LazyViewPager.java

package com.yidianling.dynamic.adapter;

import android.content.Context;
import android.content.res.TypedArray;
import androidx.viewpager.widget.PagerAdapter;
import androidx.viewpager.widget.ViewPager;
import android.util.AttributeSet;
import android.view.MotionEvent;

import com.yidianling.dynamic.R;


/**
 * 懒加载viewPager
 * Created by xiongyu on 2017/3/27.
 */

public class LazyViewPager extends ViewPager {

    private static final float DEFAULT_OFFSET = 0.5f;

    private LazyPagerAdapter mLazyPagerAdapter;
    private float mInitLazyItemOffset = DEFAULT_OFFSET;

    public LazyViewPager(Context context) {
        super(context);
    }

    public LazyViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);

        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.LazyViewPager);
        setInitLazyItemOffset(a.getFloat(R.styleable.LazyViewPager_init_lazy_item_offset, DEFAULT_OFFSET));
        a.recycle();
    }

    /**
     * change the initLazyItemOffset
     * @param initLazyItemOffset set mInitLazyItemOffset if {@code 0 < initLazyItemOffset <= 1}
     */
    public void setInitLazyItemOffset(float initLazyItemOffset) {
        if (initLazyItemOffset > 0 && initLazyItemOffset <= 1) {
            mInitLazyItemOffset = initLazyItemOffset;
        }
    }

    @Override
    public void setAdapter(PagerAdapter adapter) {
        super.setAdapter(adapter);
        mLazyPagerAdapter = adapter != null && adapter instanceof LazyPagerAdapter ? (LazyPagerAdapter) adapter : null;
    }

    @Override
    protected void onPageScrolled(int position, float offset, int offsetPixels) {
        if (mLazyPagerAdapter != null) {
            if (getCurrentItem() == position) {
                int lazyPosition = position + 1;
                if (offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
                    mLazyPagerAdapter.startUpdate(this);
                    mLazyPagerAdapter.addLazyItem(this, lazyPosition);
                    mLazyPagerAdapter.finishUpdate(this);
                }
            } else if (getCurrentItem() > position) {
                int lazyPosition = position;
                if (1 - offset >= mInitLazyItemOffset && mLazyPagerAdapter.isLazyItem(lazyPosition)) {
                    mLazyPagerAdapter.startUpdate(this);
                    mLazyPagerAdapter.addLazyItem(this, lazyPosition);
                    mLazyPagerAdapter.finishUpdate(this);
                }
            }
        }
        super.onPageScrolled(position, offset, offsetPixels);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        int curPosition;

        switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN:
                getParent().requestDisallowInterceptTouchEvent(true);
                break;
            case MotionEvent.ACTION_MOVE:
                curPosition = this.getCurrentItem();
                int count = this.getAdapter().getCount();
//                Log.i(TAG, "curPosition:=" +curPosition);
                // 当当前页面在最后一页和第0页的时候，由父亲拦截触摸事件
                if (curPosition == count - 1|| curPosition==0) {
                    getParent().requestDisallowInterceptTouchEvent(false);
                } else {//其他情况，由孩子拦截触摸事件
                    getParent().requestDisallowInterceptTouchEvent(true);
                }

        }
        return super.dispatchTouchEvent(ev);
    }

}