Drag and Drop Details and Implementation in Android

Android drag and drop framework allows users to move data from one View to another View in the current layout using a graphical drag and drop gesture. It includes following three important components to support drag & drop functionality:

1.) Drag event class

2.) Drag listeners

3.) Helper methods and classes

Drag and Drop Process
There are four basic steps in the drag and drop process:

Started: This event occurs when you start dragging an item in a layout, your application calls startDrag() method to tell the system to start a drag. The system first responds by calling back to your application to get a drag shadow. It then displays the drag shadow on the device. Next, the system sends a drag event with action type ACTION_DRAG_STARTED to the registered drag event listeners for all the View objects in the current layout.

To continue to receive drag events, including a possible drop event, a drag event listener must return true, If the drag event listener returns false, then it will not receive drag events for the current operation until the system sends a drag event with action type ACTION_DRAG_ENDED.

Continuing: The user continues the drag. System sends ACTION_DRAG_ENTERED action followed by ACTION_DRAG_LOCATION action to the registered drag event listener for the View where dragging point enters. The listener may choose to alter its View object’s appearance in response to the event or can react by highlighting its View.

Dropped: The user releases the dragged item within the bounding box of a View. The system sends the View object’s listener a drag event with action type ACTION_DROP.

Ended: Just after the action type ACTION_DROP, the system sends out a drag event with action type ACTION_DRAG_ENDED to indicate that the drag operation is over.

DragEvent Class

The DragEvent represents an event that is sent out by the system at various times during a drag and drop operation. This class provides few Constants and important methods which we use during the drag and drop process.

Listener

If you want your view should respond Drag event then your view either implements View.OnDragListener or setup onDragEvent(DragEvent) callback method. When the system calls the method or listener, it passes to them a DragEvent object explained above. You can have both a listener and a callback method for View object. If this occurs, the system first calls the listener and then defined callback as long as listener returns true.

Helper methods

You start with creating a ClipData and ClipData.Item for the data being moved. As part of the ClipData object, supply metadata that is stored in a ClipDescription object within the ClipData. For a drag and drop operation that does not represent data movement, you may want to use null instead of an actual object.

Next either you can extend extend View.DragShadowBuilder to create a drag shadow for dragging the view or simply you can use View.DragShadowBuilder(View) to create a default drag shadow that’s the same size as the View argument passed to it, with the touch point centered in the drag shadow.

Example

Steps:

1.) Create a new project by File-> New -> Android Project name it DragAndDrop.

2.) Write following into main.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/container"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >

<ImageView
android:id="@+id/iv_logo"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/thumb"
android:contentDescription="Click to drag and drop" />

</RelativeLayout>

3.) Open DragAndDropActivity.java file and write following code there:

package com.example.draganddrop;

import android.os.Bundle;
import android.app.Activity;
import android.content.ClipData;
import android.content.ClipDescription;
import android.util.Log;
import android.view.DragEvent;
import android.view.View;
import android.view.View.DragShadowBuilder;
import android.view.View.OnDragListener;
import android.widget.*;

public class DragAndDropActivity extends Activity{
ImageView ima;
private static final String IMAGEVIEW_TAG = "Android Logo";
String msg;

private android.widget.RelativeLayout.LayoutParams layoutParams;

@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

ima = (ImageView)findViewById(R.id.iv_logo);
// Sets the tag
ima.setTag(IMAGEVIEW_TAG);

ima.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);

// Instantiates the drag shadow builder.
View.DragShadowBuilder myShadow = new DragShadowBuilder(ima);

// Starts the drag
v.startDrag(dragData, // the data to be dragged
myShadow, // the drag shadow builder
null, // no need to use local data
0 // flags (not currently used, set to 0)
);
return true;
}
});

// Create and set the drag event listener for the View
ima.setOnDragListener( new 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;
}
});
}
}

When you run this application it will display following Emulator window:

Now do long click on the displayed android logo and you will see that logo image moves a little after 1 seconds long click from its place, its the time when you should start dragging the image. You can drag it around the screen and drop it at a new location.