Android -- 處理ViewPager的notifyDataSetChanged無重新整理

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Viewpager在調用notifyDataSetChanged()時，介面無重新整理

Viewpager在調用notifyDataSetChanged()時，介面無重新整理，它確實影響我們功能的實現了。可能選擇為Viewpager重新設定一遍適配器adapter，達到重新整理的目的。但是這種方法在大多數情況下，是有問題的。

查看方法

super.notifyDataSetChanged()調用的是PagerAdapter.notifyDataSetChanged()

/**     * This method should be called by the application if the data backing this adapter has changed     * and associated views should update.     */    public void notifyDataSetChanged() {        mObservable.notifyChanged();    }

注釋裡說到，當附加在適配器上的資料發生變化時，應該調用該方法重新整理資料。該方法調用了一個mObservable .notifyChanged()；

我們繼續跟進這個方法，進入DataSetObservable類中，發現這樣一段代碼：

/**     * Invokes {@link DataSetObserver#onChanged} on each observer.     * Called when the contents of the data set have changed.  The recipient     * will obtain the new contents the next time it queries the data set.     */    public void notifyChanged() {        synchronized(mObservers ) {            // since onChanged() is implemented by the app, it could do anything, including            // removing itself from {@link mObservers} - and that could cause problems if            // an iterator is used on the ArrayList {@link mObservers}.            // to avoid such problems, just march thru the list in the reverse order.            for (int i = mObservers .size() - 1; i >= 0; i--) {                mObservers.get(i).onChanged();            }        }    }

這都不是重點，重點我們來看這個mObservers的類型是一個抽象類別DataSetObserver，裡面只有兩個未實現的方法，都有誰使用了這個抽象類別呢，其中我們發現了Viewpager的身影。進入viewpager，我們終於找到了viewpager中控制資料變更的重點方法dataSetChanged ，這個方法如下：

void dataSetChanged () {        // This method only gets called if our observer is attached, so mAdapter is non-null.         boolean needPopulate = mItems .size() < mOffscreenPageLimit * 2 + 1 &&                mItems.size() < mAdapter.getCount();        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 );             if (newPos == PagerAdapter.POSITION_UNCHANGED ) {                continue;            }             if (newPos == PagerAdapter.POSITION_NONE) {                mItems.remove(i);                i--;                 if (!isUpdating) {                    mAdapter.startUpdate( this);                    isUpdating = true;                }                 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, mAdapter.getCount() - 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();        }    }

重點看這樣一行代碼：

final int newPos = mAdapter.getItemPosition(ii.object );    if (newPos == PagerAdapter.POSITION_UNCHANGED ) {         continue ;   }

  Called when the host view is attempting to determine if an item’s position has changed. Returns POSITION_UNCHANGED if the position of the given item has not changed orPOSITION_NONE if the item is no longer present in the adapter.

The default implementation assumes that items will never change position and always returnsPOSITION_UNCHANGED.

意思是如果item的位置如果沒有發生變化，則返回POSITION_UNCHANGED。如果返回了POSITION_NONE，表示該位置的item已經不存在了。預設的實現是假設item的位置永遠不會發生變化，而返回POSITION_UNCHANGED

解決方案

所以我們可以嘗試著修改適配器的寫法，覆蓋getItemPosition()方法，當調用notifyDataSetChanged時，讓getItemPosition方法人為的返回POSITION_NONE，從而達到強迫viewpager重繪所有item的目的。

class SearchAdapter extends PagerAdapter {         private int mChildCount = 0;      @Override     public void notifyDataSetChanged() {                    mChildCount = getCount();           super.notifyDataSetChanged();     }      @Override     public int getItemPosition(Object object)   {                     if ( mChildCount > 0) {           mChildCount --;           return POSITION_NONE;           }           return super.getItemPosition(object);     } }

我是天王蓋地虎的分割線

Android -- 處理ViewPager的notifyDataSetChanged無重新整理