listview自己制造bug分析如何解决

网上很多listview加载不同的布局 采用getItemViewType 方式行不通 也不知道他们怎么写的blog.能先自己用一下在发表吗 复制粘贴的东西 不自己分析对错，就这么用吗？

采用所有类型整合到一个item中控制布局显示隐藏的方式，虽然维护不方便，但至少不会出错。

我为了测试，使用了pulltorefreshlistview ,我的blog有讲。

错误源代码：

public class MainActivity extends AppCompatActivity {

PullToRefreshListView listView;
ImageView imageView;
MyAdapter mAdapter;
List<MDTO> list = new ArrayList<>();

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

imageView = (ImageView) findViewById(R.id.iv);

imageView.setImageResource(R.drawable.proxyimg);

//控件
listView = (PullToRefreshListView) findViewById(R.id.listview);

for (int i = 0; i < 50; i++) {
MDTO mdto = new MDTO();
mdto.setId(R.drawable.ic_launcher);
mdto.setText("text" + i);
mdto.setType(i % 3);
list.add(mdto);
}

mAdapter = new MyAdapter(list);
listView.setAdapter(mAdapter);

//控件模式
listView.setMode(PullToRefreshBase.Mode.PULL_FROM_START);

//控件滚动行为
listView.setOnScrollListener(new AbsListView.OnScrollListener() {
@Override
public void onScrollStateChanged(AbsListView absListView, int i) {

}

@Override
public void onScroll(AbsListView absListView, int i, int i1, int i2) {

}
});
//控件刷新行为
listView.setOnRefreshListener(new PullToRefreshBase.OnRefreshListener<ListView>() {
@Override
public void onRefresh(PullToRefreshBase<ListView> refreshView) {

}
});

}

class MyAdapter extends BaseAdapter {

List<MDTO> mList = new ArrayList<>();

MyAdapter(List<MDTO> mdtoList) {

mList.clear();
this.mList.addAll(mdtoList);

for (int i = 0; i < mList.size(); i++) {

Log.i("id", mList.get(i).getId() + "");
Log.i("type", mList.get(i).getType() + "");

}

notifyDataSetChanged();
}

@Override
public int getCount() {
return mList.size();
}

@Override
public Object getItem(int i) {
return mList.get(i);
}

@Override
public long getItemId(int i) {
return i;
}

@Override
public View getView(int i, View view, ViewGroup viewGroup) {

int type = mList.get(i).getType();
ViewHolder_One viewHolder_one;

if (view == null) {

view = LayoutInflater.from(MainActivity.this).inflate(R.layout.item_one, null);
viewHolder_one = new ViewHolder_One();
viewHolder_one.iv = (ImageView) view.findViewById(R.id.one_iv);
viewHolder_one.tv = (TextView) view.findViewById(R.id.one_tv);

} else {

viewHolder_one = (ViewHolder_One) view.getTag();

}

if (type == 0) {

viewHolder_one.iv.setVisibility(View.VISIBLE);
viewHolder_one.tv.setVisibility(View.VISIBLE);
viewHolder_one.iv.setImageResource(mList.get(i).getId());
viewHolder_one.tv.setText(mList.get(i).getText());
view.setBackgroundColor(Color.RED);

} else if (type == 1) {
viewHolder_one.tv.setVisibility(View.VISIBLE);
viewHolder_one.iv.setVisibility(View.GONE);
viewHolder_one.tv.setText(mList.get(i).getText());
view.setBackgroundColor(Color.BLUE);
} else {
viewHolder_one.iv.setVisibility(View.VISIBLE);
viewHolder_one.tv.setVisibility(View.GONE);
viewHolder_one.iv.setImageResource(mList.get(i).getId());
view.setBackgroundColor(Color.WHITE);
}

return view;
}

class ViewHolder_One {
TextView tv;
ImageView iv;
}

}

}

当然 熟悉的同学一眼就看出问题所在，如果你忘了那么办呢？

报错处

java.lang.NullPointerException: Attempt to read from field 'android.widget.TextView com.lcldream.www.MainActivity$MyAdapter$ViewHolder_One.tv' on a null object reference
at com.lcldream.www.MainActivity$MyAdapter.getView(MainActivity.java:141)
at android.widget.HeaderViewListAdapter.getView(HeaderViewListAdapter.java:220)
at android.widget.AbsListView.obtainView(AbsListView.java:2575)
at android.widget.ListView.makeAndAddView(ListView.java:1956)
at android.widget.ListView.fillDown(ListView.java:757)
at android.widget.ListView.fillGap(ListView.java:721)
at android.widget.AbsListView.trackMotionScroll(AbsListView.java:5733)
at android.widget.AbsListView.scrollIfNeeded(AbsListView.java:3780)
at android.widget.AbsListView.onTouchMove(AbsListView.java:4277)
at android.widget.AbsListView.onTouchEvent(AbsListView.java:4052)
at android.widget.ListView.onTouchEvent(ListView.java:4287)
at android.view.View.dispatchTouchEvent(View.java:8677)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2517)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2161)
at com.handmark.pulltorefresh.library.PullToRefreshListView$InternalListView.dispatchTouchEvent(PullToRefreshListView.java:312)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2523)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2175)
at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2539)
at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1807)
at android.app.Activity.dispatchTouchEvent(Activity.java:2823)
at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:71)
at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2496)
at android.view.View.dispatchPointerEvent(View.java:8883)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4779)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4611)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4087)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4140)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4106)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4243)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4114)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4300)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4087)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4140)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4106)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4114)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4087)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6598)
at a

我们一看 空指针 果断debug,debug是遇到任何问题都首先想到的。

进入了else{

viewHolder_one = (ViewHolder_One) view.getTag();

}

然后执行

对应type

报的空指针

那么至此问题定位就明确了

viewHolder_one 出问题了

我们在看viewholder代码发现没有setTag.添上再运行 很流畅。

那么没有tag为什么会报错呢 第一屏为什么没有报错呢？

涉及到listview复用机制

listview 为了达到优化目的 使用了复用机制 其实第一屏都是 执行view == null 判断 都是重新加载布局 第二屏开始慢慢移除旧的 此时 view不再为空 所以会重用tag中的内容，类似于日历表 可以上下滑动 但是 中间显示就一个 也就是容量有限 不用的就回收。