[Qgis-user] Qgis2.8 mapcomposer related plugin

[邓冲]

My environment is :window 7, Qgis2.8.2, standalone install.
I'm developing a qgis python plugin about mapcomposer.
My qgis project has some mapcomposer template, which contains some soild combination of base shp layers.For some reason, i have to choose the mapcomposer mapitem's "Lock layers for map item" and "Lock layer styles for map item" options, then i need add a shp layer to mainwindow's canvas and update that layer to the mapcomposer item. What makes me mad is, when i choose that two options, i can't add the layer to mapitem; when i cancle them, it will update all mainwindow's layer to the mapitem, which is useless. 
Here is my question, how to add a layer to the locked map composer item.
If this is impossible, i have thought another way, that is add a new composer map item which contains the one layer i want, then make the item transparent and the same extent with the former item. Could anybody tell me if that is workable or not? If that works, may be you can tell me how to add a mapitem with a soild layer, not all layers in the main canvas. 
Any help is grateful, thank you!! 		 	   		  
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