您可以回應可能會觸發拖曳開始及回應放置事件的事件，在檢視畫面中實作拖曳程序。
開始拖曳
使用者以手勢開始拖曳，通常可透過輕觸或按住要拖曳的項目來完成。
如要在 View 中處理這種情況，請為要移動的資料建立 ClipData 物件和 ClipData.Item 物件。在 ClipData 中，提供在 ClipData 內儲存於 ClipDescription 物件的中繼資料。如果不是代表資料移動的拖曳作業，建議您使用 null，而非實際物件。
例如，此程式碼片段說明如何建立包含 ImageView 標記 (或標籤) 的 ClipData 物件，藉此回應 ImageView 的按住手勢：
Kotlin
// Create a string for the ImageView label.
val IMAGEVIEW_TAG = "icon bitmap"
...
val imageView = ImageView(context).apply {
// Set the bitmap for the ImageView from an icon bitmap defined elsewhere.
setImageBitmap(iconBitmap)
tag = IMAGEVIEW_TAG
setOnLongClickListener { v ->
// Create a new ClipData. This is done in two steps to provide
// clarity. The convenience method ClipData.newPlainText() can
// create a plain text ClipData in one step.
// Create a new ClipData.Item from the ImageView object's tag.
val item = ClipData.Item(v.tag as? CharSequence)
// Create a new ClipData using the tag as a label, the plain text
// MIME type, and the already-created item. This creates a new
// ClipDescription object within the ClipData and sets its MIME type
// to "text/plain".
val dragData = ClipData(
v.tag as? CharSequence,
arrayOf(ClipDescription.MIMETYPE_TEXT_PLAIN),
item)
// Instantiate the drag shadow builder. We use this imageView object
// to create the default builder.
val myShadow = View.DragShadowBuilder(view: this)
// Start the drag.
v.startDragAndDrop(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.
)
// Indicate that the long-click is handled.
true
}
}
Java
// Create a string for the ImageView label.
private static final String IMAGEVIEW_TAG = "icon bitmap";
...
// Create a new ImageView.
ImageView imageView = new ImageView(context);
// Set the bitmap for the ImageView from an icon bitmap defined elsewhere.
imageView.setImageBitmap(iconBitmap);
// Set the tag.
imageView.setTag(IMAGEVIEW_TAG);
// Set a long-click listener for the ImageView using an anonymous listener
// object that implements the OnLongClickListener interface.
imageView.setOnLongClickListener( v -> {
// Create a new ClipData. This is done in two steps to provide clarity. The
// convenience method ClipData.newPlainText() can create a plain text
// ClipData in one step.
// Create a new ClipData.Item from the ImageView object's tag.
ClipData.Item item = new ClipData.Item((CharSequence) v.getTag());
// Create a new ClipData using the tag as a label, the plain text MIME type,
// and the already-created item. This creates a new ClipDescription object
// within the ClipData and sets its MIME type to "text/plain".
ClipData dragData = new ClipData(
(CharSequence) v.getTag(),
new String[] { ClipDescription.MIMETYPE_TEXT_PLAIN },
item);
// Instantiate the drag shadow builder. We use this imageView object
// to create the default builder.
View.DragShadowBuilder myShadow = new View.DragShadowBuilder(imageView);
// Start the drag.
v.startDragAndDrop(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.
);
// Indicate that the long-click is handled.
