android activity向fragment通信，获取fragment的三种方法

三种方法

第一种：静态fragment

如果是在layout文件中定义的fragment，则使用

getSupportFragmentManager().findFragmentById();

第二种：动态fragment

如果还在程序中代码变更了fragment，则使用

getSupportFragmentManager().findFragmentByTag();

第三种：使用ViewPager装载的fragment

如果使用ViewPager装载fragment时，则需要修改Adapter

• 定义SmartFragmentStatePagerAdapter.java

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.util.SparseArray;
import android.view.ViewGroup;

/* 
Extension of FragmentStatePagerAdapter which intelligently caches 
all active fragments and manages the fragment lifecycles. 
Usage involves extending from SmartFragmentStatePagerAdapter as you would any other PagerAdapter.
*/
public abstract class SmartFragmentStatePagerAdapter extends FragmentStatePagerAdapter {
    // Sparse array to keep track of registered fragments in memory
    private SparseArray registeredFragments = new SparseArray();

    public SmartFragmentStatePagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

    // Register the fragment when the item is instantiated
    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment fragment = (Fragment) super.instantiateItem(container, position);
        registeredFragments.put(position, fragment);
        return fragment;
    }

    // Unregister when the item is inactive
    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        registeredFragments.remove(position);
        super.destroyItem(container, position, object);
    }

    // Returns the fragment for the position (if instantiated)
    public Fragment getRegisteredFragment(int position) {
        return registeredFragments.get(position);
    }
}

• 将SectionsPagerAdapter继承于SmartFragmentStatePagerAdapter

/**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends SmartFragmentStatePagerAdapter {

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

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            switch (position) {
                case 0:
                    return Fragment1.newInstance(position);
                case 1:
                    return Fragment2.newInstance(position);
                default:
                    return null;
            }
        }

        @Override
        public int getCount() {
            // Show 2 total pages.
            return 2;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "SECTION 0";
                case 1:
                    return "SECTION 1";
            }
            return null;
        }
    }

• 在onCreate()等中加入初始化代码

mSmartFragmentStatePagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.fragment_container);
mViewPager.setAdapter(mSmartFragmentStatePagerAdapter);

• 使用的时候只需下面的代码即可

((Fragment1) mSmartFragmentStatePagerAdapter.getRegisteredFragment(1));