AutoScrollHelper

Added in 1.1.0

abstract class AutoScrollHelper : View.OnTouchListener

Known direct subclasses
ListViewAutoScrollHelper

An implementation of AutoScrollHelper that knows how to scroll through a ListView.


AutoScrollHelper is a utility class for adding automatic edge-triggered scrolling to Views.

Note: Implementing classes are responsible for overriding the scrollTargetBy, canTargetScrollHorizontally, and canTargetScrollVertically methods. See ListViewAutoScrollHelper for a android.widget.ListView -specific implementation.

Activation

Automatic scrolling starts when the user touches within an activation area. By default, activation areas are defined as the top, left, right, and bottom 20% of the host view's total area. Touching within the top activation area scrolls up, left scrolls to the left, and so on.

As the user touches closer to the extreme edge of the activation area, scrolling accelerates up to a maximum velocity. When using the default edge type, EDGE_TYPE_INSIDE_EXTEND, moving outside of the view bounds will scroll at the maximum velocity.

The following activation properties may be configured:

Scrolling

When automatic scrolling is active, the helper will repeatedly call scrollTargetBy to apply new scrolling offsets.

The following scrolling properties may be configured:

  • Acceleration ramp-up duration, see setRampUpDuration. Default value is 500 milliseconds.
  • Acceleration ramp-down duration, see setRampDownDuration. Default value is 500 milliseconds.
  • Target velocity relative to view size, see setRelativeVelocity. Default value is 100% per second for both vertical and horizontal.
  • Minimum velocity used to constrain relative velocity, see setMinimumVelocity. When set, scrolling will accelerate to the larger of either this value or the relative target value. Default value is approximately 5 centimeters or 315 dips per second.
  • Maximum velocity used to constrain relative velocity, see setMaximumVelocity. Default value is approximately 25 centimeters or 1575 dips per second.

Summary

Constants

const Int

Edge type that specifies an activation area starting at the view bounds and extending inward.

const Int

Edge type that specifies an activation area starting at the view bounds and extending inward.

const Int

Edge type that specifies an activation area starting at the view bounds and extending outward.

const Float
NO_MAX = 3.4028235E38f

Constant passed to setMaximumEdges, setMaximumVelocity, or setMinimumVelocity.

const Float
NO_MIN = 0.0f

Constant passed to setMaximumEdges, or setMaximumVelocity, or setMinimumVelocity.

const Float

Constant passed to setRelativeEdges or setRelativeVelocity.

Public constructors

Creates a new helper for scrolling the specified target view.

Public functions

abstract Boolean

Override this method to return whether the target view can be scrolled horizontally in a certain direction.

abstract Boolean

Override this method to return whether the target view can be scrolled vertically in a certain direction.

Boolean
Boolean

Indicates whether the scroll helper handles touch events exclusively during scrolling.

Boolean
onTouch(v: View!, event: MotionEvent!)

Handles touch events by activating automatic scrolling, adjusting scroll velocity, or stopping.

abstract Unit
scrollTargetBy(deltaX: Int, deltaY: Int)

Override this method to scroll the target view by the specified number of pixels.

AutoScrollHelper
setActivationDelay(delayMillis: Int)

Sets the delay after entering an activation edge before activation of auto-scrolling.

AutoScrollHelper

Sets the activation edge type, one of:

AutoScrollHelper!
setEnabled(enabled: Boolean)

Sets whether the scroll helper is enabled and should respond to touch events.

AutoScrollHelper!
setExclusive(exclusive: Boolean)

Enables or disables exclusive handling of touch events during scrolling.

AutoScrollHelper
setMaximumEdges(horizontalMax: Float, verticalMax: Float)

Sets the absolute maximum edge size.

AutoScrollHelper
setMaximumVelocity(horizontalMax: Float, verticalMax: Float)

Sets the absolute maximum scrolling velocity.

AutoScrollHelper
setMinimumVelocity(horizontalMin: Float, verticalMin: Float)

Sets the absolute minimum scrolling velocity.

AutoScrollHelper
setRampDownDuration(durationMillis: Int)

Sets the amount of time after de-activation of auto-scrolling that is takes to slow to a stop.

AutoScrollHelper
setRampUpDuration(durationMillis: Int)

Sets the amount of time after activation of auto-scrolling that is takes to reach target velocity for the current touch position.

AutoScrollHelper
setRelativeEdges(horizontal: Float, vertical: Float)

Sets the activation edge size relative to the host view's dimensions.

AutoScrollHelper
setRelativeVelocity(horizontal: Float, vertical: Float)

Sets the target scrolling velocity relative to the host view's dimensions.

