其實(shí)實(shí)現(xiàn)這種效果有兩種方法：
View.startDrag(), 然后給需要監(jiān)聽拖拽的控件setOnDragListener.
ItemTouchHelper，這種實(shí)現(xiàn)方法更為簡(jiǎn)單，具體可參考鏈接描述
這里我是用的第一種方法，因?yàn)楦杏X第二種方法已經(jīng)爛大街了。。況且第二種方法只能在RecycleView內(nèi)部移動(dòng)。跟其他控件結(jié)合的話就爆炸。
具體實(shí)現(xiàn)步驟
給RecycleView.ViewHolder實(shí)現(xiàn)OnClickListener()方法，長(zhǎng)按的時(shí)候開始拖動(dòng)。
拖動(dòng)的時(shí)候給不同的DragEvent做不同的操作。分別有DragEvent.ACTION_DRAG_STARTED（拖動(dòng)開始時(shí)）
DragEvent.ACTION_DRAG_ENTERED（拖動(dòng)的View進(jìn)入監(jiān)聽的View時(shí)），DragEvent.ACTION_DRAG_LOCATION（拖動(dòng)的View在監(jiān)聽的View中改變位置時(shí)），DragEvent.ACTION_DRAG_EXITED（拖動(dòng)的View離開在監(jiān)聽的View中時(shí)），DragEvent.ACTION_DROP（拖動(dòng)放下時(shí)），DragEvent.ACTION_DRAG_ENDED（拖動(dòng)結(jié)束時(shí)）
實(shí)現(xiàn)RecleView在拖動(dòng)中排序
這幾步中，最重要的還是第二步:
@Override
? ? public boolean onDrag(View v, DragEvent event) {
? ? ? ? switch (event.getAction()) {
? ? ? ? ? ? case DragEvent.ACTION_DRAG_STARTED:
? ? ? ? ? ? ? ? //開始時(shí)，讓拖動(dòng)的Item變白
? ? ? ? ? ? ? ? break;
? ? ? ? ? ? case DragEvent.ACTION_DRAG_ENTERED:
? ? ? ? ? ? ? ? //進(jìn)入時(shí)，這個(gè)Demo不需要用到
? ? ? ? ? ? ? ? break;
? ? ? ? ? ? case DragEvent.ACTION_DRAG_LOCATION:
? ? ? ? ? ? ? ? //處理RecycleView的滑動(dòng)
? ? ? ? ? ? ? ? //處理Item之間的交換
? ? ? ? ? ? ? ? break;
? ? ? ? ? ? case DragEvent.ACTION_DRAG_EXITED:
? ? ? ? ? ? case DragEvent.ACTION_DRAG_ENDED:
? ? ? ? ? ? ? ? //善后工作? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? break;
? ? ? ? }
? ? ? ? //一定要return true
? ? ? ? return true;
? ? }
、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、、
Android中實(shí)現(xiàn)拖拽其實(shí)很簡(jiǎn)單，系統(tǒng)早已經(jīng)提供了api讓我使用，主要用到了View的startDrag（startDragAndDrop API24+）方法以及OnDragListener。
startDrag
先來看下方法介紹：
/**
?????* 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)
看到英文就頭大？沒事，我來翻譯解釋一下。
啟動(dòng)拖放操作。當(dāng)應(yīng)用程序調(diào)用此方法時(shí)，它將傳遞一個(gè)DragShadowBuilder對(duì)象到系統(tǒng)。系統(tǒng)調(diào)用此對(duì)象的onProvideShadowMetrics(Point, Point)方法獲取拖動(dòng)陰影的參數(shù)指標(biāo)，然后調(diào)用onDrawShadow(Canvas)來繪制陰影。一旦系統(tǒng)有了拖動(dòng)陰影，它就開始拖拽操作，通過將拖拽事件發(fā)送到當(dāng)前可見的應(yīng)用程序中的所有視圖對(duì)象。這些視圖可以通過設(shè)置OnDragListener在或者實(shí)現(xiàn)onDragEvent方法接受DragEvent（事件）來響應(yīng)和拖拽事件。
可以看到有四個(gè)參數(shù)：
ClipData data
其實(shí)就是一個(gè)封裝數(shù)據(jù)的對(duì)象，通過拖放操作傳遞給接受者。該對(duì)象可以存放一個(gè)Item的集合，Item可以存放如下數(shù)據(jù)：
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，通常可以將參數(shù)存入intent，然后通過靜態(tài)方法直接創(chuàng)建ClipData對(duì)象：
ClipData clipData = ClipData.newIntent("label", intent);
該數(shù)據(jù)可以在監(jiān)聽的中的DragEvent獲取
ClipData clipData = event.getClipData();
簡(jiǎn)單點(diǎn)說就是可以將一些數(shù)據(jù)傳遞給拖拽的接受者，該拖拽其實(shí)可以跨Activity的，如果只是同一個(gè)Activity可以使用第三個(gè)參數(shù)傳遞數(shù)據(jù)。
DragShadowBuilder shadowBuilder
用于創(chuàng)建拖拽view是的陰影，也就是跟隨手指移動(dòng)的視圖，通常直接使用默認(rèn)即可生成與一個(gè)原始view相同，帶有透明度的陰影：
View.DragShadowBuilder shadowBuilder = new?View.DragShadowBuilder(v);
Object myLocalState
當(dāng)你的拖拽行為是在同一個(gè)Activity中進(jìn)行時(shí)可以傳遞一個(gè)任意對(duì)象，在監(jiān)聽中可以通過{@link android.view.DragEvent#getLocalState()}獲得。如果是跨Activity拖拽中無法訪問此數(shù)據(jù)，getLocalState()將返回null。
int flags
控制拖放操作的標(biāo)志。因?yàn)闆]有標(biāo)志可以設(shè)置為0，flag標(biāo)志拖動(dòng)是否可以跨越窗口以及一些訪問權(quán)限（需要API24+）。
了解了方法參數(shù)含義，接下來就是啟用拖拽了，通常會(huì)通過長(zhǎng)按來觸發(fā)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);
