Android中实现拖拽其实很简单，系统早已经提供了api让我使用，主要用到了View的startDrag（startDragAndDrop API24+） 方法以及OnDragListener。

startDrag

先来看下方法介绍：

/*** Starts a drag and drop operation. When your application calls this method, it passes a* {@link android.view.View.DragShadowBuilder} object to the system. The* system calls this object's {@link DragShadowBuilder#onProvideShadowMetrics(Point, Point)}* to get metrics for the drag shadow, and then calls the object's* {@link DragShadowBuilder#onDrawShadow(Canvas)} to draw the drag shadow itself.* <p>*  Once the system has the drag shadow, it begins the drag and drop operation by sending*  drag events to all the View objects in your application that are currently visible. It does*  this either by calling the View object's drag listener (an implementation of*  {@link android.view.View.OnDragListener#onDrag(View,DragEvent) onDrag()} or by calling the*  View object's {@link android.view.View#onDragEvent(DragEvent) onDragEvent()} method.*  Both are passed a {@link android.view.DragEvent} object that has a*  {@link android.view.DragEvent#getAction()} value of*  {@link android.view.DragEvent#ACTION_DRAG_STARTED}.* </p>* <p>* Your application can invoke {@link #startDragAndDrop(ClipData, DragShadowBuilder, Object,* int) startDragAndDrop()} on any attached View object. The View object does not need to be* the one used in {@link android.view.View.DragShadowBuilder}, nor does it need to be related* to the View the user selected for dragging.* </p>* @param data A {@link android.content.ClipData} object pointing to the data to be* transferred by the drag and drop operation.* @param shadowBuilder A {@link android.view.View.DragShadowBuilder} object for building the* drag shadow.* @param myLocalState An {@link java.lang.Object} containing local data about the drag and* drop operation. When dispatching drag events to views in the same activity this object* will be available through {@link android.view.DragEvent#getLocalState()}. Views in other* activities will not have access to this data ({@link android.view.DragEvent#getLocalState()}* will return null).* <p>* myLocalState is a lightweight mechanism for the sending information from the dragged View* to the target Views. For example, it can contain flags that differentiate between a* a copy operation and a move operation.* </p>* @param flags Flags that control the drag and drop operation. This can be set to 0 for no* flags, or any combination of the following:*     <ul>*         <li>{@link #DRAG_FLAG_GLOBAL}</li>*         <li>{@link #DRAG_FLAG_GLOBAL_PERSISTABLE_URI_PERMISSION}</li>*         <li>{@link #DRAG_FLAG_GLOBAL_PREFIX_URI_PERMISSION}</li>*         <li>{@link #DRAG_FLAG_GLOBAL_URI_READ}</li>*         <li>{@link #DRAG_FLAG_GLOBAL_URI_WRITE}</li>*         <li>{@link #DRAG_FLAG_OPAQUE}</li>*     </ul>* @return {@code true} if the method completes successfully, or* {@code false} if it fails anywhere. Returning {@code false} means the system was unable to* do a drag, and so no drag operation is in progress.*/public final boolean startDragAndDrop(ClipData data, DragShadowBuilder shadowBuilder,Object myLocalState, int flags)
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看到英文就头大？没事，我来翻译解释一下。

启动拖放操作。当应用程序调用此方法时，它将传递一个DragShadowBuilder对象到系统。系统调用此对象的onProvideShadowMetrics(Point, Point)方法获取拖动阴影的参数指标，然后调用onDrawShadow(Canvas)来绘制阴影。一旦系统有了拖动阴影，它就开始拖拽操作，通过将拖拽事件发送到当前可见的应用程序中的所有视图对象。这些视图可以通过设置OnDragListener在或者实现onDragEvent方法接受DragEvent（事件）来响应和拖拽事件。

可以看到有四个参数：

ClipData data

其实就是一个封装数据的对象，通过拖放操作传递给接受者。该对象可以存放一个Item的集合，Item可以存放如下数据：

public static class Item {final CharSequence mText;final String mHtmlText;final Intent mIntent;Uri mUri;
}
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注意到可以存放Intent，因此，通常可以将参数存入intent，然后通过静态方法直接创建ClipData对象：

ClipData clipData = ClipData.newIntent("label", intent);
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该数据可以在监听的中的DragEvent获取

ClipData clipData = event.getClipData();
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简单点说就是可以将一些数据传递给拖拽的接受者，该拖拽其实可以跨Activity的，如果只是同一个Activity可以使用第三个参数传递数据。

DragShadowBuilder shadowBuilder

用于创建拖拽view是的阴影，也就是跟随手指移动的视图，通常直接使用默认即可生成与一个原始view相同，带有透明度的阴影：

View.DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(v);
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Object myLocalState

