listview数据刷新分析(三)

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5.1,概念

当adapter中的数据更新时,会调用notifyDataSetChanged方法来刷新listview,这应该是典型的观察者模式,背后的原理是什么呢?

5.2,注册

注册流程图如下,

调用listview的setadapter方法时,开始注册, setAdapter方法部分源码如下,

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

AdapterDataSetObserver是AbsListView的内部类,定义如下,

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();            }        }    }

其中,有2个回调方法, onChanged和onInvalidated,当数据库发生改变时,应该会回调onChanged方法。

BaseAdapter中的registerDataSetObserver方法如下,

public void registerDataSetObserver(DataSetObserver observer) {        mDataSetObservable.registerObserver(observer);    }

mDataSetObservable 是一个DataSetObservable对象,其父类Observable的registerObserver方法如下,

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);        }    }

mObservers是一个ArrayList,存放所有注册的观察者,

protected final ArrayList<T> mObservers = new ArrayList<T>();

这里观察者模式使用的很巧妙,直接使用一个内部类。

5.3, notifyDataSetChanged

调用流程图如下,

BaseAdapter 的notifyDataSetChanged方法如下,

public void notifyDataSetChanged() {        mDataSetObservable.notifyChanged(); //直接调用DataSetObservable的notifyChanged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();            }        }}

取出mObservers中所有元素,然后调用其onChanged方法,

最后Adapterview的AdapterDataSetObserver内部类的onChanged方法如下,

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(); // 重新绘制        }

requestLayout和invalidate方法的区别,

1, requestLayout会重新对view的布局进行测量,布局以及绘制;而invalidate只是对view进行重绘。

2,如果view只需重绘而不需测量等,使用invalidate会更高效。

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