Some Android Developer may not know well of Linearlayout . This may help you to know sth about the gravity of its chilren;
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1) wahts the diffrence between "gravity " android "layout_gravity"?
gravity: is sth to itself; this tell how you want its content to layout; Its see a TextView ; provided its wd is 200; and its content (text ) measured wd is 150; This Gravity setting will make its text align left ? top? center ? etc...
layout_gravity: is sth to its parent ; this tells it;s parent how it wants to be layed out; Again this TextView in its parent Linearlayout; Provided parent wd is 300, ht is 400;
2) WHY Does my layout_gravity not work?
layout_gravity is sth relevent to its parent orentation;
If its orentation is horizontal , top , btm , center_vertical will be effective;
else if is vertical : left , right ,center_horizontal is effective;
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THAT ALL FOR TODAY ! MY POOR UBUNTU INPUT METHOD!!!
Android LinearLayout Why Gravity Not Work
原文：http://my.oschina.net/u/592116/blog/396102