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：

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
;

}

}
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

}

}