Constants

EDGE_TYPE_INSIDE

Added in 1.1.0
const val EDGE_TYPE_INSIDE = 0: Int

Edge type that specifies an activation area starting at the view bounds and extending inward. Moving outside the view bounds will stop scrolling.

See also
setEdgeType

EDGE_TYPE_INSIDE_EXTEND

Added in 1.1.0
const val EDGE_TYPE_INSIDE_EXTEND = 1: Int

Edge type that specifies an activation area starting at the view bounds and extending inward. After activation begins, moving outside the view bounds will continue scrolling.

See also
setEdgeType

EDGE_TYPE_OUTSIDE

Added in 1.1.0
const val EDGE_TYPE_OUTSIDE = 2: Int

Edge type that specifies an activation area starting at the view bounds and extending outward. Moving inside the view bounds will stop scrolling.

See also
setEdgeType

NO_MAX

Added in 1.1.0
const val NO_MAX = 3.4028235E38f: Float

Constant passed to setMaximumEdges, setMaximumVelocity, or setMinimumVelocity. Using this value ensures that the computed relative value is always used without constraining to a particular minimum or maximum value.

NO_MIN

Added in 1.1.0
const val NO_MIN = 0.0f: Float

Constant passed to setMaximumEdges, or setMaximumVelocity, or setMinimumVelocity. Using this value ensures that the computed relative value is always used without constraining to a particular minimum or maximum value.

RELATIVE_UNSPECIFIED

Added in 1.1.0
const val RELATIVE_UNSPECIFIED = 0.0f: Float

Constant passed to setRelativeEdges or setRelativeVelocity. Using this value ensures that the computed relative value is ignored and the absolute maximum value is always used.

Public constructors

AutoScrollHelper

Added in 1.1.0
AutoScrollHelper(target: View)

Creates a new helper for scrolling the specified target view.

The resulting helper may be configured by chaining setter calls and should be set as a touch listener on the target view.

By default, the helper is disabled and will not respond to touch events until it is enabled using setEnabled.

Parameters
target: View

The view to automatically scroll.

Public functions

canTargetScrollHorizontally

Added in 1.1.0
abstract fun canTargetScrollHorizontally(direction: Int): Boolean

Override this method to return whether the target view can be scrolled horizontally in a certain direction.

Parameters
direction: Int

Negative to check scrolling left, positive to check scrolling right.

Returns
Boolean

true if the target view is able to horizontally scroll in the specified direction.

canTargetScrollVertically

Added in 1.1.0
abstract fun canTargetScrollVertically(direction: Int): Boolean

Override this method to return whether the target view can be scrolled vertically in a certain direction.

Parameters
direction: Int

Negative to check scrolling up, positive to check scrolling down.

Returns
Boolean

true if the target view is able to vertically scroll in the specified direction.

isEnabled

Added in 1.1.0
fun isEnabled(): Boolean
Returns
Boolean

True if this helper is enabled and responding to touch events.

isExclusive

Added in 1.1.0
fun isExclusive(): Boolean

Indicates whether the scroll helper handles touch events exclusively during scrolling.

Returns
Boolean

True if exclusive handling of touch events during scrolling is enabled, false otherwise.

See also
setExclusive

onTouch

Added in 1.1.0
fun onTouch(v: View!, event: MotionEvent!): Boolean

Handles touch events by activating automatic scrolling, adjusting scroll velocity, or stopping.

If isExclusive is false, always returns false so that the host view may handle touch events. Otherwise, returns true when automatic scrolling is active and false otherwise.

scrollTargetBy

Added in 1.1.0
abstract fun scrollTargetBy(deltaX: Int, deltaY: Int): Unit

Override this method to scroll the target view by the specified number of pixels.

Parameters
deltaX: Int

The number of pixels to scroll by horizontally.

deltaY: Int

The number of pixels to scroll by vertically.

setActivationDelay

Added in 1.1.0
fun setActivationDelay(delayMillis: Int): AutoScrollHelper

Sets the delay after entering an activation edge before activation of auto-scrolling. By default, the activation delay is set to getTapTimeout.

Specifying a delay of zero will start auto-scrolling immediately after the touch position enters an activation edge.

Parameters
delayMillis: Int

The activation delay in milliseconds.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setEdgeType

Added in 1.1.0
fun setEdgeType(type: Int): AutoScrollHelper

Sets the activation edge type, one of:

  • EDGE_TYPE_INSIDE for edges that respond to touches inside the bounds of the host view. If touch moves outside the bounds, scrolling will stop.
  • EDGE_TYPE_INSIDE_EXTEND for inside edges that continued to scroll when touch moves outside the bounds of the host view.
  • EDGE_TYPE_OUTSIDE for edges that only respond to touches that move outside the bounds of the host view.
Parameters
type: Int

The type of edge to use.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setEnabled

Added in 1.1.0
fun setEnabled(enabled: Boolean): AutoScrollHelper!

