Question

我想检测我使用ViewPager设置的三个片段的特定片段何时出现在屏幕上。我在viewpager中有3个片段，并使用

设置了viewpager

mViewPager.setOffscreenPageLimit(2);

因此所有三个片段一次构建（因为它在每一侧存储2个）。我检查了Fragment.onResume（）只被调用一次而不是当每个片段到达前台时。是否有另一个回调以确保我可以捕获此事件并为其发送一些分析信号？

由于

Answer 1

You can use the ViewPager.setOnPageChangeListener() to set a ViewPager.OnPageChangeListener().

Use the listener for the selected page based on its position in the PagerAdapter.

Here's my sample app:

package net.ynotapps.testviewpagerfragment;

import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;

import butterknife.ButterKnife;
import butterknife.InjectView;


public class MainActivity extends ActionBarActivity {


    @InjectView(R.id.pager)
    ViewPager pager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ButterKnife.inject(this);

        CustomPagerAdapter adapter = new CustomPagerAdapter(getSupportFragmentManager());
        pager.setAdapter(adapter);
        pager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

            }

            @Override
            public void onPageSelected(int position) {

                // Put Code here //

                // Example code //
                if (position == 2) {
                    Toast.makeText(MainActivity.this, "Found Position 2 fragment", Toast.LENGTH_SHORT).show();
                }
            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });
    }


    public static class CustomPagerAdapter extends FragmentPagerAdapter {

        public CustomPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            CustomFragment fragment = new CustomFragment();
            Bundle bundle = new Bundle();
            bundle.putString("display_text", String.format("Fragment %d is here.", position));
            fragment.setArguments(bundle);
            return fragment;
        }

        @Override
        public int getCount() {
            return 5;
        }
    }

    public static class CustomFragment extends Fragment {

        @Override
        public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            TextView tv = new TextView(getActivity());
            tv.setText(getArguments().getString("display_text"));
            return tv;
        }
    }
}

Answer 2

ViewPager源代码：

// Dispatch the change to any listeners
        if (mAdapterChangeListeners != null && !mAdapterChangeListeners.isEmpty()) {
            for (int i = 0, count = mAdapterChangeListeners.size(); i < count; i++) {
                mAdapterChangeListeners.get(i).onAdapterChanged(this, oldAdapter, adapter);
            }
        }

因此，您可以使用ViewPager.addOnAdapterChangeListener来监听回调。

当片段寻呼机适配器的片段进入屏幕时，会调用哪个生命周期回调？

2 个答案: