Android基礎——Activity生命週期

1.ViewPager事件

ViewPager通常和Fragment配合一起使用，能夠使用適當的方式去管理每一頁的生命週期，並使用PagerAdapter去生成每一頁顯示的view數據，向左向右可以滑出每頁視圖。

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.
     */
    public 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.
     */
    // 一個新頁被調用時執行,仍爲原來的page時，該方法不被調用
    public void onPageSelected(int position) {
        for (int i = 0; i < imageViews.length; i++) {
            imageViews[position]
                    .setBackgroundResource(R.drawable.guide_dot_green);
            if (position != i) {
                imageViews[i]
                        .setBackgroundResource(R.drawable.guide_dot_normal);
            }
        }
    }

    /**
     * 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
     */
    /*
     * SCROLL_STATE_IDLE: pager處於空閒狀態
     * SCROLL_STATE_DRAGGING： pager處於正在拖拽中
     * SCROLL_STATE_SETTLING： pager正在自動沉降，相當於鬆手後，pager恢復到一個完整pager的過程
     */
    public void onPageScrollStateChanged(int state) {
        // System.out.println("state:" + state);

    }
    /*
     * { private void setScrollState(int newState) { // ...
     * if(mOnPageChangeListener != null) {
     * mOnPageChangeListener.onPageScrollStateChanged(newState);//調用 } }
     */
}

2.PagerAdapter

//PagerAdapter是object的子類
class MyAdapter extends PagerAdapter {

    /**
     * PagerAdapter管理數據大小
     */
    @Override
    public int getCount() {
        return views.size();
    }

    /**
     * 關聯key 與 obj是否相等，即是否爲同一個對象
     */
    @Override
    public boolean isViewFromObject(View view, Object obj) {
        return view == obj; // key
    }

    /**
     * 銷燬當前page的相隔2個及2個以上的item時調用
     */
    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((View) object); // 將view 類型 的object熊容器中移除,根據key
    }

    /**
     * 當前的page的前一頁和後一頁也會被調用，如果還沒有調用或者已經調用了destroyItem
     */
    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        container.addView(views.get(position));
        return views.get(position); // 返回該view對象，作爲key
    }
}

3.FragmentPagerAdapter

用戶訪問的每頁數據都保存在內存中，因此適合訪問頁數不是很多的情況！使用至需要複寫getItem(int) and getCount()方法，如果頁數很多可以使用FragmentStatePagerAdapter，通過bundle保存每頁的狀態信息，因而佔用內存就很小。

                        

public class MainActivity extends FragmentActivity implements TabListener,
        OnPageChangeListener {

    private ActionBar actionBar;
    private final static int TYPE_YULE = 0;
    private final static int TYPE_NEWS = 1;
    private final static int TYPE_SPORT = 2;

    private static List<Fragment> fragments;
    private ViewPager viewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        viewPager = (ViewPager) findViewById(R.id.container);
        actionBar = getActionBar();
        fragments = new ArrayList<Fragment>();
        fragments.add(new YuleFragment());
        fragments.add(new NewsFragment());
        fragments.add(new SportFragment());

        // actionBar.setBackgroundDrawable(getResources().getDrawable(R.drawable.appmgr_btn_upgrade_selected));
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        actionBar.addTab(actionBar.newTab().setText("娛樂").setTag(TYPE_YULE)
                .setTabListener(this));
        actionBar.addTab(actionBar.newTab().setText("新聞").setTag(TYPE_NEWS)
                .setTabListener(this));
        actionBar.addTab(actionBar.newTab().setText("體育").setTag(TYPE_SPORT)
                .setTabListener(this));

        viewPager.setAdapter(new MyAdapter(getSupportFragmentManager()));
        viewPager.setOnPageChangeListener(this);

    }

    private static class MyAdapter extends FragmentPagerAdapter {// static

        public MyAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            return fragments.get(position);
        }

        @Override
        public int getCount() {
            return fragments.size();
        }
    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        int tag = Integer.parseInt(tab.getTag().toString());
        //設置當前的viewpager的當前viewItem
        viewPager.setCurrentItem(tag);
    }

    @Override
    public void onPageSelected(int position) {
        //滑動時設置actionbar的當前tab
        actionBar.setSelectedNavigationItem(position);
    }

    @Override
    public void onPageScrollStateChanged(int state) {
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {
    }

    @Override
    public void onPageScrolled(int position, float positionOffset,
            int positionOffsetPixels) {
    }
}

4.FragmentStatePagerAdapter

當有大量頁數時，使用該類型的適配器很合適，類似於listview，當頁面不在可見，fragment對象有可能銷燬，但是，這些fragment的狀態信息被保存下來，類似於activity的狀態保存，從而佔用很少的內存。用法與FragmentPagerAdapter 類似