探究Fragment的onResume和onPause方法

Fragment(碎片)在Activity中的使用是非常频繁的，Activity使用Fragment主要有直接通过FragmentManager添加到Activity视图中，或者通过ViewPager加入多个Fragment，通常为了用户体验保存隐藏Fragment的状态，我们都是通过FragmentManager.hide和show方法进行切换Fragment的或者ViewPager.setOffscreenPageLimit()不销毁隐藏的Fragment，这样会导致不显示的Fragmnent也会执行onResume和onPause方法，下面我测试了一下不同方式添加Fragment的结果：
测试的Fragmnet：

public class TestFragment extends Fragment {
    private static final String TAG = "TestFragment";
    protected String mName;

    public static TestFragment create(String mName) {
        TestFragment fragment = new TestFragment();
        fragment.mName = mName;
        return fragment;
    }

    public void setName(String mName) {
        this.mName = mName;
    }

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        TextView view = new TextView(getContext());
        view.setText(mName);
        return view;
    }

    @Override
    public void onResume() {
        Log.e(TAG, mName + "==>onResume,isHidden=" + isHidden() + ",getUserVisibleHint=" + getUserVisibleHint());
        super.onResume();
    }

    @Override
    public void onPause() {
        Log.e(TAG, mName + "==>onPause,isHidden=" + isHidden() + ",getUserVisibleHint=" + getUserVisibleHint());
        super.onPause();
    }

    @Override
    public void onHiddenChanged(boolean hidden) {
        Log.e(TAG, mName + "==>onHiddenChanged,isHidden=" + hidden + ",getUserVisibleHint=" + getUserVisibleHint());
        super.onHiddenChanged(hidden);
    }

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        Log.e(TAG, mName + "==>setUserVisibleHint,isHidden=" + isHidden() + ",getUserVisibleHint=" + isVisibleToUser);
        super.setUserVisibleHint(isVisibleToUser);
    }
}

在Activity-Fragments，Fragment进行切换并退出的log:

在Activity-ViewPager-Fragments初始化log:

ViewPager进行切换并退出的log:

Activity-Fragment-ViewPager-Fragments初始化log:

ViewPager切换并退出log:

由测试得出不同点:
1、通过FragmentManager添加的Fragment在切换时会调用onHiddenChanged(boolean hidden)其中getUserVisibleHint()是一直true
2、ViewPager添加的Fragment切换时会调用setUserVisibleHint(boolean isVisibleToUser)其中isHidden()一直未false，而且初始化时我们设置了setOffscreenPageLimit(3),导致全部Fragment都初始化了

相同点：Fragment的onResume和onPause都会随Activity的onResume和onPause一起被调用

现在根据测试结果改一下TestFragment:

public class TestFragment extends Fragment {
    private static final String TAG = "TestFragment";
    protected String mName;
    private boolean isPageResume;

    public static TestFragment create(String mName) {
        TestFragment fragment = new TestFragment();
        fragment.mName = mName;
        return fragment;
    }

    public void setName(String mName) {
        this.mName = mName;
    }

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        TextView view = new TextView(getContext());
        view.setText(mName);
        return view;
    }

    @Override
    public void onResume() {
        Log.e(TAG, mName + "==>onResume,isHidden=" + isHidden() + ",getUserVisibleHint=" + getUserVisibleHint());
        if (!isPageResume && !isHidden() && getUserVisibleHint()) {
            onPageResume();
        }
        super.onResume();
    }

    @Override
    public void onPause() {
        Log.e(TAG, mName + "==>onPause,isHidden=" + isHidden() + ",getUserVisibleHint=" + getUserVisibleHint());
        if (isPageResume) {
            onPagePause();
        }
        super.onPause();
    }

    @Override
    public void onHiddenChanged(boolean hidden) {
        Log.e(TAG, mName + "==>onHiddenChanged,isHidden=" + hidden + ",getUserVisibleHint=" + getUserVisibleHint());
        if (hidden) {
            if (isPageResume && getUserVisibleHint()) {
                onPagePause();
            }
        } else {
            if (!isPageResume && getUserVisibleHint()) {
                onPageResume();
            }
        }
        super.onHiddenChanged(hidden);
    }

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        Log.e(TAG, mName + "==>setUserVisibleHint,isHidden=" + isHidden() + ",getUserVisibleHint=" + isVisibleToUser);
        if (isVisibleToUser) {
            if (!isPageResume && !isHidden()) {
                onPageResume();
            }
        } else {
            if (isPageResume && !isHidden()) {
                onPagePause();
            }
        }

        super.setUserVisibleHint(isVisibleToUser);
    }

    protected void onPageResume() {
        Log.e(TAG, mName + "==>onPageResume");
        isPageResume = true;
    }

    protected void onPagePause() {
        Log.e(TAG, mName + "==>onPagePause");
        isPageResume = false;
    }

}

这样就可以像使用Activity的onResume和onPause那样使用TestFragment的onPageResume()和onPagePause()方法。