return true;
});
回應拖曳開始事件
在拖曳作業中，系統會把拖曳事件分派到目前版面配置中 View 物件的拖曳事件監聽器。事件監聽器會呼叫 DragEvent.getAction() 來取得動作類型。拖曳開始時，此方法會傳回 ACTION_DRAG_STARTED。
為回應動作類型為 ACTION_DRAG_STARTED 的事件，拖曳事件監聽器必須執行下列操作：
呼叫
DragEvent.getClipDescription()，並在回傳的ClipDescription中使用 MIME 類型方法，確認事件監聽器是否能接受拖曳的資料。如果拖曳作業不是資料移動，就不需要進行此步驟。
如果拖曳事件監聽器可接受放置，則必須回傳
true以指示系統繼續將拖曳事件傳送至事件監聽器。如果事件監聽器不接受放置，事件監聽器必須回傳false，而系統會停止將拖曳事件傳送至事件監聽器，直到系統傳送ACTION_DRAG_ENDED完成拖曳作業。
如果是 ACTION_DRAG_STARTED 事件，則下列 DragEvent 方法無效：getClipData()、getX()、getY() 和 getResult()。
在拖曳期間處理事件
在拖曳過程中，回傳 true 以回應 ACTION_DRAG_STARTED 拖曳事件的拖曳事件監聽器，會繼續接收拖曳事件。事件監聽器在拖曳期間收到的拖曳事件類型，取決於拖曳陰影的位置和事件監聽器 View 的瀏覽權限。事件監聽器主要使用拖曳事件，判斷是否必須變更 View 的外觀。
在拖曳過程中，DragEvent.getAction() 會回傳下列三個值的其中之一：
ACTION_DRAG_ENTERED：當觸控點 (使用者手指或滑鼠下方畫面上的點) 進入事件監聽器View的定界框時，事件監聽器會收到此事件動作類型。ACTION_DRAG_LOCATION：監聽器收到ACTION_DRAG_ENTERED事件後，每次觸控點移動時都會收到新的ACTION_DRAG_LOCATION事件，直到收到ACTION_DRAG_EXITED事件為止。getX()和getY()方法會傳回觸控點的 X 和 Y 座標。ACTION_DRAG_EXITED：此事件動作類型會傳送給先前接收ACTION_DRAG_ENTERED的事件監聽器。當拖曳陰影觸控點從事件監聽器View的定界框內移動至定界框外，就會傳送事件。
拖曳事件監聽器不需要回應上述任何動作類型。如果事件監聽器回傳該值，系統會忽略該值。
以下是回應各種動作類型的方針：
- 回應
ACTION_DRAG_ENTERED或ACTION_DRAG_LOCATION時，事件監聽器可以變更View的外觀，以表示檢視畫面是潛在的放置目標。 - 動作類型為
ACTION_DRAG_LOCATION的事件包含getX()和getY()的有效資料，對應至觸控點位置。事件監聽器可使用此資訊調整觸控點的View外觀，或決定使用者可放置內容的確切位置。 - 為回應
ACTION_DRAG_EXITED，事件監聽器必須重設其對ACTION_DRAG_ENTERED或ACTION_DRAG_LOCATION套用的任何外觀變更。這會向使用者表明View不再是預期的放置目標。
回應放置
當使用者在 View 上放開拖曳陰影，且 View 先前回報可接受拖曳的內容時，系統就會將拖曳事件分派給動作類型為 ACTION_DROP 的 View。
拖曳事件監聽器必須執行下列操作：
呼叫
getClipData()以取得原先在呼叫startDragAndDrop()中提供的ClipData物件，並處理資料。如果拖曳作業不是資料移動，就不需要進行這項步驟。回傳布林值
true，表示已成功處理放置；如未成功處理，則回傳false。回傳的值會成為getResult()最終為ACTION_DRAG_ENDED事件回傳的值。如果系統未傳送ACTION_DROP事件，則getResult()為ACTION_DRAG_ENDED事件回傳的值會是false。
如果是 ACTION_DROP 事件，getX() 和 getY() 會使用接收放置的 View 座標系統，在放置時回傳觸控點的 X 和 Y 位置。
使用者可在拖曳事件監聽器未接收拖曳事件的 View 上放開拖曳陰影、應用程式 UI 的空白區域，甚至是應用程式外的區域，Android 不會傳送動作類型為 ACTION_DROP 的事件，只會傳送 ACTION_DRAG_ENDED 事件。
回應拖曳結束事件
在使用者放開拖曳陰影後，系統會立即將動作類型為 ACTION_DRAG_ENDED 的拖曳事件傳送至應用程式中的所有拖曳事件監聽器。這表示拖曳作業已完成。
每個拖曳事件監聽器都必須執行下列操作：
- 假如事件監聽器在作業期間變更外觀，則應將其重設為預設外觀，讓使用者瞭解作業已完成。
- 事件監聽器可以選擇呼叫
getResult()以進一步瞭解作業。如果事件監聽器回傳true以回應動作類型ACTION_DROP的事件，則getResult()會回傳布林值true。在所有其他情況下，getResult()都會回傳布林值false，包括系統未傳送ACTION_DROP事件的情況。 - 如要表示放置作業成功完成，事件監聽器應將布林值
true回傳系統。如果不傳回false，顯示投射陰影的視覺線索可能會返回來源，建議使用者作業失敗。
回應拖曳事件：範例
所有拖曳事件均由拖曳事件方法或事件監聽器接收。以下程式碼片段為回應拖曳事件的範例：
Kotlin
val imageView = ImageView(this)
// Set the drag event listener for the View.
