How can I achieve that the points on a Graphics element could be manipulated both by Locators (clicking and moving) and on the Manipulate panel as well.
As you can see, only one of the methods work. In this example only pt1 appears on the Manipulate panel.
So any ideas?
Here's my code:
Manipulate[ Graphics[{Polygon[{pt1, pt2, pt3}]}, PlotRangeClipping -> False, Frame -> True, PlotRange -> {{0, 10}, {0, 10}}], {{pt1, {0, 0}}}, {{pt2, {0, 1}}, Locator}, {{pt3, {1, 1}}, Locator}] 
You can attach multiple controls to a variable:
Manipulate[Graphics[{Polygon[{pt1, pt2, pt3}]}, PlotRangeClipping -> False, Frame -> True, PlotRange -> {{0, 10}, {0, 10}}], {{pt1, {0, 0}}}, {{pt2, {0, 1}}}, {{pt3, {1, 1}}}, Row[{Control@{{pt1, {0, 0}}, {0, 0}, {10, 10}, Slider2D}, Control@{{pt2, {0, 1}}, {0, 0}, {10, 10}, Slider2D}, Control@{{pt3, {1, 1}}, {0, 0}, {10, 10}, Slider2D}}, Spacer[5]], {{pt1, {0, 0}}, Locator}, {{pt2, {0, 1}}, Locator}, {{pt3, {1, 1}}, Locator}] 
Appearance -> "Labeled" to each Slider2D. $\endgroup$ Slider2D finally got the Appearance option in Version 10. In version 9, the option Appearance is not supported. I am using the free cloud version of Version 10, where neither Locator nor Slider2D works. I would be curious to see the how the Labeled Slider2D` looks/works. $\endgroup$ This is not an answer but rather an extended comment to @kguler to demonstrate Appearance -> "Labeled" with Slider2D in version 10.1
$Version "10.1.0 for Mac OS X x86 (64-bit) (March 24, 2015)"
Using @kguler answer
Manipulate[ Graphics[{Polygon[{pt1, pt2, pt3}]}, PlotRangeClipping -> False, Frame -> True, PlotRange -> {{0, 10}, {0, 10}}], {{pt1, {0, 0}}}, {{pt2, {0, 1}}}, {{pt3, {1, 1}}}, Row[{ Control@{{pt1, {0, 0}}, {0, 0}, {10, 10}, Slider2D, Appearance -> "Labeled"}, Control@{{pt2, {0, 1}}, {0, 0}, {10, 10}, Slider2D, Appearance -> "Labeled"}, Control@{{pt3, {1, 1}}, {0, 0}, {10, 10}, Slider2D, Appearance -> "Labeled"}}, Spacer[5]], {{pt1, {0, 0}}, Locator}, {{pt2, {0, 1}}, Locator}, {{pt3, {1, 1}}, Locator}] 
