How: Drag the image into another image above

I was in a UIView inside plus two subview (imageview), I want one picture to another by dragging the picture above, the effect is now dragged to another picture when it directly to him below, and be covered . How to make a picture by dragging the image displayed above fixed?
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First calculate the coordinates of the clicked view in the parent view, and then move it to modify the coordinates in the parent view click view it.
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Two pictures the same size it? If you like you can be the first picture removeFromSubView, and then the second picture addSubView.
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Adjusted index