您可以藉由回應事件，在檢視畫面中實作拖曳程序 可能觸發拖曳開始、回應和消耗放置事件。
開始拖曳
使用者以手勢開始拖曳，通常是輕觸或點選， 然後按住要拖曳的項目
如要在 View 中處理這種情況，請建立
ClipData 物件和
ClipData.Item 物件：
要移動的資料做為 ClipData 的一部分，請提供
儲存在
ClipDescription 物件
ClipData。適用於並不代表
資料移動時，建議您使用 null 而非實際物件。
舉例來說，以下程式碼片段說明如何回應觸控與保留
ImageView 來建立包含 ClipData 物件的
ImageView 的標記 (或標籤)：
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。聽眾回應
呼叫 DragEvent.getAction() 以取得動作類型。拖曳開始時
這個方法會傳回 ACTION_DRAG_STARTED。
為了回應動作類型為 ACTION_DRAG_STARTED 的事件，拖曳事件
接聽程式必須執行下列操作：
致電
DragEvent.getClipDescription()敬上 並透過傳回的ClipDescription中的 MIME 類型方法查看 指出監聽器是否能夠接受拖曳的資料。如果拖曳作業不是資料移動， 不必要的額外區塊
如果拖曳事件監聽器可接受放置，則必須傳回
true以通知 系統會繼續將拖曳事件傳送至事件監聽器。如果事件監聽器 無法接受放置，事件監聽器必須傳回false，且系統會停止 將拖曳事件傳送至事件監聽器，直到系統傳送ACTION_DRAG_ENDED以結束拖曳作業。
如果是 ACTION_DRAG_STARTED 事件，則無法使用下列 DragEvent 方法
有效：getClipData()，
getX(),
getY()和
getResult()。
在拖曳期間處理事件
在拖曳動作期間，拖曳事件監聽器，以回應 true
ACTION_DRAG_STARTED 拖曳事件會繼續接收拖曳事件。類型
事件監聽器在拖曳期間收到的拖曳事件的多寡，取決於
拖曳陰影以及事件監聽器 View 的顯示設定。事件監聽器會使用拖曳功能
事件，主要用於判斷是否必須變更 View 的外觀。
在拖曳動作期間，DragEvent.getAction() 會傳回以下三個值的其中之一：
ACTION_DRAG_ENTERED: 監聽器會在接觸點收到此事件動作類型 在這個畫面上，請在使用者的手指或滑鼠下方輸入 事件監聽器View的定界框。ACTION_DRAG_LOCATION: 一旦事件監聽器收到ACTION_DRAG_ENTERED事件，就會收到新的ACTION_DRAG_LOCATION每次接觸點移動至觸發事件為止 就會收到ACTION_DRAG_EXITED事件getX()和getY()方法 會傳回觸控點的 X 和 Y 座標。ACTION_DRAG_EXITED: 這個事件動作類型會傳送到先前接收到ACTION_DRAG_ENTERED。拖曳陰影觸控點時，會傳送這個事件 從事件監聽器View的定界框內移至 定界框。
拖曳事件監聽器不需要回應任何這些動作類型。如果 事件監聽器會將值傳回系統，則會忽略該值。
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」的事件包含以下項目的有效資料： 與接觸點位置相對應的getX()和getY()。 事件監聽器可以利用這項資訊來調整View外觀 也可以判斷使用者在何處 內容。 - 回應
ACTION_DRAG_EXITED時，事件監聽器必須重設任何外觀 套用變更，以回應ACTION_DRAG_ENTERED或ACTION_DRAG_LOCATION。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，且先前使用 View 時
回報可接受拖曳的內容時，系統會發送
將動作類型 ACTION_DROP 拖曳到 View。
拖曳事件監聽器必須執行以下操作：
呼叫
getClipData()即可取得最初的ClipData物件 所提供的參數startDragAndDrop()敬上 以及處理資料如果拖曳作業不代表資料 這不需要的話傳回布林值
true，表示已成功處理放置。 如果不是，則使用false。傳回的值會成為 最終ACTION_DRAG_ENDED事件的getResult()。如果系統 未送出ACTION_DROP事件，getResult()傳回的值。ACTION_DRAG_ENDED事件為false。
如果是 ACTION_DROP 事件，getX() 和 getY() 會使用
接收捨棄值的 View，傳回
與主要接觸點相比。
使用者可以透過拖曳事件的 View 釋放拖曳陰影
事件監聽器未收到拖曳事件、應用程式使用者介面的空白區域，
在應用程式以外的區域，Android 不會傳送含有動作的事件
輸入 ACTION_DROP，且只會傳送 ACTION_DRAG_ENDED 事件。
回應拖曳結束事件
使用者放開拖曳陰影後，系統會立即傳送拖曳
動作類型為 ACTION_DRAG_ENDED 的事件可對所有拖曳事件監聽器
應用程式。這表示拖曳作業已完成。
每個拖曳事件監聽器都必須執行以下操作：
- 如果事件監聽器在作業期間變更外觀，應重設 重新顯示應用程式的預設外觀，以視覺化的方式告知使用者 作業完成。
- 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以回應動作事件 輸入ACTION_DROP，然後getResult()會傳回布林值true。所有其他大小getResult()會傳回布林值false，包括系統 但不會傳送ACTION_DROP事件。 - 如要表示放置作業成功完成，事件監聽器：
應將布林值
true傳回系統。在未傳回false的情況下， 顯示投射陰影返回來源的視覺提示，可能會建議 使用者表示作業失敗。
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。 以下程式碼片段是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
您可以覆寫自訂 myDragShadowBuilder，方法是覆寫
View.DragShadowBuilder。下列程式碼片段會建立一個小型的
TextView 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);
}
}