A view that allows the scrolling and zooming of its contained views.
- iOS 2.0+
- tvOS 9.0+
The central notion of a
UIScroll object (or, simply, a scroll view) is that it is a view whose origin is adjustable over the content view. It clips the content to its frame, which generally (but not necessarily) coincides with that of the application’s main window. A scroll view tracks the movements of fingers and adjusts the origin accordingly. The view that is showing its content “through” the scroll view draws that portion of itself based on the new origin, which is pinned to an offset in the content view. The scroll view itself does no drawing except for displaying vertical and horizontal scroll indicators. The scroll view must know the size of the content view so it knows when to stop scrolling; by default, it “bounces” back when scrolling exceeds the bounds of the content.
The object that manages the drawing of content displayed in a scroll view should tile the content’s subviews so that no view exceeds the size of the screen. As users scroll in the scroll view, this object should add and remove subviews as necessary.
Because a scroll view has no scroll bars, it must know whether a touch signals an intent to scroll versus an intent to track a subview in the content. To make this determination, it temporarily intercepts a touch-down event by starting a timer and, before the timer fires, seeing if the touching finger makes any movement. If the timer fires without a significant change in position, the scroll view sends tracking events to the touched subview of the content view. If the user then drags their finger far enough before the timer elapses, the scroll view cancels any tracking in the subview and performs the scrolling itself. Subclasses can override the
touches methods (which are called by the scroll view) to affect how the scroll view handles scrolling gestures.
A scroll view also handles zooming and panning of content. As the user makes a pinch-in or pinch-out gesture, the scroll view adjusts the offset and the scale of the content. When the gesture ends, the object managing the content view should update subviews of the content as necessary. (Note that the gesture can end and a finger could still be down.) While the gesture is in progress, the scroll view does not send any tracking calls to the subview.
UIScroll class can have a delegate that must adopt the
UIScroll protocol. For zooming and panning to work, the delegate must implement both
scroll; in addition, the maximum (
maximum) and minimum (
minimum) zoom scale must be different.
If you assign a value to this view’s
restoration property, it attempts to preserve its scrolling-related information between app launches. Specifically, the values of the
content properties are preserved. During restoration, the scroll view restores these values so that the content appears scrolled to the same position as before. For more information about how state preservation and restoration works, see App Programming Guide for iOS.