Android Handler+ViewPager实现图片视频混轮播

       项目中要用到广告轮播，轮播的内容有图片，也有视频。百度一番无果，自己手撸一个。

先看一下效果图，PS:录制的gif，由于上传大小不能超2M，所以效果不太好，视频一闪而过，否则超出限制。

Handler + ViewPager实现轮播图的demo百度一堆，Timer定时，利用handler更新UI。本篇也是一样，不过要注意的是ViewPager的预加载。预加载功能的好处不言而喻，但也要分情况。纯图片的轮播当然不用考虑这一点，但是如果加上视频，混播的话就要格外注意了，因为会出现“未见其图，先闻其声”的问题。

解决方式：除了禁用ViewPager的预加载之外，没有想到更好的解决方式，再次也抛砖引玉，乞求各位大佬告知更好的解决办法，或者更好的实现混轮播的方式。

ViewPager预加载禁用

直接调用ViewPager的setOffscreenPageLimit（int limit），来设置预加载的数量是没有用的。

为什么呢？？看viewPager的源码

如果你调用此方法传递进来的值小于1是无效的，limit也会设置为1，并且DEFAULT_OFFSCREEN_PAGES这个是private修饰，子类继承ViewPager也是不可见的，所以说，单纯的设置是没有卵用的。

亲测可用的方法：

将v4包里的ViewPager的源码完全copy一份，将其中DEFAULT_OFFSCREEN_PAGES的值改为0即可。

在这里直接给大家提供一个我亲测可用的源码，大家可以直接拷贝使用，拿走不谢

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


import android.content.Context;
import android.database.DataSetObserver;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.os.Parcel;
import android.os.Parcelable;
import android.os.SystemClock;
import android.support.v4.os.ParcelableCompat;
import android.support.v4.os.ParcelableCompatCreatorCallbacks;
import android.support.v4.view.KeyEventCompat;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.VelocityTrackerCompat;
import android.support.v4.view.ViewCompat;
import android.support.v4.view.ViewConfigurationCompat;
import android.support.v4.widget.EdgeEffectCompat;
import android.util.AttributeSet;
import android.util.Log;
import android.view.FocusFinder;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.SoundEffectConstants;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.accessibility.AccessibilityEvent;
import android.view.animation.Interpolator;
import android.widget.Scroller;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

/**
 * Layout manager that allows the user to flip left and right
 * through pages of data.  You supply an implementation of a
 * {@link android.support.v4.view.PagerAdapter} to generate the pages that the view shows.
 *
 * <p>Note this class is currently under early design and
 * development.  The API will likely change in later updates of
 * the compatibility library, requiring changes to the source code
 * of apps when they are compiled against the newer version.</p>
 */
public class ViewPager extends ViewGroup {
    private static final String TAG = "NoPreLoadViewPager";
    private static final boolean DEBUG = false;

    private static final boolean USE_CACHE = false;

    private static final int DEFAULT_OFFSCREEN_PAGES = 0;//默认是1，这里直接改为0
    private static final int MAX_SETTLE_DURATION = 600; // ms

    static class ItemInfo {
        Object object;
        int position;
        boolean scrolling;
    }

    private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>(){
        @Override
        public int compare(ItemInfo lhs, ItemInfo rhs) {
            return lhs.position - rhs.position;
        }};

    private static final Interpolator sInterpolator = new Interpolator() {
        public float getInterpolation(float t) {
            // _o(t) = t * t * ((tension + 1) * t + tension)
            // o(t) = _o(t - 1) + 1
            t -= 1.0f;
            return t * t * t + 1.0f;
        }
    };

    private final ArrayList<ItemInfo> mItems = new ArrayList<ItemInfo>();

    private PagerAdapter mAdapter;
    private int mCurItem;   // Index of currently displayed page.
    private int mRestoredCurItem = -1;
    private Parcelable mRestoredAdapterState = null;
    private ClassLoader mRestoredClassLoader = null;
    private Scroller mScroller;
    private PagerObserver mObserver;

