Canvas Draw Line Issue

I have a android tablet and I am having problems dragging prevX to x1, prevY to y1, currentX to x2 and currentY to y2 and I am trying to create a DigitalDraw1 project

Is there a way I could fix this problem?

See this tutorial

http://appinventor.mit.edu/explore/ai2/paintpot-part1

Those orange potato eyes (parameters?) at the top of the event block are not meant to be dragged directly.

Instead, you must hover your cursor over the potato eye you want, and see a getter and a setter block pop up, which you can then drag. The potato eyes in themselves are not draggable.

image

This is helpful, but my problem is when I press prevX it does not let me select it and drag it to x1, when I press it instead, it drags the whole block, not prevX.

Assuming you use computer, hover with mouse, then the dropdown will appear. not sure how it works if using touchscreen?

Yeah, on touch device it works differently. You have to hold the touch on the parameter(in your case eg: prevX), then it gives the choice of get, or set parameter, and then you can drag whichever you want, but on my phone the hold touch is also collapse the draw block so you have to tap on it again to see the whole block and than you can drag the get or set block to position. I suppose it works the same way on tablet too.