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)
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;
}
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);
该数据可以在监听的中的DragEvent获取
ClipData clipData = event.getClipData();
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);
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() {
@Override
public 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;
}
});
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);
}
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() {
@Override
public boolean onDrag(View v, DragEvent event) {
//v 永远是设置该监听的view，这里即fl_blue
String 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;
}
});
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
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;
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