    private int mPageMargin;
    private Drawable mMarginDrawable;

    private int mChildWidthMeasureSpec;
    private int mChildHeightMeasureSpec;
    private boolean mInLayout;

    private boolean mScrollingCacheEnabled;

    private boolean mPopulatePending;
    private boolean mScrolling;
    private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;

    private boolean mIsBeingDragged;
    private boolean mIsUnableToDrag;
    private int mTouchSlop;
    private float mInitialMotionX;
    /**
     * Position of the last motion event.
     */
    private float mLastMotionX;
    private float mLastMotionY;
    /**
     * ID of the active pointer. This is used to retain consistency during
     * drags/flings if multiple pointers are used.
     */
    private int mActivePointerId = INVALID_POINTER;
    /**
     * Sentinel value for no current active pointer.
     * Used by {@link #mActivePointerId}.
     */
    private static final int INVALID_POINTER = -1;

    /**
     * Determines speed during touch scrolling
     */
    private VelocityTracker mVelocityTracker;
    private int mMinimumVelocity;
    private int mMaximumVelocity;
    private float mBaseLineFlingVelocity;
    private float mFlingVelocityInfluence;

    private boolean mFakeDragging;
    private long mFakeDragBeginTime;

    private EdgeEffectCompat mLeftEdge;
    private EdgeEffectCompat mRightEdge;

    private boolean mFirstLayout = true;

    private OnPageChangeListener mOnPageChangeListener;

    /**
     * Indicates that the pager is in an idle, settled state. The current page
     * is fully in view and no animation is in progress.
     */
    public static final int SCROLL_STATE_IDLE = 0;

    /**
     * Indicates that the pager is currently being dragged by the user.
     */
    public static final int SCROLL_STATE_DRAGGING = 1;

    /**
     * Indicates that the pager is in the process of settling to a final position.
     */
    public static final int SCROLL_STATE_SETTLING = 2;

    private int mScrollState = SCROLL_STATE_IDLE;

    /**
     * Callback interface for responding to changing state of the selected page.
     */
    public interface OnPageChangeListener {

        /**
         * This method will be invoked when the current page is scrolled, either as part
         * of a programmatically initiated smooth scroll or a user initiated touch scroll.
         *
         * @param position Position index of the first page currently being displayed.
         *                 Page position+1 will be visible if positionOffset is nonzero.
         * @param positionOffset Value from [0, 1) indicating the offset from the page at position.
         * @param positionOffsetPixels Value in pixels indicating the offset from position.
         */
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);

        /**
         * This method will be invoked when a new page becomes selected. Animation is not
         * necessarily complete.
         *
         * @param position Position index of the new selected page.
         */
        public void onPageSelected(int position);

        /**
         * Called when the scroll state changes. Useful for discovering when the user
         * begins dragging, when the pager is automatically settling to the current page,
         * or when it is fully stopped/idle.
         *
         * @param state The new scroll state.
         * @see android.support.v4.view.ViewPager#SCROLL_STATE_IDLE
         * @see android.support.v4.view.ViewPager#SCROLL_STATE_DRAGGING
         * @see android.support.v4.view.ViewPager#SCROLL_STATE_SETTLING
         */
        public void onPageScrollStateChanged(int state);
    }

    /**
     * Simple implementation of the {@link android.support.v4.view.LazyViewPager.OnPageChangeListener} interface with stub
     * implementations of each method. Extend this if you do not intend to override
     * every method of {@link android.support.v4.view.LazyViewPager.OnPageChangeListener}.
     */
    public static class SimpleOnPageChangeListener implements OnPageChangeListener {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            // This space for rent
        }

