拖拽问题

Question

我正在尝试在android studio 3.0.1中实现执行拖放的示例。当我拖动图像时，它会消失。我是android新手，无法弄清楚发生了什么。我想要关注的示例在此链接上提供：https://www.tutorialspoint.com/android/android_drag_and_drop.htm

这是我的MainActivity代码：

<div data-role="popup" id="positionWindow" class="ui-content popup" data-theme="b" data-overlay-theme="a">
      <a href="#" data-rel="back" data-role="button" data-theme="b" data-overlay-theme="a" data-icon="delete" data-iconpos="notext" class="ui-btn-right">Close</a>
      <p> Replace the text here depending on which button is pressed </p>
</div>

这是activity_main.xml的代码

package com.example.osman.dragdrop2;

import android.content.ClipData;
import android.content.ClipDescription;
import android.graphics.drawable.Drawable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.DragEvent;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.RelativeLayout;

public class MainActivity extends AppCompatActivity
{
    ImageView img;
    String msg;
    private android.widget.RelativeLayout.LayoutParams layoutParams;

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


        img=(ImageView)findViewById(R.id.imageView);

        img.setOnLongClickListener(new View.OnLongClickListener() {
            @Override
            public boolean onLongClick(View v) {
                ClipData.Item item = new ClipData.Item((CharSequence)v.getTag());
                String[] mimeTypes = {ClipDescription.MIMETYPE_TEXT_PLAIN};

                ClipData dragData = new ClipData(v.getTag().toString(),mimeTypes, item);
                View.DragShadowBuilder myShadow = new View.DragShadowBuilder(img);

                v.startDrag(dragData,myShadow,null,0);
                return true;
            }
        });

        img.setOnDragListener(new View.OnDragListener() {
            @Override
            public boolean onDrag(View v, DragEvent event) {
                switch(event.getAction()) {
                    case DragEvent.ACTION_DRAG_STARTED:
                        layoutParams = (RelativeLayout.LayoutParams)v.getLayoutParams();
                        Log.d(msg, "Action is DragEvent.ACTION_DRAG_STARTED");

                        // Do nothing
                        break;

                    case DragEvent.ACTION_DRAG_ENTERED:
                        Log.d(msg, "Action is DragEvent.ACTION_DRAG_ENTERED");
                        int x_cord = (int) event.getX();
                        int y_cord = (int) event.getY();
                        break;

                    case DragEvent.ACTION_DRAG_EXITED :
                        Log.d(msg, "Action is DragEvent.ACTION_DRAG_EXITED");
                        x_cord = (int) event.getX();
                        y_cord = (int) event.getY();
                        layoutParams.leftMargin = x_cord;
                        layoutParams.topMargin = y_cord;
                        v.setLayoutParams(layoutParams);
                        break;

                    case DragEvent.ACTION_DRAG_LOCATION  :
                        Log.d(msg, "Action is DragEvent.ACTION_DRAG_LOCATION");
                        x_cord = (int) event.getX();
                        y_cord = (int) event.getY();
                        break;

                    case DragEvent.ACTION_DRAG_ENDED   :
                        Log.d(msg, "Action is DragEvent.ACTION_DRAG_ENDED");

                        // Do nothing
                        break;

                    case DragEvent.ACTION_DROP:
                        Log.d(msg, "ACTION_DROP event");

                        // Do nothing
                        break;
                    default: break;
                }
                return true;
            }
        });

        img.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                if (event.getAction() == MotionEvent.ACTION_DOWN) {
                    ClipData data = ClipData.newPlainText("", "");
                    View.DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(img);

                    img.startDrag(data, shadowBuilder, img, 0);
                    img.setVisibility(View.INVISIBLE);
                    return true;
                } else {
                    return false;
                }
            }
        });
    }



    }

提前致谢。