我有一个布局default_label.xml，如下所示：
<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/label"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
然后，我有了这个类，基本上可以让我为微调器设置默认按钮文本：
public class NothingSelectedSpinnerAdapter implements SpinnerAdapter, ListAdapter {
protected static final int EXTRA = 1;
protected SpinnerAdapter adapter;
protected Context context;
protected int nothingSelectedLayout;
protected int nothingSelectedDropdownLayout;
protected LayoutInflater layoutInflater;
protected TextView label;
/**
* Use this constructor to have NO 'Select One...' item, instead use
* the standard prompt or nothing at all.
*
* @param spinnerAdapter wrapped Adapter.
* @param nothingSelectedLayout layout for nothing selected, perhaps
* you want text grayed out like a prompt...
* @param context Context
*/
public NothingSelectedSpinnerAdapter(
SpinnerAdapter spinnerAdapter,
int nothingSelectedLayout, Context context) {
this(spinnerAdapter, nothingSelectedLayout, -1, context);
}
/**
* Use this constructor to Define your 'Select One...' layout as the first
* row in the returned choices.
* If you do this, you probably don't want a prompt on your spinner or it'll
* have two 'Select' rows.
*
* @param spinnerAdapter wrapped Adapter. Should probably return false for isEnabled(0)
* @param nothingSelectedLayout layout for nothing selected, perhaps you want
* text grayed out like a prompt...
* @param nothingSelectedDropdownLayout layout for your 'Select an Item...' in
* the dropdown.
* @param context Context
*/
public NothingSelectedSpinnerAdapter(SpinnerAdapter spinnerAdapter,
int nothingSelectedLayout, int nothingSelectedDropdownLayout, Context context) {
this.adapter = spinnerAdapter;
this.context = context;
this.nothingSelectedLayout = nothingSelectedLayout;
this.nothingSelectedDropdownLayout = nothingSelectedDropdownLayout;
layoutInflater = LayoutInflater.from(context);
}
@Override
public final View getView(int position, View convertView, ViewGroup parent) {
// This provides the View for the Selected Item in the Spinner, not
// the dropdown (unless dropdownView is not set).
if (position == 0) {
return getNothingSelectedView(parent);
}
return adapter.getView(position - EXTRA, null, parent); // Could re-use
// the convertView if possible.
}
public int getPosition(String value) {
int index = 0;
for (int i = 1; i < getCount() + EXTRA; i++) {
if (getItem(i).equals(value)) {
index = i;
break;
}
}
return index;
}
public TextView getNothingSelectedView(ViewGroup parent) {
label = (TextView) layoutInflater.inflate(nothingSelectedLayout, parent, false);
return label;
}
@Override
public View getDropDownView(int position, View convertView, ViewGroup parent) {
// Android BUG! http://code.google.com/p/android/issues/detail?id=17128 -
// Spinner does not support multiple view types
if (position == 0) {
return new View(context);
}
if (adapter.getItem(position-EXTRA).toString().equals("")){
View view = nothingSelectedDropdownLayout == -1 ?
new View(context) :
getNothingSelectedDropdownView(parent);
view.setEnabled(false);
view.setOnClickListener(null);
return view;
}
// Could re-use the convertView if possible, use setTag...
return adapter.getDropDownView(position - EXTRA, null, parent);
}
protected View getNothingSelectedDropdownView(ViewGroup parent) {
return layoutInflater.inflate(nothingSelectedDropdownLayout, parent, false);
}
@Override
public int getCount() {
int count = adapter.getCount();
return count == 0 ? 0 : count + EXTRA;
}
@Override
public Object getItem(int position) {
return position == 0 ? null : adapter.getItem(position - EXTRA);
}
@Override
public int getItemViewType(int position) {
return 0;
}
@Override
public int getViewTypeCount() {
return 1;
}
@Override
public long getItemId(int position) {
return position >= EXTRA ? adapter.getItemId(position - EXTRA) : position - EXTRA;
}
@Override
public boolean hasStableIds() {
return adapter.hasStableIds();
}
@Override
public boolean isEmpty() {
return adapter.isEmpty();
}
@Override
public void registerDataSetObserver(DataSetObserver observer) {
adapter.registerDataSetObserver(observer);
}
@Override
public void unregisterDataSetObserver(DataSetObserver observer) {
adapter.unregisterDataSetObserver(observer);
}
@Override
public boolean areAllItemsEnabled() {
return false;
}
@Override
public boolean isEnabled(int position) {
return position != 0; // Don't allow the 'nothing selected'
// item to be picked.
}
}
我想做的就是更改TextView的文本，例如：
NothingSelectedSpinnerAdapter myAdapter = new NothingSelectedSpinnerAdapter(adapter, R.layout.default_label, getContext());
myAdapter.setText("Custom Label");
mySpinner.setAdapter(myAdapter);
但是，当我尝试将以下方法添加到上述NothingSelectedSpinnerAdapter类中时：
public void setText(String text) {
label.setText(text);
}
我收到以下错误：
java.lang.NullPointerException：尝试调用虚拟方法'void android.widget.TextView.setText（java.lang.CharSequence）'上的null 对象引用
我应该改变什么？
答案 0 :(得分：0)
看看以下三行：
NothingSelectedSpinnerAdapter myAdapter = new NothingSelectedSpinnerAdapter(adapter, R.layout.default_label, getContext());
实例化适配器，标签为null
myAdapter.setText("Custom Label");
TextView 标签仍为null，这就是label.setText();导致NullPointerException
mySpinner.setAdapter(myAdapter);
现在，运行时将能够绘制Spinner项目。这涉及到重复调用getView()（对于应该绘制的每个项目至少调用一次）。为getView()执行position = 0之后，您将为 label 分配一个值。从现在开始，您可以安全地致电setText()。
如果您希望随时可以调用myAdapter.setText("Custom Label");而不用担心适配器的内部工作，那么可以引入一个字段private String mLabelText并实现以下方法
public void setText(String labelText){
mLabelText = labelText;
if (label != null){
label.setText(labelText);
}
}
最后一步：别忘了在初始化 label
后立即设置文本public TextView getNothingSelectedView(ViewGroup parent) {
label = (TextView) layoutInflater.inflate(nothingSelectedLayout, parent, false);
label.setText(mLabelText);
return label;
}