        @Override
        public void onPageSelected(int position) {
            // This space for rent
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            // This space for rent
        }
    }

    public NoPreloadViewPager(Context context) {
        super(context);
        initViewPager();
    }

    public NoPreloadViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
        initViewPager();
    }

    void initViewPager() {
        setWillNotDraw(false);
        setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
        setFocusable(true);
        final Context context = getContext();
        mScroller = new Scroller(context, sInterpolator);
        final ViewConfiguration configuration = ViewConfiguration.get(context);
        mTouchSlop = ViewConfigurationCompat.getScaledPagingTouchSlop(configuration);
        mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
        mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
        mLeftEdge = new EdgeEffectCompat(context);
        mRightEdge = new EdgeEffectCompat(context);

        float density = context.getResources().getDisplayMetrics().density;
        mBaseLineFlingVelocity = 2500.0f * density;
        mFlingVelocityInfluence = 0.4f;
    }

    private void setScrollState(int newState) {
        if (mScrollState == newState) {
            return;
        }

        mScrollState = newState;
        if (mOnPageChangeListener != null) {
            mOnPageChangeListener.onPageScrollStateChanged(newState);
        }
    }

    public void setAdapter(PagerAdapter adapter) {
        if (mAdapter != null) {
//            mAdapter.unregisterDataSetObserver(mObserver);
            mAdapter.startUpdate(this);
            for (int i = 0; i < mItems.size(); i++) {
                final ItemInfo ii = mItems.get(i);
                mAdapter.destroyItem(this, ii.position, ii.object);
            }
            mAdapter.finishUpdate(this);
            mItems.clear();
            removeAllViews();
            mCurItem = 0;
            scrollTo(0, 0);
        }

        mAdapter = adapter;

        if (mAdapter != null) {
            if (mObserver == null) {
                mObserver = new PagerObserver();
            }
//            mAdapter.registerDataSetObserver(mObserver);
            mPopulatePending = false;
            if (mRestoredCurItem >= 0) {
                mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
                setCurrentItemInternal(mRestoredCurItem, false, true);
                mRestoredCurItem = -1;
                mRestoredAdapterState = null;
                mRestoredClassLoader = null;
            } else {
                populate();
            }
        }
    }

    public PagerAdapter getAdapter() {
        return mAdapter;
    }

    /**
     * Set the currently selected page. If the ViewPager has already been through its first
     * layout there will be a smooth animated transition between the current item and the
     * specified item.
     *
     * @param item Item index to select
     */
    public void setCurrentItem(int item) {
        mPopulatePending = false;
        setCurrentItemInternal(item, !mFirstLayout, false);
    }

    /**
     * Set the currently selected page.
     *
     * @param item Item index to select
     * @param smoothScroll True to smoothly scroll to the new item, false to transition immediately
     */
    public void setCurrentItem(int item, boolean smoothScroll) {
        mPopulatePending = false;
        setCurrentItemInternal(item, smoothScroll, false);
    }

    public int getCurrentItem() {
        return mCurItem;
    }

    void setCurrentItemInternal(int item, boolean smoothScroll, boolean always) {
        setCurrentItemInternal(item, smoothScroll, always, 0);
    }

    void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity) {
        if (mAdapter == null || mAdapter.getCount() <= 0) {
            setScrollingCacheEnabled(false);
            return;
        }
        if (!always && mCurItem == item && mItems.size() != 0) {
            setScrollingCacheEnabled(false);
            return;
        }
        if (item < 0) {
            item = 0;
        } else if (item >= mAdapter.getCount()) {
            item = mAdapter.getCount() - 1;
        }
        final int pageLimit = mOffscreenPageLimit;
        if (item > (mCurItem + pageLimit) || item < (mCurItem - pageLimit)) {
            // We are doing a jump by more than one page.  To avoid
            // glitches, we want to keep all current pages in the view
            // until the scroll ends.
            for (int i=0; i<mItems.size(); i++) {
                mItems.get(i).scrolling = true;
            }
        }

        final bool