Sets whether the scroll helper is enabled and should respond to touch events.

Parameters
enabled: Boolean

Whether the scroll helper is enabled.

Returns
AutoScrollHelper!

The scroll helper, which may used to chain setter calls.

setExclusive

Added in 1.1.0
fun setExclusive(exclusive: Boolean): AutoScrollHelper!

Enables or disables exclusive handling of touch events during scrolling. By default, exclusive handling is disabled and the target view receives all touch events.

When enabled, onTouch will return true if the helper is currently scrolling and false otherwise.

Parameters
exclusive: Boolean

True to exclusively handle touch events during scrolling, false to allow the target view to receive all touch events.

Returns
AutoScrollHelper!

The scroll helper, which may used to chain setter calls.

setMaximumEdges

Added in 1.1.0
fun setMaximumEdges(horizontalMax: Float, verticalMax: Float): AutoScrollHelper

Sets the absolute maximum edge size.

If relative edge size is not specified, activation edges will always be the maximum edge size. If both relative and maximum edges are specified, the maximum edge will be used to constrain the calculated relative edge size.

Parameters
horizontalMax: Float

The maximum horizontal edge size in pixels, or NO_MAX to use the unconstrained calculated relative value.

verticalMax: Float

The maximum vertical edge size in pixels, or NO_MAX to use the unconstrained calculated relative value.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setMaximumVelocity

Added in 1.1.0
fun setMaximumVelocity(horizontalMax: Float, verticalMax: Float): AutoScrollHelper

Sets the absolute maximum scrolling velocity.

If relative velocity is not specified, scrolling will always reach the same maximum velocity. If both relative and maximum velocities are specified, the maximum velocity will be used to clamp the calculated relative velocity.

Parameters
horizontalMax: Float

The maximum horizontal scrolling velocity, or NO_MAX to leave the relative value unconstrained.

verticalMax: Float

The maximum vertical scrolling velocity, or NO_MAX to leave the relative value unconstrained.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setMinimumVelocity

Added in 1.1.0
fun setMinimumVelocity(horizontalMin: Float, verticalMin: Float): AutoScrollHelper

Sets the absolute minimum scrolling velocity.

If both relative and minimum velocities are specified, the minimum velocity will be used to clamp the calculated relative velocity.

Parameters
horizontalMin: Float

The minimum horizontal scrolling velocity, or NO_MIN to leave the relative value unconstrained.

verticalMin: Float

The minimum vertical scrolling velocity, or NO_MIN to leave the relative value unconstrained.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setRampDownDuration

Added in 1.1.0
fun setRampDownDuration(durationMillis: Int): AutoScrollHelper

Sets the amount of time after de-activation of auto-scrolling that is takes to slow to a stop.

Specifying a duration greater than zero prevents sudden jumps in velocity.

Parameters
durationMillis: Int

The ramp-down duration in milliseconds.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setRampUpDuration

Added in 1.1.0
fun setRampUpDuration(durationMillis: Int): AutoScrollHelper

Sets the amount of time after activation of auto-scrolling that is takes to reach target velocity for the current touch position.

Specifying a duration greater than zero prevents sudden jumps in velocity.

Parameters
durationMillis: Int

The ramp-up duration in milliseconds.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setRelativeEdges

Added in 1.1.0
fun setRelativeEdges(horizontal: Float, vertical: Float): AutoScrollHelper

Sets the activation edge size relative to the host view's dimensions.

If both relative and maximum edges are specified, the maximum edge will be used to constrain the calculated relative edge size.

Parameters
horizontal: Float

The horizontal edge size as a fraction of the host view width, or RELATIVE_UNSPECIFIED to always use the maximum value.

vertical: Float

The vertical edge size as a fraction of the host view height, or RELATIVE_UNSPECIFIED to always use the maximum value.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.

setRelativeVelocity

Added in 1.1.0
fun setRelativeVelocity(horizontal: Float, vertical: Float): AutoScrollHelper

Sets the target scrolling velocity relative to the host view's dimensions.

If both relative and maximum velocities are specified, the maximum velocity will be used to clamp the calculated relative velocity.

Parameters
horizontal: Float

The target horizontal velocity as a fraction of the host view width per second, or RELATIVE_UNSPECIFIED to ignore.

vertical: Float

The target vertical velocity as a fraction of the host view height per second, or RELATIVE_UNSPECIFIED to ignore.

Returns
AutoScrollHelper

The scroll helper, which may used to chain setter calls.