如何只實現addOnPageChangeListener的一個或兩個方法

前言

我們知道，監聽ViewPager滑動，需要通過addOnPageChangeListener來監聽，addOnPageChangeListener需要實現以下3個方法：

/**
     * Callback interface for responding to changing state of the selected page.
     */
    public interface OnPageChangeListener {

        /**
         * This method will be invoked when the current page is scrolled, either as part
         * of a programmatically initiated smooth scroll or a user initiated touch scroll.
         *
         * @param position Position index of the first page currently being displayed.
         *                 Page position+1 will be visible if positionOffset is nonzero.
         * @param positionOffset Value from [0, 1) indicating the offset from the page at position.
         * @param positionOffsetPixels Value in pixels indicating the offset from position.
         */
        void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);

        /**
         * This method will be invoked when a new page becomes selected. Animation is not
         * necessarily complete.
         *
         * @param position Position index of the new selected page.
         */
        void onPageSelected(int position);

        /**
         * Called when the scroll state changes. Useful for discovering when the user
         * begins dragging, when the pager is automatically settling to the current page,
         * or when it is fully stopped/idle.
         *
         * @param state The new scroll state.
         * @see ViewPager#SCROLL_STATE_IDLE
         * @see ViewPager#SCROLL_STATE_DRAGGING
         * @see ViewPager#SCROLL_STATE_SETTLING
         */
        void onPageScrollStateChanged(int state);
    }

在實現開發中，很多時候我們並不需要都實現，爲了減少代碼量，那怎麼才能只實現其中一個方法或者實現其中兩個方法呢？

方式

通過源碼，我們可以找到以下代碼：

/**
     * Simple implementation of the {@link OnPageChangeListener} interface with stub
     * implementations of each method. Extend this if you do not intend to override
     * every method of {@link OnPageChangeListener}.
     */
    public static class SimpleOnPageChangeListener implements OnPageChangeListener {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            // This space for rent
        }

        @Override
        public void onPageSelected(int position) {
            // This space for rent
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            // This space for rent
        }
    }

如同註釋所說，當你不需要實現所有方法時，就是用SimpleOnPageChangeListener，在代碼中只需要這樣調用即可：

viewPager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
               
            }
        });

雖然文章很短，但如果文章對你有幫助或者心頭有了“又學到了”的感慨，希望點個喜歡支持一下，謝謝。