imageView.setOnDragListener { v, e ->
// Handle each of the expected events.
when (e.action) {
DragEvent.ACTION_DRAG_STARTED -> {
// Determine whether this View can accept the dragged data.
if (e.clipDescription.hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) {
// As an example, apply a blue color tint to the View to
// indicate that it can accept data.
(v as? ImageView)?.setColorFilter(Color.BLUE)
// Invalidate the view to force a redraw in the new tint.
v.invalidate()
// Return true to indicate that the View can accept the dragged
// data.
true
} else {
// Return false to indicate that, during the current drag and
// drop operation, this View doesn't receive events again until
// ACTION_DRAG_ENDED is sent.
false
}
}
DragEvent.ACTION_DRAG_ENTERED -> {
// Apply a green tint to the View.
(v as? ImageView)?.setColorFilter(Color.GREEN)
// Invalidate the view to force a redraw in the new tint.
v.invalidate()
// Return true. The value is ignored.
true
}
DragEvent.ACTION_DRAG_LOCATION ->
// Ignore the event.
true
DragEvent.ACTION_DRAG_EXITED -> {
// Reset the color tint to blue.
(v as? ImageView)?.setColorFilter(Color.BLUE)
// Invalidate the view to force a redraw in the new tint.
v.invalidate()
// Return true. The value is ignored.
true
}
DragEvent.ACTION_DROP -> {
// Get the item containing the dragged data.
val item: ClipData.Item = e.clipData.getItemAt(0)
// Get the text data from the item.
val dragData = item.text
// Display a message containing the dragged data.
Toast.makeText(this, "Dragged data is $dragData", Toast.LENGTH_LONG).show()
// Turn off color tints.
(v as? ImageView)?.clearColorFilter()
// Invalidate the view to force a redraw.
v.invalidate()
// Return true. DragEvent.getResult() returns true.
true
}
DragEvent.ACTION_DRAG_ENDED -> {
// Turn off color tinting.
(v as? ImageView)?.clearColorFilter()
// Invalidate the view to force a redraw.
v.invalidate()
// Do a getResult() and display what happens.
when(e.result) {
true ->
Toast.makeText(this, "The drop was handled.", Toast.LENGTH_LONG)
else ->
Toast.makeText(this, "The drop didn't work.", Toast.LENGTH_LONG)
}.show()
// Return true. The value is ignored.
true
}
else -> {
// An unknown action type is received.
Log.e("DragDrop Example", "Unknown action type received by View.OnDragListener.")
false
}
}
}
Java
View imageView = new ImageView(this);
// Set the drag event listener for the View.
imageView.setOnDragListener( (v, e) -> {
// Handle each of the expected events.
switch(e.getAction()) {
case DragEvent.ACTION_DRAG_STARTED:
// Determine whether this View can accept the dragged data.
if (e.getClipDescription().hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) {
// As an example, apply a blue color tint to the View to
// indicate that it can accept data.
((ImageView)v).setColorFilter(Color.BLUE);
// Invalidate the view to force a redraw in the new tint.
v.invalidate();
// Return true to indicate that the View can accept the dragged
// data.
return true;
}
// Return false to indicate that, during the current drag-and-drop
// operation, this View doesn't receive events again until
// ACTION_DRAG_ENDED is sent.
return false;
case DragEvent.ACTION_DRAG_ENTERED:
// Apply a green tint to the View.
((ImageView)v).setColorFilter(Color.GREEN);
// Invalidate the view to force a redraw in the new tint.
v.invalidate();
// Return true. The value is ignored.
return true;
case DragEvent.ACTION_DRAG_LOCATION:
// Ignore the event.
return true;
case DragEvent.ACTION_DRAG_EXITED:
// Reset the color tint to blue.
