場景：開發的過程，如果一個頁面子View比較多，一個螢幕放不下，此時我們大多會採用ScrollView來實現。然後產品可能會提這種需求，比如某個子View在最底部，產品想要頁面進入就直接滑到最底部；或者是當頁面滑到最底部時，點選某個按鈕直接滑到頂部。

還是先呈上方法，然後再來具體分析

mScrollView.fullScroll(ScrollView.FOCUS_UP);//滑到頂部
mScrollView.fullScroll(ScrollView.FOCUS_DOWN);//滑到底部

為什麼可以實現這種效果，我們來看下fullScroll()這個方法的原始碼

/**
  * <p>Handles scrolling in
 
 response to a "home/end" shortcut press. This
  * method will scroll the view to the top or bottom and give the focus
  * to the topmost/bottommost component in the new visible area. If no
  * component is a good candidate for focus, this scrollview reclaims the
  * focus.</p>
  *
  * @param direction the
 
 scroll direction: {@link android.view.View#FOCUS_UP}
  *                  to go the top of the view or
  *                  {@link android.view.View#FOCUS_DOWN} to go the bottom
  * @return true if the key event is consumed by this method, false otherwise
  */
    public boolean fullScroll(int direction) {
        boolean
 
 down = direction == View.FOCUS_DOWN;
        int height = getHeight();

        mTempRect.top = 0;
        mTempRect.bottom = height;

        if (down) {
            int count = getChildCount();
            if (count > 0) {
                View view = getChildAt(count - 1);
                mTempRect.bottom = view.getBottom() + mPaddingBottom;
                mTempRect.top = mTempRect.bottom - height;
            }
        }

        return scrollAndFocus(direction, mTempRect.top, mTempRect.bottom);
    }

這裡我們可以看到實際上是呼叫了scrollAndFocus()方法，繼續來看原始碼

/**
  * <p>Scrolls the view to make the area defined by <code>top</code> and
  * <code>bottom</code> visible. This method attempts to give the focus
  * to a component visible in this area. If no component can be focused in
  * the new visible area, the focus is reclaimed by this ScrollView.</p>
  *
  * @param direction the scroll direction: {@link android.view.View#FOCUS_UP}
  *                  to go upward, {@link android.view.View#FOCUS_DOWN} to downward
  * @param top       the top offset of the new area to be made visible
  * @param bottom    the bottom offset of the new area to be made visible
  * @return true if the key event is consumed by this method, false otherwise
  */
    private boolean scrollAndFocus(int direction, int top, int bottom) {
        boolean handled = true;

        int height = getHeight();
        int containerTop = getScrollY();
        int containerBottom = containerTop + height;
        boolean up = direction == View.FOCUS_UP;

        View newFocused = findFocusableViewInBounds(up, top, bottom);
        if (newFocused == null) {
            newFocused = this;
        }

        if (top >= containerTop && bottom <= containerBottom) {
            handled = false;
        } else {
            int delta = up ? (top - containerTop) : (bottom - containerBottom);
            doScrollY(delta);
        }

        if (newFocused != findFocus()) newFocused.requestFocus(direction);

        return handled;
    }

看到這裡就很明白了吧，最終是呼叫doScrollY()這個方法，所以能夠實現滑動到頂部或者底部的效果

還沒有結束，重點在這裡，請仔細看完 - - ->

fullScroll()這個方法不能直接被呼叫，因為Android中的佈局載入需要時間（跟佈局的巢狀度以及頁面的複雜度等都有關），頁面建立不等於view馬上就會顯示，而是在等待繪製完成，雖然很快，但是如果立即呼叫fullScroll()，此時view可能還沒有完成，這樣就會報異常，應該通過handler來呼叫fullScroll()，具體程式碼如下：

new Handler().post(new Runnable() {
            @Override
            public void run() {
                mScrollView.fullScroll(ScrollView.FOCUS_UP);//滑到頂部
                //mScrollView.fullScroll(ScrollView.FOCUS_DOWN);//滑到底部
            }
        });