箭头的使用

  <Button
                         android:layout_width="48dip"
                         android:layout_height="48dip"
                         android:layout_toLeftOf="@id/next"
                         android:layout_centerVertical="true"
                         android:text="◀"
                         android:textAppearance="@android:style/TextAppearance.Large"
                         android:id="@+id/previous"
                        
                 />