安卓Fragment资源懒加载

          多个Fragment放到一个Activity里面，Activity启动时会同时加载所有Fragment资源，造成卡顿现象。实际情况我们想要的是只加载当前显示的Fragment资源即可，即懒加载模式，使用Fragment的setUserVisibleHint和getUserVisibleHint 。

下面是一段官方的描述：

* Set a hint to the system about whether this fragment's UI is currently visible

* to the user. This hint defaults to true and is persistent across fragment instance

* state save and restore.

*

* <p>An app may set this to false to indicate that the fragment's UI is

* scrolled out of visibility or is otherwise not directly visible to the user.

* This may be used by the system to prioritize operations such as fragment lifecycle updates

* or loader ordering behavior.</p>

*

* @param isVisibleToUser true if this fragment's UI is currently visible to the user (default),

*                        false if it is not.

          public void setUserVisibleHint(boolean isVisibleToUser)

/**

* @return The current value of the user-visible hint on this fragment.

* @see #setUserVisibleHint( boolean)

*/

public boolean getUserVisibleHint() {

    return mUserVisibleHint;

}

总结一句就是isVisibleToUser为Ture时此fragment可见，false时不可见

          Fragment懒加载实现思路就是把Fragment的资源加载写到一个LazyLoad（）函数里面，当fragment可见时对资源进行加载。示例代码如下：

1.           在Fragement中重写setUserVisibleHint方法
   @Override
   public void setUserVisibleHint(boolean isVisibleToUser) {
       super.setUserVisibleHint(isVisibleToUser);
       if(getUserVisibleHint()){
           mIsVisible = true ;
           lazyLoad();
       }else{
           mIsVisible = false ;
       }
   }
2. lazyLoad 负责进行耗时操作，这里需要注意判断是否执行过Create，系统执行流程会先执行到setUserVisibleHint 然后执行onCreate最后再去执行setUserVisibleHint，初始化工作放到onCreate里面，加载资源时判断是否已经执行过onCreate方法，否则第一次执行 setUserVisibleHint  方法时会因为没有初始化报空指针异常。
   private void lazyLoad(){
       if(mIsVisible &&getIsCrearte ()){
            //网络请求，资源加载等费事操作
       }
   }