BaseAdapter.notifyDataSetChanged()之观察者设计模式及源码分析

BaseAdapter.notifyDataSetChanged()的实现涉及到设计模式-观察者模式，详情请参考我之前的博文设计模式之观察者模式

Ok,回到notifyDataSetChanged进行源码分析，

 Adapter本身不是被观察者Observable，而是adapter里面有一个被观察者DataSetObservable mDataSetObservable

 public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }

 notifyChanged源码分析，notifyChanged实际上是去调用了观察者Observer的onChanged方法.

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();
            }
        }
    }

被观察者subject ListAdapter添加注册观察者Observer AdapterDataSetObserver

1    //ListView.java - setAdapter()
2    //被观察者subject ListAdapter添加注册观察者Observer AdapterDataSetObserver
3    mDataSetObserver = new AdapterDataSetObserver();
4    mAdapter.registerDataSetObserver(mDataSetObserver);

AdapterDataSetObserver的onChanged方法里最后调用了requestLayout去更新ListView

//AdapterView.java
    class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;

            checkFocus();
            requestLayout();
        }

        public void clearSavedState() {
            mInstanceState = null;
        }
    }