Android Adapter原理解析

1、先看一下Adapter的继承关系：

2、Adapter的实现原理其实就是一个观察者模式的应用。先上一幅简图：

3、首先看观察者抽象类：DataSetObserver，它有两个方法onChange()和onInvalidated()，源码：

public abstract class DataSetObserver {    /**     * This method is called when the entire data set has changed,     * most likely through a call to {@link Cursor#requery()} on a {@link Cursor}.     */    public void onChanged() {        // Do nothing    }    /**     * This method is called when the entire data becomes invalid,     * most likely through a call to {@link Cursor#deactivate()} or {@link Cursor#close()} on a     * {@link Cursor}.     */    public void onInvalidated() {        // Do nothing    }}

4、被观察者抽象类Observable，提供了注册观察者、注销观察者和清空观察者的方法。

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);        }    }    /**     * Removes a previously registered observer. The observer must not be null and it     * must already have been registered.     * @param observer the observer to unregister     * @throws IllegalArgumentException the observer is null     * @throws IllegalStateException the observer is not yet registered     */    public void unregisterObserver(T observer) {        if (observer == null) {            throw new IllegalArgumentException("The observer is null.");        }        synchronized(mObservers) {            int index = mObservers.indexOf(observer);            if (index == -1) {                throw new IllegalStateException("Observer " + observer + " was not registered.");            }            mObservers.remove(index);        }    }    /**     * Remove all registered observers.     */    public void unregisterAll() {        synchronized(mObservers) {            mObservers.clear();        }    }}

该抽象类有一个继承类DataSetObservable，新增了两个方法，notifyChanged()和notifyInvalidated()用来提醒观察者更新操作。

public class DataSetObservable extends Observable<DataSetObserver> {    /**     * 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();            }        }    }    /**     * Invokes {@link DataSetObserver#onInvalidated} on each observer.     * Called when the data set is no longer valid and cannot be queried again,     * such as when the data set has been closed.     */    public void notifyInvalidated() {        synchronized (mObservers) {            for (int i = mObservers.size() - 1; i >= 0; i--) {                mObservers.get(i).onInvalidated();            }        }    }}

5、Adapter基类的部分代码：

public interface Adapter {    /**     * Register an observer that is called when changes happen to the data used by this adapter.     *     * @param observer the object that gets notified when the data set changes.     */    void registerDataSetObserver(DataSetObserver observer);    /**     * Unregister an observer that has previously been registered with this     * adapter via {@link #registerDataSetObserver}.     *     * @param observer the object to unregister.     */    void unregisterDataSetObserver(DataSetObserver observer);}

可以看到有注册观察者和注销观察者的方法。

6、来看一下BaseAdapter的部分代码：

public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {    private final DataSetObservable mDataSetObservable = new DataSetObservable();    public void registerDataSetObserver(DataSetObserver observer) {        mDataSetObservable.registerObserver(observer);    }    public void unregisterDataSetObserver(DataSetObserver observer) {        mDataSetObservable.unregisterObserver(observer);    }    /**     * 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();    }}

可以看到，内部有个被观察者变量DataSetObservable，注册和注销观察者的方法，还有我们熟悉的notifyDataSetChanged()和notifyDataSetInvalidated()方法。我们通过ListView.setAdapter(Adapter adapter)来设置适配器，该适配器则通过notifyDataSetChanged()和notifyDataSetInvalidated()方法来通知被观察者调用相应的notifyChanged（）和notifyInvalidated（）去做更新操作。

7、附上ListView.setAdapter（）的代码：

 @Override    public void setAdapter(ListAdapter adapter) {        if (mAdapter != null && mDataSetObserver != null) {            mAdapter.unregisterDataSetObserver(mDataSetObserver);        }        resetList();        mRecycler.clear();        if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {            mAdapter = new HeaderViewListAdapter(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();    }

注意到下面两句：
mDataSetObserver = new AdapterDataSetObserver();
mAdapter.registerDataSetObserver(mDataSetObserver);

这个观察者是重新实例化了一个AdapterDataSetObserver类的对象，AdapterDataSetObserver类是AbsListView的一个内部类。

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();            }        }    }

可以看到，更新时，又调用了父类的方法，来看看父类在更新时具体的操作。AdapterDataSetObserver类是AdapterView的一个内部类：

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;        }    }

可以看到，最后是调用了 requestLayout()来更新界面以达到更新数据的目的。

8、最后，Adapter和ListView是怎样实现数据更新的呢，平时我们是通过getView()来返回一个item的view给ListView，但是又在何时调用呢，首先来查看一下ListView的onMeasure()源码：

@Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        // Sets up mListPadding        super.onMeasure(widthMeasureSpec, heightMeasureSpec);        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);        int widthSize = MeasureSpec.getSize(widthMeasureSpec);        int heightSize = MeasureSpec.getSize(heightMeasureSpec);        int childWidth = 0;        int childHeight = 0;        int childState = 0;        mItemCount = mAdapter == null ? 0 : mAdapter.getCount();        if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED                || heightMode == MeasureSpec.UNSPECIFIED)) {            final View child = obtainView(0, mIsScrap);            // Lay out child directly against the parent measure spec so that            // we can obtain exected minimum width and height.            measureScrapChild(child, 0, widthMeasureSpec, heightSize);            childWidth = child.getMeasuredWidth();            childHeight = child.getMeasuredHeight();            childState = combineMeasuredStates(childState, child.getMeasuredState());            if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(                    ((LayoutParams) child.getLayoutParams()).viewType)) {                mRecycler.addScrapView(child, 0);            }        }        if (widthMode == MeasureSpec.UNSPECIFIED) {            widthSize = mListPadding.left + mListPadding.right + childWidth +                    getVerticalScrollbarWidth();        } else {            widthSize |= (childState & MEASURED_STATE_MASK);        }        if (heightMode == MeasureSpec.UNSPECIFIED) {            heightSize = mListPadding.top + mListPadding.bottom + childHeight +                    getVerticalFadingEdgeLength() * 2;        }        if (heightMode == MeasureSpec.AT_MOST) {            // TODO: after first layout we should maybe start at the first visible position, not 0            heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);        }        setMeasuredDimension(widthSize, heightSize);        mWidthMeasureSpec = widthMeasureSpec;    }

可以注意到final View child = obtainView(0, mIsScrap);查看一下obtainView（）这个方法的部分源码，这个方法在AbsListView类里面：

View obtainView(int position, boolean[] isScrap) {        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");        isScrap[0] = false;        // Check whether we have a transient state view. Attempt to re-bind the        // data and discard the view if we fail.        final View transientView = mRecycler.getTransientStateView(position);        if (transientView != null) {            final LayoutParams params = (LayoutParams) transientView.getLayoutParams();            // If the view type hasn't changed, attempt to re-bind the data.            if (params.viewType == mAdapter.getItemViewType(position)) {                final View updatedView = mAdapter.getView(position, transientView, this);                // If we failed to re-bind the data, scrap the obtained view.                if (updatedView != transientView) {                    setItemViewLayoutParams(updatedView, position);                    mRecycler.addScrapView(updatedView, position);                }            }            isScrap[0] = true;            // Finish the temporary detach started in addScrapView().            transientView.dispatchFinishTemporaryDetach();            return transientView;        }}

注意到final View updatedView = mAdapter.getView(position, transientView, this);这句话，我们的getView()就是在这里被调用的。

综上所述，Adapter的大概原理基本就是这样实现的。