Android深入研究Adapter重绘

一直以来Adapter的使用都只是流于表面，只知道要实现几个抽象的方法，把Adapter设置给某种listView，就可以很好的工作起来。所谓理解只是建立在主观的猜想上面，觉得应该是这样，对，ok，就这样，恩， 明白了。但是事实上却没有正真的懂它。这可以说Android的设计模式真的做得很好，无需了解实现就可以很好的运用，不过总感觉没有深入的追究还是少了点什么。那就看看它的一些细节吧。尤其来看看我一直疑惑的为什么调用notifyDataSetChanged  notifyDataSetInvalidated
就可以重绘View。

首先还是从我掌握的有限的线索开始，notifyDataSetChanged 方法是怎么实现的呢？

根据经验发现，几乎我们用到的Adapter都是继承自BaseAdapter，借助eclipse 的Type Hierarchy View 也可以确认确实是这么回事，ok，那看看它的实现：

    /**
     * Notifies the attached observers that the underlying data has been changed
     * and any View reflecting the data set should refresh itself.
     */
    public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
    }

    /**
     * Notifies the attached observers that the underlying data is no longer valid
     * or available. Once invoked this adapter is no longer valid and should
     * not report further data set changes.
     */
    public void notifyDataSetInvalidated() {
        mDataSetObservable.notifyInvalidated();
    }

原来是个Observer的实现，这个倒是和我之前的猜想差不多。

    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();
            }
        }
    }

看来可以挂很多个Observer，继续看看onChanged实现发现时个abstract的，怎么办，到底是谁呢？看来要找个具体的实现。看看比较熟悉的ListView

和比较熟悉的ListAdapter，在ListView的setAdapter里发现

            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

AdapterDataSetObserver的实现在AdapterView：

 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;
        }
    }

ok，原来是这样来更新的，明白了。

总结起来看，所有的Adapter都是BaseAdapter的子类，所有的带Adapter的view都是AdapterView的子类。AbsListView也是AdapterView的子类。

在View和Adapter关联的时候会把view的Oberver注册到adapter上，所以在adapter数据变化的时候。view就可以通过observer了解到，从而可以及时更新。

看来Adapter 模式的实现用了Oberver模式，隐隐约约还用到桥模式，不知道我的认识是不是正确呢。