Question

I am working on an application that can read pdf documents and I am trying to implement zooming and panning on a pdf page.

The page is loaded as a bitmap and displayed in an ImageView. There are also some other functionalities already implemented such as navigating the pages in a document with a bottom custom navigation bar that can be scrolled.

Also, the bottom bar appears when the user taps on the page and disappears with the next tap and when flinging, users should be able to navigate to next/previous page in the document.

All functionality such as the scroll, tap and fling is handled by implementing OnGestureListener in the reader activity and I am trying to do the zoom and pan with an OnTouchListener implementation that is set on the ImageView containing the page.

The code seems to work, however it appears that some of the other events, especially the scroll, are interfering with it which makes it slow.

My question is if there is a better way to go about reconciling everything, since the OnGestureListener is used for the GestureDetector’s handling of onFling, onTapUp and onScroll, but I can’t find a good way to add the zoom/pan code in one of the methods supported by this.

So, if anyone has some experience on handling all these events for one view or some good suggestions/tutorials on this, I would very much appreciate it.

Answer 1

I’m not sure what you mean by ‘some other events such as the scroll make it slow’, could you elaborate?

You should be able to perform zoom and pan using onTouch fairly trivially, without scrolling being an issue if you choose to use Canvas. Full implementation code can be found at Touch and drag image in android for scrolling, and http://www.zdnet.com/blog/burnette/how-to-use-multi-touch-in-android-2-part-6-implementing-the-pinch-zoom-gesture/1847 for zooming.

It’s generally not common to see advanced interactive features applied solely on an ImageView, but that doesn’t mean people haven’t done it (see How can I get zoom functionality for images? for zoom examples including support for multi-touch or Adding Fling Gesture to an image view – Android ).

Regardless of which way you do it, you’ll probably want a GestureDetector in your onTouch function. You should be able to just cut and splice together sections of the tutorials I’ve linked to in order to get full gesture support + zoom/pan. However, if I were doing it, I would probably just use onTouch with Canvas (since Canvas will give better performance) and use the core MotionEvents such as DOWN MOVE UP for greater control, or if I was feeling lazy, use a GestureDetector for trickier bits like flinging.