iPhone - cleaning the context of a UIView outside drawRect -

i have uiview subclassed , implemented own drawrect method.

when drawrect method runs first time, grab context in variable using

ctx = uigraphicsgetcurrentcontext(); 

later on code, outside drawrect, trying clean whole context filling transparent color, do:

cgcontextclearrect(ctx, self.bounds); [self setneedsdisplay]; 

the problem context not erased , continues before.

at point ctx not nil.

am missing something?

thanks

is not possible move clearing drawrect? i'm not sure if drawing out side of drawrect possible, , if is, it's not how it's intended work.