Android面试题避坑：ViewPager中的Framgent如何实现懒加载？

@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);
    if (getUserVisibleHint()) {
        isVisible = true;
        onVisible();
    } else {
        isVisible = false;
        onInVisible();
    }
}

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?, persistentState: PersistableBundle?) {
        super.onCreate(savedInstanceState, persistentState)
        setContentView(R.layout.activity_main)
        val viewPager: ViewPager = findViewById(R.id.viewpager)
        val fragmentList: MutableList<Fragment> = ArrayList()
        fragmentList.add(Fragment1())
        fragmentList.add(Fragment2())
        fragmentList.add(Fragment3())
        // 为MyPagerAdapter适配器设置FragmentPagerAdapter.BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT 参数
        val myPagerAdapter: MyPagerAdapter = MyPagerAdapter(
            getSupportFragmentManager(),
            FragmentPagerAdapter.BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT, fragmentList
        )
        viewPager.setAdapter(myPagerAdapter)
        // 设置预加载为3页，来测试懒加载是否成功
        viewPager.offscreenPageLimit = 3
    }

    class MyPagerAdapter(
        fm: FragmentManager,
        behavior: Int,
        val fragmentList: List<Fragment>
    ) :
        FragmentPagerAdapter(fm, behavior) {

        override fun getCount() = fragmentList.size
        override fun getItem(position: Int) = fragmentList[position]

    }
}

//FragmentPagerAdapter.java

 public FragmentPagerAdapter(@NonNull FragmentManager fm,
        @Behavior int behavior) {
    mFragmentManager = fm;
    mBehavior = behavior;
}

@Override
public Object instantiateItem(@NonNull ViewGroup container, int position) {
    ...
    if (fragment != mCurrentPrimaryItem) {
        fragment.setMenuVisibility(false);
        // mBehaviour为1的时候走新逻辑
        if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
            // 初始化item时将其生命周期限制为STARTED
            mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.STARTED);
        } else {
            // 兼容旧版逻辑
            fragment.setUserVisibleHint(false);
        }
    }

    return fragment;
}

@Override
public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
    Fragment fragment = (Fragment)object;
    if (fragment != mCurrentPrimaryItem) {
        if (mCurrentPrimaryItem != null) {
            mCurrentPrimaryItem.setMenuVisibility(false);
            if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
                ...
                // 滑走的会变成非主item, 设置其Lifecycle为STARTED
                mCurTransaction.setMaxLifecycle(mCurrentPrimaryItem, Lifecycle.State.STARTED);
            } else {
                mCurrentPrimaryItem.setUserVisibleHint(false);
            }
        }
        fragment.setMenuVisibility(true);
        if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
            ...
            // 设置新滑到的主item的Lifecycle为RESUMED
            mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.RESUMED);
        } else {
            fragment.setUserVisibleHint(true);
        }
        mCurrentPrimaryItem = fragment;
    }
}

@NonNull
public FragmentTransaction setMaxLifecycle(@NonNull Fragment fragment,
        @NonNull Lifecycle.State state) {
    addOp(new Op(OP_SET_MAX_LIFECYCLE, fragment, state));
    return this;
}

void executeOps() {
    final int numOps = mOps.size();
    for (int opNum = 0; opNum < numOps; opNum++) {
        final Op op = mOps.get(opNum);
        final Fragment f = op.mFragment;
        //...
        switch (op.mCmd) {
            //...
            // 新引入的这个Op类型, 在这里会给这个Fragment设置允许的生命周期上限
            case OP_SET_MAX_LIFECYCLE:
                mManager.setMaxLifecycle(f, op.mCurrentMaxState);
                break;
            //...
        }
}

void setMaxLifecycle(@NonNull Fragment f, @NonNull Lifecycle.State state) {
    //...
    f.mMaxState = state;
}

void moveToExpectedState() {
    try {
        ...
        // 循环计算声明周期是否可以推进
        while ((newState = computeExpectedState()) != mFragment.mState) {
            if (newState > mFragment.mState) {
                // 生命周期向前推进
                int nextStep = mFragment.mState + 1;
                //...
                switch (nextStep) {
                    //...
                    case Fragment.ACTIVITY_CREATED:
                        //...
                    case Fragment.STARTED:
                        start();
                        break;
                    //...
                    case Fragment.RESUMED:
                        resume();
                        break;
                }
            } else {
                // 如果应有的生命周期小于当前, 后退
                int nextStep = mFragment.mState - 1;
                //...
                switch (nextStep) {
                   // 与上面的switch类似
                   //...
                }
            }
        }
        ...
    }
    ...
}

int computeExpectedState() {
    // 其他计算expected state的逻辑, 算出maxState
    //...

    // mMaxState 对生命周期做出限制
    switch (mFragment.mMaxState) {
        case RESUMED:
            break;
        case STARTED:
            maxState = Math.min(maxState, Fragment.STARTED);
            break;
        case CREATED:
            maxState = Math.min(maxState, Fragment.CREATED);
            break;
        default:
            maxState = Math.min(maxState, Fragment.INITIALIZING);
    }

    // 其他计算expected state的逻辑, 算出 maxState
    // ...
    return maxState;
}