android.support.v4.view.ViewPager 如何动态更换显示内容

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看一段简单的代码，在一个pager里面显示两个TextView

List<View> items = new ArrayList<View>();
adapter = new MyPagerAdapter();

			TextView tv = new TextView(getActivity());
			tv.setText("第一页");
			items.add(tv);
			tv = new TextView(getActivity());
			tv.setText("第二页");
			items.add(tv);
			pager.setAdapter(adapter);	

Adapter代码

		class MyPagerAdapter extends PagerAdapter{
			
			
			@Override
			public Object instantiateItem(ViewGroup container, int position) {
				View layout = items.get(position);
				container.addView(layout);
				return layout;
			}

			@Override
			public void destroyItem(ViewGroup container, int position, Object object) {
				View layout = items.get(position);
				container.removeView(layout);
			}

			@Override
			public boolean isViewFromObject(View arg0, Object arg1) {
				return arg0 == arg1;

			}

			@Override
			public int getCount() {
				return items.size();
			}

			@Override
			public int getItemPosition(Object object) {
				// TODO Auto-generated method stub
				return super.getItemPosition(object);
			}
		}

代码运行正常，左右滑动都OK。

好，现在需求来了，我需要把“第一页”和“第二页”干掉，再重新添加两个TextView，分别是“第三页”和“第四页”，于是按常理写了如下代码

					items.clear();
					TextView tv = new TextView(getActivity());
					tv.setText("第三页");
					items.add(tv);
					tv = new TextView(getActivity());
					tv.setText("第四页");
					items.add(tv);
					adapter.notifyDataSetChanged();

那我们想是不是ViewPager需要清一下原有的试图呢，好，再加一句

pager.removeAllViews();

蒙了吧，到底什么原因，就是getItemPosition，让我看看当ViewPager的Adapter发生变化时，到底干了啥，Adapter执行notifyDataSetChanged时，会触发ViewPager的dataSetChanged方法，下面是源码，：

    void dataSetChanged() {
        // This method only gets called if our observer is attached, so mAdapter is non-null.

        final int adapterCount = mAdapter.getCount();
        mExpectedAdapterCount = adapterCount;
        boolean needPopulate = mItems.size() < mOffscreenPageLimit * 2 + 1 &&
                mItems.size() < adapterCount;
        int newCurrItem = mCurItem;

        boolean isUpdating = false;
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            final int newPos = mAdapter.getItemPosition(ii.object);
				//看这句话，如果是PagerAdapter.POSITION_UNCHANGED标志，直接返回，不做任何处理
            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }
				//只有返回PagerAdapter.POSITION_NONE标志时，才会对内部视图进行调整
            if (newPos == PagerAdapter.POSITION_NONE) {
				//先从数据源中清除
                mItems.remove(i);
                i--;

                if (!isUpdating) {
                    mAdapter.startUpdate(this);
                    isUpdating = true;
                }
				//在调用Adapter的destroyItem，去销毁视图
                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                    needPopulate = true;
                }
                continue;
            }

            if (ii.position != newPos) {
                if (ii.position == mCurItem) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii.position = newPos;
                needPopulate = true;
            }
        }

        if (isUpdating) {
            mAdapter.finishUpdate(this);
        }

        Collections.sort(mItems, COMPARATOR);

        if (needPopulate) {
            // Reset our known page widths; populate will recompute them.
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor) {
                    lp.widthFactor = 0.f;
                }
            }

            setCurrentItemInternal(newCurrItem, false, true);
            requestLayout();
        }
    }

PagerAdapter.POSITION_UNCHANGED，这个标记决定着视图是否被重新创建，Apdapter默认的代码如下

    public int getItemPosition(Object object) {
        return POSITION_UNCHANGED;
    }

			@Override
			public int getItemPosition(Object object) {
				// TODO Auto-generated method stub
				return POSITION_NONE;
			}

如果返回了POSITION_NONE，但是没有pager.removeAllViews()，那么“第一页”和“第二页”没有消失，“第三页”和“第四页”也上去了，分别和前两页重合了，看图

android.support.v4.view.ViewPager 如何动态更换显示内容

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原文地址：http://blog.csdn.net/bdmh/article/details/48464097