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:

• Delay after entering activation area before auto-scrolling begins, see setActivationDelay. Default value is getTapTimeout to avoid conflicting with taps.
• Location of activation areas, see setEdgeType. Default value is EDGE_TYPE_INSIDE_EXTEND.
• Size of activation areas relative to view size, see setRelativeEdges. Default value is 20% for both vertical and horizontal edges.
• Maximum size used to constrain relative size, see setMaximumEdges. Default value is NO_MAX.

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.

Inheritors