Android -- 處理ViewPager的notifyDataSetChanged無刷新

Viewpager在調用notifyDataSetChanged()時，界面無刷新

Viewpager在調用notifyDataSetChanged()時，界面無刷新，它確實影響咱們功能的實現了。可能選擇爲Viewpager從新設置一遍適配器adapter，達到刷新的目的。可是這種方法在大多數狀況下，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();
    }

註釋裏說到，當附加在適配器上的數據發生變化時，應該調用該方法刷新數據。該方法調用了一個mObservable .notifyChanged()；

咱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，重點咱們來看這個mObservers的類型是一個抽象類DataSetObserver，裏面只有兩個未實現的方法，都有誰使用了這個抽象類呢，其中咱們發現了Viewpager的身影。進入viewpager，咱們終於找到了viewpager中控制數據變動的重點方法dataSetChanged ，這個方法以下：

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.

意思是若是item的位置若是沒有發生變化，則返回POSITION_UNCHANGED。若是返回了POSITION_NONE，表示該位置的item已經不存在了。默認的實現是假設item的位置永遠不會發生變化，而返回POSITION_UNCHANGED

解決方案

因此咱們能夠嘗試着修改適配器的寫法，覆蓋getItemPosition()方法，當調用notifyDataSetChanged時，讓getItemPosition方法人爲的返回POSITION_NONE，從而達到強迫viewpager重繪全部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