Android开发：拖拽

google官网的training和API两个地方都提到了拖拽的实现，两种方法不太一样。

方法一

training（https://developer.android.com/training/gestures/scale.html）中提到的方法是监听onTouchEvent，在ACTION_DOWN的时候记录位置，在ACTION_MOVE的时候获取坐标，改变拖拽的控件位置。

方法二

在android3.0及以上可以使用View.OnDragListener。

下面是我写的简单的demo:

activity_drag_demo.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/activity_drag_demo"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.topsports.testapplication.DragDemoActivity">
<ImageView
    android:id="@+id/imageView"
    android:layout_width="100dp"
    android:layout_height="100dp"
    android:background="@drawable/c1"/>

    <FrameLayout
        android:background="#c7f6ff"
        android:layout_width="500dp"
        android:layout_height="500dp"
        android:id="@+id/content_fragment"
        android:layout_alignParentBottom="true"></FrameLayout>

    <FrameLayout
        android:background="#dce9c1"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:id="@+id/content_fragment2"
        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"></FrameLayout>


</RelativeLayout>

在官网示例基础上修改的DragDemoActivity:package com.topsports.testapplication;

import android.content.ClipData;
import android.content.ClipDescription;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Point;
import android.graphics.drawable.ColorDrawable;
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.View;
import android.view.ViewGroup;
import android.widget.FrameLayout;
import android.widget.ImageView;
import android.widget.Toast;

public class DragDemoActivity extends AppCompatActivity {

    private ImageView imageView;

    private FrameLayout frameLayout;
    private FrameLayout frameLayout2;

    private static final String IMAGEVIEW_TAG = "icon bitmap";

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

        imageView=(ImageView)findViewById(R.id.imageView);
        frameLayout=(FrameLayout)findViewById(R.id.content_fragment);
        frameLayout2=(FrameLayout)findViewById(R.id.content_fragment2);

        imageView.setTag(IMAGEVIEW_TAG);

        imageView.setOnLongClickListener(new View.OnLongClickListener() {

             public boolean onLongClick(View v) {

                 v.getBackground().setAlpha(100);

                 ClipData.Item item = new ClipData.Item(v.getTag().toString());


                 ClipData dragData = new ClipData(v.getTag().toString(),new String[]{ClipDescription.MIMETYPE_TEXT_PLAIN},item);

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

             }
        });

        frameLayout.setOnDragListener(new myDragEventListener());
        frameLayout2.setOnDragListener(new myDragEventListener());

        imageView.setOnDragListener(new myDragEventListener());

    }protected class myDragEventListener implements View.OnDragListener {

        // This is the method that the system calls when it dispatches a drag event to the
        // listener.
        public boolean onDrag(View v, DragEvent event) {

            // Defines a variable to store the action type for the incoming event
            final int action = event.getAction();

            // Handles each of the expected events
            switch(action) {

                case DragEvent.ACTION_DRAG_STARTED:

                    // Determines if this View can accept the dragged data
                    if (event.getClipDescription().hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) {

                        v.invalidate();

                        // returns true to indicate that the View can accept the dragged data.
                        return true;

                    }

                    // Returns false. During the current drag and drop operation, this View will
                    // not receive events again until ACTION_DRAG_ENDED is sent.
                    return false;

                case DragEvent.ACTION_DRAG_ENTERED:

                    // Applies a green tint to the View. Return true; the return value is ignored

                    // Invalidate the view to force a redraw in the new tint
                    v.invalidate();

                    return true;

                case DragEvent.ACTION_DRAG_LOCATION:

                    // Ignore the event
                    return true;

                case DragEvent.ACTION_DRAG_EXITED:

                    // Re-sets the color tint to blue. Returns true; the return value is ignored.

                    // Invalidate the view to force a redraw in the new tint
                    v.invalidate();

                    return true;

                case DragEvent.ACTION_DROP:

                    // Gets the item containing the dragged data
                    ClipData.Item item = event.getClipData().getItemAt(0);

                    // Gets the text data from the item.
                    String dragData = item.getText().toString();

                    // Displays a message containing the dragged data.
                    Toast.makeText(DragDemoActivity.this, "Dragged data is " + dragData, Toast.LENGTH_LONG).show();

                    // Turns off any color tints
                    if(v instanceof FrameLayout){
                        ((ViewGroup)imageView.getParent()).removeView(imageView);
                        ((FrameLayout)v).addView(imageView);
                    }
                    // Invalidates the view to force a redraw
                    v.invalidate();

                    // Returns true. DragEvent.getResult() will return true.
                    return true;

                case DragEvent.ACTION_DRAG_ENDED:

                    // Turns off any color tinting

                    // Invalidates the view to force a redraw
                    v.invalidate();

                    // Does a getResult(), and displays what happened.
                    if (event.getResult()) {
                        Toast.makeText(DragDemoActivity.this, "The drop was handled.", Toast.LENGTH_LONG).show();

                    } else {
                        Toast.makeText(DragDemoActivity.this, "The drop didn't work.", Toast.LENGTH_LONG).show();

                    }
                    imageView.getBackground().setAlpha(255);
                    // returns true; the value is ignored.
                    return true;

                // An unknown action type was received.
                default:
                    Log.e("DragDrop Example","Unknown action type received by OnDragListener.");
                    break;
            }

            return false;
        }
    }
}

实现的是将图片拖进两个layout中，并在两个layout间来回拖动。

代码做简单的解释

在长按imageview的时候开始拖拽事件

ClipData.Item item = new ClipData.Item(v.getTag().toString());

ClipData dragData = new ClipData(v.getTag().toString(),new String[]{ClipDescription.MIMETYPE_TEXT_PLAIN},item);

这两句是为拖动的view创建传递的数据(可以省略)

View.DragShadowBuilder myShadow = new View.DragShadowBuilder(v)

这句根据imageview(v)创建一个拖拽的阴影，就像在桌面上拖动应用图标一样，产生一个view的图像副本，产生拖动效果。

frameLayout.setOnDragListener(new myDragEventListener());

frameLayout2.setOnDragListener(new myDragEventListener());

imageView.setOnDragListener(new myDragEventListener());

设置拖拽的事件监听（拖动的对象和被放置的对象）实现View.OnDragListener的onDrag方法，判断event.getAction()的值进行不同的操作。