((ImageView)v).setColorFilter(Color.BLUE);
// Invalidate the view to force a redraw in the new tint.
v.invalidate();
// Return true. The value is ignored.
return true;
case DragEvent.ACTION_DROP:
// Get the item containing the dragged data.
ClipData.Item item = e.getClipData().getItemAt(0);
// Get the text data from the item.
CharSequence dragData = item.getText();
// Display a message containing the dragged data.
Toast.makeText(this, "Dragged data is " + dragData, Toast.LENGTH_LONG).show();
// Turn off color tints.
((ImageView)v).clearColorFilter();
// Invalidate the view to force a redraw.
v.invalidate();
// Return true. DragEvent.getResult() returns true.
return true;
case DragEvent.ACTION_DRAG_ENDED:
// Turn off color tinting.
((ImageView)v).clearColorFilter();
// Invalidate the view to force a redraw.
v.invalidate();
// Do a getResult() and displays what happens.
if (e.getResult()) {
Toast.makeText(this, "The drop was handled.", Toast.LENGTH_LONG).show();
} else {
Toast.makeText(this, "The drop didn't work.", Toast.LENGTH_LONG).show();
}
// Return true. The value is ignored.
return true;
// An unknown action type is received.
default:
Log.e("DragDrop Example","Unknown action type received by View.OnDragListener.");
break;
}
return false;
});
自訂拖曳陰影
您可以覆寫 View.DragShadowBuilder 中的方法，定義自訂的 myDragShadowBuilder。下列程式碼片段會為 TextView 建立小型的矩形灰色拖曳陰影：
Kotlin
private class MyDragShadowBuilder(view: View) : View.DragShadowBuilder(view) {
private val shadow = ColorDrawable(Color.LTGRAY)
// Define a callback that sends the drag shadow dimensions and touch point
// back to the system.
override fun onProvideShadowMetrics(size: Point, touch: Point) {
// Set the width of the shadow to half the width of the original
// View.
val width: Int = view.width / 2
// Set the height of the shadow to half the height of the original
// View.
val height: Int = view.height / 2
// The drag shadow is a ColorDrawable. Set its dimensions to
// be the same as the Canvas that the system provides. As a result,
// the drag shadow fills the Canvas.
shadow.setBounds(0, 0, width, height)
// Set the size parameter's width and height values. These get back
// to the system through the size parameter.
size.set(width, height)
// Set the touch point's position to be in the middle of the drag
// shadow.
touch.set(width / 2, height / 2)
}
// Define a callback that draws the drag shadow in a Canvas that the system
// constructs from the dimensions passed to onProvideShadowMetrics().
override fun onDrawShadow(canvas: Canvas) {
// Draw the ColorDrawable on the Canvas passed in from the system.
shadow.draw(canvas)
}
}
Java
private static class MyDragShadowBuilder extends View.DragShadowBuilder {
// The drag shadow image, defined as a drawable object.
private static Drawable shadow;
// Constructor.
public MyDragShadowBuilder(View view) {
// Store the View parameter.
super(view);
// Create a draggable image that fills the Canvas provided by the
// system.
shadow = new ColorDrawable(Color.LTGRAY);
}
// Define a callback that sends the drag shadow dimensions and touch point
// back to the system.
@Override
public void onProvideShadowMetrics (Point size, Point touch) {
// Define local variables.
int width, height;
// Set the width of the shadow to half the width of the original
// View.
width = getView().getWidth() / 2;
// Set the height of the shadow to half the height of the original
// View.
height = getView().getHeight() / 2;
// The drag shadow is a ColorDrawable. Set its dimensions to
// be the same as the Canvas that the system provides. As a result,
// the drag shadow fills the Canvas.
shadow.setBounds(0, 0, width, height);
// Set the size parameter's width and height values. These get back
// to the system through the size parameter.
size.set(width, height);
// Set the touch point's position to be in the middle of the drag
// shadow.
touch.set(width / 2, height / 2);
}
// Define a callback that draws the drag shadow in a Canvas that the system
// constructs from the dimensions passed to onProvideShadowMetrics().
@Override
public void onDrawShadow(Canvas canvas) {
// Draw the ColorDrawable on the Canvas passed in from the system.
shadow.draw(canvas);
}
}