Class methods that draw the scroll bar track , scroll box also known as the thumb , and scroll box grip 类方法的参数，该类的方法绘制滚动条轨道、滚动框（也称为滚动块）和滚动框手柄。
The user clicked the scroll bar to the left horizontal or above vertical the scroll box , or pressed the page up key 用户在滚动条上单击了滚动框左侧（水平）或上方（垂直） ，或者按了page up键。
When the user presses the page up or page down key or clicks in the scroll bar track on either side of the scroll box , the 当用户按下page up键或page down键或者在滚动框的任何一边单击滚动条轨迹时，
The user clicked the scroll bar to the right horizontal or below vertical the scroll box , or pressed the page down key . last 用户在滚动条上单击了滚动框右侧（水平）或下方（垂直） ，或者按了page down键。
Similarly , a value halfway between the bottom and top of the range places the leading edge of the scroll box in the middle of the scroll bar 同理，在值范围的底端和顶端中间的值会使滚动框的前端位于滚动条的中间。
In addition to using mouse clicks to change the scroll bar value , a user can also drag the scroll box to any point along the bar 除了可以通过鼠标单击来更改滚动条值以外，用户还可以沿着滚动条将滚动框拖动到任何点。
When the scroll box position is at the minimum value , it moves to the left - most position for horizontal scroll bars or the top position for vertical scroll bars 值。当滚动框的位置值为最小值时，滚动框移到最左端位置（对于水平滚动条） ，或移到顶端位置（对于垂直滚动条） 。
