ListView观察者模式

概述

ListView的源码中使用了多种设计模式，我们先来看一下观察者模式的应用。
先来看一张UML图：

从图中可以看到ListView通过setAdapter方法将观察者对象传递到BaseAdapter中，然后BaseAdapter通过registerDataSetObserver()方法框观察者对象加入到mDataSetObservable列表中，而通过调用Adapter的notifyDatasetChanged方法通知观察者进行notifyChanged方法调用，从而进行界面的刷新。

源码分析

先看ListView的setAdapter方法：

public void setAdapter(ListAdapter adapter) {
        if (mAdapter != null && mDataSetObserver != null) {
            mAdapter.unregisterDataSetObserver(mDataSetObserver);
        }

        resetList();
        mRecycler.clear();

        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {
            mAdapter = wrapHeaderListAdapterInternal(mHeaderViewInfos, mFooterViewInfos, adapter);
        } else {
            mAdapter = adapter;
        }

        mOldSelectedPosition = INVALID_POSITION;
        mOldSelectedRowId = INVALID_ROW_ID;

        // AbsListView#setAdapter will update choice mode states.
        super.setAdapter(adapter);

        if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
            checkFocus();

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

            mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

            int position;
            if (mStackFromBottom) {
                position = lookForSelectablePosition(mItemCount - 1, false);
            } else {
                position = lookForSelectablePosition(0, true);
            }
            setSelectedPositionInt(position);
            setNextSelectedPositionInt(position);

            if (mItemCount == 0) {
                // Nothing selected
                checkSelectionChanged();
            }
        } else {
            mAreAllItemsSelectable = true;
            checkFocus();
            // Nothing selected
            checkSelectionChanged();
        }

        requestLayout();
    }

mAdapter.unregisterDataSetObserver(mDataSetObserver);
当mAdapter和mDataSetObserver都不为空时，将原来的mDataSetObserver进行反注册，即从观察者列表中去除掉。这种情况一般出现在当我们重复设置Adapter时。
mDataSetObserver = new AdapterDataSetObserver();
mAdapter.registerDataSetObserver(mDataSetObserver);
这里生成了一个AdapterDataSetObserver对象。

看一下AdapterDataSetObserver的源码：

class AdapterDataSetObserver extends AdapterView.AdapterDataSetObserver {
        @Override
        public void onChanged() {
            super.onChanged();
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }

        @Override
        public void onInvalidated() {
            super.onInvalidated();
            if (mFastScroll != null) {
                mFastScroll.onSectionsChanged();
            }
        }
    }

AdapterDataSetObserver又继承了 AdapterView.AdapterDataSetObserver。
这个 AdapterView.AdapterDataSetObserver到底又是什么东西呢？
继续看源码：

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;
        }
    }

到这一步，DataSetObserver终于出现了。经历了千辛万苦最终还是生成的是一个DataSetObserver对象。
这个对象作为一个观察者，我们回到ListView的setAdapter方法中，看到mAdapter调用registerDataSetObserver方法将DataSetObserver对象传递进去。而registerDataSetObserver方法的源码是：

private final DataSetObservable mDataSetObservable = new DataSetObservable();
public void registerDataSetObserver(DataSetObserver observer) {
        mDataSetObservable.registerObserver(observer);
 }

这里又调用了Observable中的registerObserver方法，将观察者添加到观察者列表中：

public abstract class Observable {
    /**
     * The list of observers.  An observer can be in the list at most
     * once and will never be null.
     */
    protected final ArrayList mObservers = new ArrayList();

    /**
     * Adds an observer to the list. The observer cannot be null and it must not already
     * be registered.
     * @param observer the observer to register
     * @throws IllegalArgumentException the observer is null
     * @throws IllegalStateException the observer is already registered
     */
    public void registerObserver(T observer) {
        if (observer == null) {
            throw new IllegalArgumentException("The observer is null.");
        }
        synchronized(mObservers) {
            if (mObservers.contains(observer)) {
                throw new IllegalStateException("Observer " + observer + " is already registered.");
            }
            mObservers.add(observer);
        }
    }
}

到这里算是将观察者进行注册了。

我们再去看Adapter的notifyDataSetChanged方法：

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

这里调用的是Adapter中的mDataSetObservable的notifyChanged()方法。而DataSetObserval对象的notifyChanged方法则遍历观察者列表然后调用观察者的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();
            }
        }
    }

而回到上面AdapterDataSetObserver类的方法中重写了onChanged方法，该方法最终调用的是requestLayout()对界面进行刷新。到这里就全部实现了观察者模式的全部流程。