????????????????//震動(dòng)反饋
????????????????v.performHapticFeedback(HapticFeedbackConstants.LONG_PRESS, HapticFeedbackConstants.FLAG_IGNORE_GLOBAL_SETTING);
????????????????return?true;
????????????}
????????});
開始拖拽后還要有來接受這些拖拽事件，這就需要OnDragListener了。
OnDragListener
OnDragListener是在View中定義的接口，用于響應(yīng)拖拽事件,可以通過View的setOnDragListener方法設(shè)置監(jiān)聽，有點(diǎn)類似于點(diǎn)擊事件。
public?interface OnDragListener {
????????boolean onDrag(View v, DragEvent event);
}
設(shè)置監(jiān)聽，實(shí)現(xiàn)onDrag(View v, DragEvent event)方法，其中View是設(shè)置該監(jiān)聽的view，DragEvent是拖拽事件，可以通過event.getAction()獲取具體事件類型，這和TouchEvent非常類似，具體事件類型有如下幾種：
fl_blue.setOnDragListener(new?View.OnDragListener() {
????????????@Override
????????????public boolean onDrag(View v, DragEvent event) {
????????????????//v 永遠(yuǎn)是設(shè)置該監(jiān)聽的view，這里即fl_blue
????????????????String simpleName = v.getClass().getSimpleName();
????????????????Log.w(BLUE, "view name:"?+ simpleName);
????????????????//獲取事件
????????????????int?action = event.getAction();
????????????????switch?(action) {
????????????????????case?DragEvent.ACTION_DRAG_STARTED:
????????????????????????Log.i(BLUE, "開始拖拽");
????????????????????????break;
????????????????????case?DragEvent.ACTION_DRAG_ENDED:
????????????????????????Log.i(BLUE, "結(jié)束拖拽");
????????????????????????break;
????????????????????case?DragEvent.ACTION_DRAG_ENTERED:
????????????????????????Log.i(BLUE, "拖拽的view進(jìn)入監(jiān)聽的view時(shí)");
????????????????????????break;
????????????????????case?DragEvent.ACTION_DRAG_EXITED:
????????????????????????Log.i(BLUE, "拖拽的view離開監(jiān)聽的view時(shí)");
????????????????????????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在監(jiān)聽view中的位置:x ="?+ x + ",y="?+ y);
????????????????????????break;
????????????????????case?DragEvent.ACTION_DROP:
????????????????????????Log.i(BLUE, "釋放拖拽的view");
????????????????????????break;
????????????????}
????????????????//是否響應(yīng)拖拽事件，true響應(yīng)，返回false只能接受到ACTION_DRAG_STARTED事件，后續(xù)事件不會(huì)收到
????????????????return?true;
????????????}
????????});
此處通過event.getX(); event.getY();獲取的x，y是手指（也即是被拖拽view的中心點(diǎn)）在監(jiān)聽view的位置。
釋放手指會(huì)觸發(fā)ACTION_DRAG_ENDED事件，如果此時(shí)被拖拽的view正好在監(jiān)聽的view中，則會(huì)先觸發(fā)ACTION_DROP事件。
這里寫圖片描述
可以同時(shí)有多個(gè)view設(shè)置拖拽監(jiān)聽接受事件，我給紅色和藍(lán)色view都設(shè)置了OnDragListener，然后拖動(dòng)Android圖片到藍(lán)色區(qū)域后釋放，可以看到日志如下：
03-09 14:53:54.518?12937-12937/com.huburt.app.androiddrag I/RED:開始拖拽
03-09 14:53:54.518?12937-12937/com.huburt.app.androiddrag I/BLUE:開始拖拽
03-09 14:53:55.689?12937-12937/com.huburt.app.androiddrag I/BLUE:拖拽的view進(jìn)入監(jiān)聽的view時(shí)
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:釋放拖拽的view
03-09 14:53:56.017?12937-12937/com.huburt.app.androiddrag I/RED:結(jié)束拖拽
03-09 14:53:56.017?12937-12937/com.huburt.app.androiddrag I/BLUE:結(jié)束拖拽
現(xiàn)在我們已經(jīng)可以把Android圖片拖出來，但是還不能把它放入目標(biāo)view，其實(shí)也挺簡(jiǎn)單的，只需要在ACTION_DROP事件做一些處理即可：
????????????case?DragEvent.ACTION_DROP:
????????????????????????Log.i(BLUE, "釋放拖拽的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;
這里因?yàn)槭窃谕粋€(gè)Activity中，我是將拖拽的view直接傳遞過來了，當(dāng)然也可以只傳遞圖片，然后在接收的view中重新new一個(gè)imageview現(xiàn)實(shí)圖片。
運(yùn)行一下就可以看到view可以拖拽到目標(biāo)位置了。
AndroidDrag-master ? ? ? ?DragDropTwoRecyclerViews-master
