I used AGSSimpleMarkerSymbol, but I dong know how to auto zoom it when the mapscale changes
Looking at the title of the post, I don't think you're talking about Reference Scale.
I think you want to place a circular polygon. You'd do this as follows:
You shouldn't create a new AGSGraphicsOverlay each time you create a circle. An overlay can contain many graphics. I think you're already doing steps 3, 4, and 5 but with a point graphic rather than a polygon graphic.
Can you provide some more information please? Are you trying to represent a fixed physical area with the circle, or are you talking more about a reference scale?
Reference Scale means that the symbol is a specific screen size at a specific map scale, and as you zoom the map the symbol's screen size changes appropriately to match.
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