Implementing pinch-zoom and pan/drag in an Android view on the canvas

I was trying to get pinch-zoom and panning working on an Android view today. Basically I was trying to implement the same behavior you see when you use Google Maps (for example). You can zoom in and pan around until the edge of the image, but no further. Also, if the image is fully zoomed out, you can’t pan the image. Implementing the pinch-zoom functionality was pretty easy. I found an example on StackOverflow. I then wanted to implement panning (or dragging) as well. However, I wasn’t able to easily find examples and tutorials for this functionality. I started with this example that comes from the third edition of the Hello, Android! book but I didn’t get too far. So I started playing around a little bit with the events and started writing some code from scratch (using the example from Hello, Android!) so that I could have a better idea of what was happening.

As I mentioned before, getting zoom to work was pretty easy. Implementing panning/dragging was the hard part. The major issues I encountered and subsequently fixed were the following:

  1. Panning continues indefinitely in all directions.
  2. When you zoom and then pan, stop, and then start again, the view jerks to a new position instead of panning from the existing position.
  3. Excessive panning towards the left and top can be constrained, but panning towards the right and bottom is not so easily constrained.

Once I fixed all the problems, I figured that it would be nice to document it for future reference, and I also think it would be a useful resource for others who have the same problem. Now a little disclaimer before I go any further: I’m not an Android expert and I’m really not that great with graphics; I just started it learning to program for Android this semester for one of my Masters electives. So there might be a better way of doing all this, and if there is, please let me know! Also, if you want to skip all the explanations and just see the code, you can skip to the last page.

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