Fake dragging

Examining the ViewPager #3

This article is part of a series of articles about the ViewPager component. Click here to see a list of all articles of this series.

Horizontal scrolling pages

Have you ever tried putting horizontal scrolling components inside a ViewPager? Well, since revision 9 of the support library this is supported by the ViewPager. As long as the inner component can scroll horizontally this component will be scrolled. Whenever the component can't be further scrolled the ViewPager will handle the touch events and you start to switch to the next page. This works out-of-the-box for scrolling view components of Android like the WebView.

Internally the ViewPager uses ViewCompat.canScrollHorizontally(View v, int direction) to determine if a child view can be scrolled horizontally and should receive the according touch events. Unfortunately this method is only implemented for Android 4.0 (API level 14) and above. For all earlier versions this method will always return false and therefore never scroll the components inside the ViewPager.

 

Fake dragging

The ViewPager supports fake dragging. Fake dragging can be used to simulate a dragging event/animation, e.g. for detecting drag events on a different component and delegating these to the ViewPager.

You have to signal the ViewPager when to start or end a fake drag by calling beginFakeDrag() and endFakeDrag() on it. After starting a fake drag you can use fakeDragBy(float) to drag the ViewPager by the given amount of pixels along the x axis (negative values to the left and positive values to the right).

The following example uses a SeekBar whose current progress state is used to fake drag a ViewPager by the given percentage.