当你的拖拽行为是在同一个Activity中进行时可以传递一个任意对象，在监听中可以通过{@link android.view.DragEvent#getLocalState()}获得。如果是跨Activity拖拽中无法访问此数据，getLocalState()将返回null。

int flags

控制拖放操作的标志。因为没有标志可以设置为0，flag标志拖动是否可以跨越窗口以及一些访问权限（需要API24+）。

了解了方法参数含义，接下来就是启用拖拽了，通常会通过长按来触发拖拽：

iv.setOnLongClickListener(new View.OnLongClickListener() {@Overridepublic boolean onLongClick(View v) {View.DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(v);v.startDrag(null, shadowBuilder, null, 0);//震动反馈v.performHapticFeedback(HapticFeedbackConstants.LONG_PRESS, HapticFeedbackConstants.FLAG_IGNORE_GLOBAL_SETTING);return true;}});
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开始拖拽后还要有来接受这些拖拽事件，这就需要OnDragListener了。

OnDragListener

OnDragListener是在View中定义的接口，用于响应拖拽事件,可以通过View的setOnDragListener 方法设置监听，有点类似于点击事件。

public interface OnDragListener { boolean onDrag(View v, DragEvent event);
}
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设置监听，实现onDrag(View v, DragEvent event)方法，其中View是设置该监听的view，DragEvent是拖拽事件，可以通过event.getAction() 获取具体事件类型，这和TouchEvent非常类似，具体事件类型有如下几种：

fl_blue.setOnDragListener(new View.OnDragListener() {@Overridepublic boolean onDrag(View v, DragEvent event) {//v 永远是设置该监听的view，这里即fl_blueString simpleName = v.getClass().getSimpleName();Log.w(BLUE, "view name:" + simpleName);//获取事件int action = event.getAction();switch (action) {case DragEvent.ACTION_DRAG_STARTED:Log.i(BLUE, "开始拖拽");break;case DragEvent.ACTION_DRAG_ENDED:Log.i(BLUE, "结束拖拽");break;case DragEvent.ACTION_DRAG_ENTERED:Log.i(BLUE, "拖拽的view进入监听的view时");break;case DragEvent.ACTION_DRAG_EXITED:Log.i(BLUE, "拖拽的view离开监听的view时");break;case DragEvent.ACTION_DRAG_LOCATION:float x = event.getX();float y = event.getY();long l = SystemClock.currentThreadTimeMillis();Log.i(BLUE, "拖拽的view在监听view中的位置:x =" + x + ",y=" + y);break;case DragEvent.ACTION_DROP:Log.i(BLUE, "释放拖拽的view");break;}//是否响应拖拽事件，true响应，返回false只能接受到ACTION_DRAG_STARTED事件，后续事件不会收到return true;}});
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此处通过event.getX(); event.getY(); 获取的x，y是手指（也即是被拖拽view的中心点）在监听view的位置。

释放手指会触发ACTION_DRAG_ENDED 事件，如果此时被拖拽的view正好在监听的view中，则会先触发ACTION_DROP 事件。

可以同时有多个view设置拖拽监听接受事件，我给红色和蓝色view都设置了OnDragListener，然后拖动Android图片到蓝色区域后释放，可以看到日志如下：

03-09 14:53:54.518 12937-12937/com.huburt.app.androiddrag I/RED: 开始拖拽
03-09 14:53:54.518 12937-12937/com.huburt.app.androiddrag I/BLUE: 开始拖拽
03-09 14:53:55.689 12937-12937/com.huburt.app.androiddrag I/BLUE: 拖拽的view进入监听的view时
03-09 14:53:55.689 12937-12937/com.huburt.app.androiddrag I/BLUE: 拖拽的view在BLUE中的位置:x =111.0,y=2.0
03-09 14:53:55.870 12937-12937/com.huburt.app.androiddrag I/BLUE: 拖拽的view在BLUE中的位置:x =112.0,y=23.0
03-09 14:53:56.014 12937-12937/com.huburt.app.androiddrag I/BLUE: 释放拖拽的view
03-09 14:53:56.017 12937-12937/com.huburt.app.androiddrag I/RED: 结束拖拽
03-09 14:53:56.017 12937-12937/com.huburt.app.androiddrag I/BLUE: 结束拖拽
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现在我们已经可以把Android图片拖出来，但是还不能把它放入目标view，其实也挺简单的，只需要在ACTION_DROP事件做一些处理即可：

         case DragEvent.ACTION_DROP:Log.i(BLUE, "释放拖拽的view");ImageView localState = (ImageView) event.getLocalState();FrameLayout.LayoutParams layoutParams = new FrameLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);layoutParams.topMargin = (int) event.getY() - localState.getWidth() / 2;layoutParams.leftMargin = (int) event.getX() - localState.getHeight() / 2;((ViewGroup) localState.getParent()).removeView(localState);fl_blue.addView(localState, layoutParams);break;
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这里因为是在同一个Activity中，我是将拖拽的view直接传递过来了，当然也可以只传递图片，然后在接收的view中重新new一个imageview现实图片。

运行一下就可以看到view可以拖拽到目标位置了。

可能文字描述不是特别清楚，请看demo