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?？梢詫?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。

可能文字描述不是特別清楚，請(qǐng)看demo