ListView觀察者模式

概述

ListView的源碼中使用了多種設計模式，咱們先來看一下觀察者模式的應用。 先來看一張UML圖：

從圖中能夠看到ListView經過setAdapter方法將觀察者對象傳遞到BaseAdapter中，而後BaseAdapter經過registerDataSetObserver()方法框觀察者對象加入到mDataSetObservable列表中，而經過調用Adapter的notifyDatasetChanged方法通知觀察者進行notifyChanged方法調用，從而進行界面的刷新。

源碼分析

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();
    }
複製代碼

mAdapter.unregisterDataSetObserver(mDataSetObserver); 當mAdapter和mDataSetObserver都不爲空時，將原來的mDataSetObserver進行反註冊，即從觀察者列表中去除掉。這種狀況通常出如今當咱們重複設置Adapter時。 mDataSetObserver = new AdapterDataSetObserver(); mAdapter.registerDataSetObserver(mDataSetObserver); 這裏生成了一個AdapterDataSetObserver對象。

看一下AdapterDataSetObserver的源碼：

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

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

AdapterDataSetObserver又繼承了 AdapterView.AdapterDataSetObserver。 這個 AdapterView.AdapterDataSetObserver到底又是什麼東西呢？ 繼續看源碼：

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;
        }
    }
複製代碼

到這一步，DataSetObserver終於出現了。經歷了千辛萬苦最終仍是生成的是一個DataSetObserver對象。 這個對象做爲一個觀察者，咱們回到ListView的setAdapter方法中，看到mAdapter調用registerDataSetObserver方法將DataSetObserver對象傳遞進去。而registerDataSetObserver方法的源碼是：

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

這裏又調用了Observable中的registerObserver方法，將觀察者添加到觀察者列表中：

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

    /**
     * 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);
        }
    }
}
複製代碼

到這裏算是將觀察者進行註冊了。

咱們再去看Adapter的notifyDataSetChanged方法：

public void notifyDataSetChanged() {
        mDataSetObservable.notifyChanged();
}
複製代碼

這裏調用的是Adapter中的mDataSetObservable的notifyChanged()方法。而DataSetObserval對象的notifyChanged方法則遍歷觀察者列表而後調用觀察者的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();
            }
        }
    }
複製代碼

而回到上面AdapterDataSetObserver類的方法中重寫了onChanged方法，該方法最終調用的是requestLayout()對界面進行刷新。到這裏就所有實現了觀察者模式的所有流程。