v;
            switch (event.getAction() & MotionEvent.ACTION_MASK) {
                // 单指按下
                case MotionEvent.ACTION_DOWN:
                    matrix.set(view.getImageMatrix());
                    savedMatrix.set(matrix);
                    startPoint.set(event.getX(), event.getY());
                    mode = DRAG;
                    break;
                // 双指按下
                case MotionEvent.ACTION_POINTER_DOWN:
                    oriDis = distance(event);
                    if (oriDis > 10f) {
                        savedMatrix.set(matrix);
                        midPoint = middle(event);
                        mode = ZOOM;
                    }
                    break;
                // 手指放开
                case MotionEvent.ACTION_UP:
                case MotionEvent.ACTION_POINTER_UP:
                    mode = NONE;
                    break;
                // 单指滑动事件
                case MotionEvent.ACTION_MOVE:
                    if (mode == DRAG) {
                        // 是一个手指拖动
                        matrix.set(savedMatrix);
                        matrix.postTranslate(event.getX() - startPoint.x, event.getY() - startPoint.y);
                    } else if (mode == ZOOM) {
                        // 两个手指滑动
                        float newDist = distance(event);
                        if (newDist > 10f) {
                            matrix.set(savedMatrix);
                            float scale = newDist / oriDis; // 压缩比例
                            matrix.postScale(scale, scale, midPoint.x, midPoint.y);
                        }
                    }
                    break;
                default :
                    break;
            }
            // 设置ImageView的Matrix
            view.setImageMatrix(matrix);
        }

        return true;
    }
}