android Fragment单页面加载，避免重复加载（懒加载）分析

2022-01-02 00:22:21

上面两个页面都是 ViewPager + Fragment实现，Viewpager + Fragment情况下，fragment的生命周期因Viewpager的缓存机制而失去了具体意义

目前问题：每次进入4个Fragment的生命周期都会走一遍，数据同时请求，这就会造成UI初始化较慢

需求：每次进入只加载当前看到界面数据、切换时请求当前选项卡数据、重复切换只加载一次

具体实现原理：

使用Fragment类自带方法setUserVisibleHint()判断当前fragment是否对用户可见，根据回调的isVisibleToUser参数来进行相关的逻辑判断。重写该方法，创建变量isVisible拿到是否可见标志。

但是直接根据isVisible判断就加载数据，可能onCreateView()方法并未执行完毕,此时就会出现NullPointerException空指针异常。所以就需要满足控件初始化完成，用户可见，才能加载数据。

为了方便继承调用，将实现封装到了BaseFragment中

package com.jky.mobilebzt.fragment;
 
import android.content.Context;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.text.TextUtils;
import android.view.ContextThemeWrapper;
import android.view.View;
import android.widget.Toast;
 
 
public  class BaseNewFragment extends Fragment {
 
    private static final String TAG = BaseFragment.class.getSimpleName();
 
    private boolean isFragmentVisible;
    private boolean isReuseView;
    private boolean isFirstVisible;
    private View rootView;
    public Context context;
 
    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        //setUserVisibleHint()有可能在fragment的生命周期外被调用
        if (rootView == null) {
            return;
        }
        if (isFirstVisible && isVisibleToUser) {
            onFragmentFirstVisible();
            isFirstVisible = false;
        }
        if (isVisibleToUser) {
            onFragmentVisibleChange(true);
            isFragmentVisible = true;
            return;
        }
        if (isFragmentVisible) {
            isFragmentVisible = false;
            onFragmentVisibleChange(false);
        }
    }
 
    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        context = new ContextThemeWrapper(getActivity(), android.R.style.Theme_Holo_Light);
        initVariable();
    }
 
    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        if (rootView == null) {
            rootView = view;
            if (getUserVisibleHint()) {
                if (isFirstVisible) {
                    onFragmentFirstVisible();
                    isFirstVisible = false;
                }
                onFragmentVisibleChange(true);
                isFragmentVisible = true;
            }
        }
        super.onViewCreated(isReuseView ? rootView : view, savedInstanceState);
    }
 
    @Override
    public void onDestroyView() {
        super.onDestroyView();
    }
 
    @Override
    public void onDestroy() {
        super.onDestroy();
        initVariable();
    }
 
    private void initVariable() {
        isFirstVisible = true;
        isFragmentVisible = false;
        rootView = null;
        isReuseView = true;
    }
 
 
    private Toast toast;
 
    public void showShortToast(String text) {
        if (!TextUtils.isEmpty(text)) {
            if (toast != null) {
                toast.cancel();
                toast = null;
            }
            if(getActivity()!=null){
                toast = Toast.makeText(getActivity(), text, Toast.LENGTH_SHORT);
                toast.show();
            }
        }
    }
 
    protected void reuseView(boolean isReuse) {
        isReuseView = isReuse;
    }
 
    protected void onFragmentVisibleChange(boolean isVisible) {
 
    }
 
    protected void onFragmentFirstVisible() {
 
    }
 
    protected boolean isFragmentVisible() {
        return isFragmentVisible;
    }
}

在fragment中继承BaseFragment并实现两个方法 onFragmentVisibleChange 和onFragmentFirstVisible

    @Override
    protected void onFragmentVisibleChange(boolean isVisible) {
    }
 
    @Override
    protected void onFragmentFirstVisible() {
    //这里面写请求数据的方法
        PAGE_INDEX = 1;
        getData(0);
    }

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