DragStartHelper

Artifact: androidx.core:core

View Source

Added in 1.1.0

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Kotlin |Java

public class DragStartHelper

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DragStartHelper is a utility class for implementing drag and drop support.

It detects gestures commonly used to start drag (long click for any input source, click and drag for mouse).

It also keeps track of the screen location where the drag started, and helps determining the hot spot position for a drag shadow.

Implement DragStartHelper.OnDragStartListener to start the drag operation:

DragStartHelper.OnDragStartListener listener = new DragStartHelper.OnDragStartListener {
    protected void onDragStart(View view, DragStartHelper helper) {
        View.DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(view) {
            public void onProvideShadowMetrics(Point shadowSize, Point shadowTouchPoint) {
                super.onProvideShadowMetrics(shadowSize, shadowTouchPoint);
                helper.getTouchPosition(shadowTouchPoint);
            }
        };
        view.startDrag(mClipData, shadowBuilder, mLocalState, mDragFlags);
    }
};
mDragStartHelper = new DragStartHelper(mDraggableView, listener);

Once created, DragStartHelper can be attached to a view (this will replace existing long click and touch listeners):

mDragStartHelper.attach();

It may also be used in combination with existing listeners:

public boolean onTouch(View view, MotionEvent event) {
    if (mDragStartHelper.onTouch(view, event)) {
        return true;
    }
    return handleTouchEvent(view, event);
}
public boolean onLongClick(View view) {
    if (mDragStartHelper.onLongClick(view)) {
        return true;
    }
    return handleLongClickEvent(view);
}

Summary

Nested types
public interface DragStartHelper.OnDragStartListener Interface definition for a callback to be invoked when a drag start gesture is detected.

Public constructors
DragStartHelper(     @NonNull View view,     @NonNull DragStartHelper.OnDragStartListener listener ) Create a DragStartHelper associated with the specified view.

Public methods
void	attach() Attach the helper to the view.
void	detach() Detach the helper from the view.
void	getTouchPosition(@NonNull Point point) Compute the position of the touch event that started the drag operation.
boolean	onLongClick(@NonNull View v) Handle a long click event.
boolean	onTouch(@NonNull View v, @NonNull MotionEvent event) Handle a touch event.

Public constructors

DragStartHelper

Added in 1.1.0

public DragStartHelper(
    @NonNull View view,
    @NonNull DragStartHelper.OnDragStartListener listener
)

Create a DragStartHelper associated with the specified view. The newly created helper is not initially attached to the view, attach must be called explicitly.

Parameters
@NonNull View view	A View
@NonNull DragStartHelper.OnDragStartListener listener	listener for the drag events.

Public methods

attach

Added in 1.1.0

public void attach()

Attach the helper to the view.

This will replace previously existing touch and long click listeners.

detach

Added in 1.1.0

public void detach()

Detach the helper from the view.

This will reset touch and long click listeners to null.

getTouchPosition

Added in 1.1.0

public void getTouchPosition(@NonNull Point point)

Compute the position of the touch event that started the drag operation.

Parameters
@NonNull Point point	The position of the touch event that started the drag operation.

onLongClick

Added in 1.1.0

public boolean onLongClick(@NonNull View v)

Handle a long click event.

Parameters
@NonNull View v	The view that was clicked and held.

Returns
boolean	true if the callback consumed the long click, false otherwise.

onTouch

Added in 1.1.0

public boolean onTouch(@NonNull View v, @NonNull MotionEvent event)

Handle a touch event.

Parameters
@NonNull View v	The view the touch event has been dispatched to.
@NonNull MotionEvent event	The MotionEvent object containing full information about the event.

Returns
boolean	True if the listener has consumed the event, false otherwise.