如何将 Spinner 默认值设置为 null？

is it possible have a spinner that loads with nothing selected

Only if there is no data. If you have 1+ items in the SpinnerAdapter, the Spinner will always have a selection.

Spinners are not designed to be command widgets. Users will not expect a selection in a Spinner to start an activity. Please consider using something else, like a ListView or GridView, instead of a Spinner.

EDIT

BTW, I forgot to mention -- you can always put an extra entry in your adapter that represents "no selection", and make it the initial selected item in the Spinner.

In my case, although size '2' is displayed in the spinner, nothing happens till some selection is done!

I have an xml file (data_sizes.xml) which lists all the spinner values.

<?xml version="1.0" encoding="utf-8"?>
<resources>
<string-array name="chunks">
<item>2</item>
<item>4</item>
<item>8</item>
<item>16</item>
<item>32</item>
</string-array>
</resources>

In main.xml file: Spinner element

<Spinner android:id="@+id/spinnerSize"
android:layout_marginLeft="50px"
android:layout_width="fill_parent"
android:drawSelectorOnTop="true"
android:layout_marginTop="5dip"
android:prompt="@string/SelectSize"
android:layout_marginRight="30px"
android:layout_height="35px" />

Then in my java code, I added:

In my activity: Declaration

Spinner spinnerSize;
ArrayAdapter adapter;

In a public void function - initControls(): Definition

spinnerSize = (Spinner)findViewById(R.id.spinnerSize);
adapter = ArrayAdapter.createFromResource(this, R.array.chunks, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinnerSize.setAdapter(adapter);
spinnerSize.setOnItemSelectedListener(new MyOnItemSelectedListener());

My spinner listener:

/* Spinner Listener */

class MyOnItemSelectedListener implements OnItemSelectedListener {


public void onItemSelected(AdapterView<?> parent,
View view, int pos, long id) {
chunkSize = new Integer(parent.getItemAtPosition(pos).toString()).intValue();
}
public void onNothingSelected(AdapterView<?> parent) {
// Dummy
}
}

Alternatively, you could override your spinner adapter, and provide an empty view for position 0 in your getView method, and a view with 0dp height in the getDropDownView method.

This way, you have an initial text such as "Select an Option..." that shows up when the spinner is first loaded, but it is not an option for the user to choose (technically it is, but because the height is 0, they can't see it).

Using a custom spinner layout like this:

<?xml version="1.0" encoding="utf-8"?>
<Spinner xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/spinnerTarget"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:textSize="14dp"
android:textColor="#000000"/>

In the activity:

    // populate the list
ArrayList<String> dataList = new ArrayList<String>();
for (int i = 0; i < 4; i++) {
dataList.add("Item");
}


// set custom layout spinner_layout.xml and adapter
Spinner spinnerObject = (Spinner) findViewById(R.id.spinnerObject);
ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this, R.drawable.spinner_layout, dataList);
dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinnerObject.setAdapter(dataAdapter);
spinnerObject.setOnTouchListener(new View.OnTouchListener() {


public boolean onTouch(View v, MotionEvent event) {
// to set value of first selection, because setOnItemSelectedListener will not dispatch if the user selects first element
TextView spinnerTarget = (TextView)v.findViewById(R.id.spinnerTarget);
spinnerTarget.setText(spinnerObject.getSelectedItem().toString());


return false;
}


});
spinnerObject.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
private boolean selectionControl = true;


public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
// just the first time
if(selectionControl){


// find TextView in layout
TextView spinnerTarget = (TextView)parent.findViewById(R.id.spinnerTarget);
// set spinner text empty
spinnerTarget.setText("");
selectionControl = false;
}
else{
// select object
}
}


public void onNothingSelected(AdapterView<?> parent) {


}
});

you can put the first cell in your array to be empty({"","some","some",...}) and do nothing if the position is 0;

public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
if(position>0) {
label.setText(MainActivity.questions[position - 1]);
}
}

• if you fill the array by xml file you can let the first item empty

My solution, in case you have a TextView as each row of the spinner:

    // TODO: add a fake item as the last one of "items"
final ArrayAdapter<String> adapter=new ArrayAdapter<String>(..,..,items)
{
@Override
public View getDropDownView(final int position,final View convertView,final ViewGroup parent)
{
final View dropDownView=super.getDropDownView(position,convertView,parent);
((TextView)dropDownView.findViewById(android.R.id.text1)).setHeight(position==getCount()-1?0:getDimensionFromAttribute(..,R.attr.dropdownListPreferredItemHeight));
dropDownView.getLayoutParams().height=position==getCount()-1?0:LayoutParams.MATCH_PARENT;
return dropDownView;
}
}


...
spinner.setAdapter(adapter);
_actionModeSpinnerView.setSelection(dataAdapter.getCount()-1,false);






public static int getDimensionFromAttribute(final Context context,final int attr)
{
final TypedValue typedValue=new TypedValue();
if(context.getTheme().resolveAttribute(attr,typedValue,true))
return TypedValue.complexToDimensionPixelSize(typedValue.data,context.getResources().getDisplayMetrics());
return 0;
}

Merge this:

private long previousItemId = 0;


@Override
public long getItemId(int position) {
long nextItemId = random.nextInt(Integer.MAX_VALUE);
while(previousItemId == nextItemId) {
nextItemId = random.nextInt(Integer.MAX_VALUE);
}
previousItemId = nextItemId;
return nextItemId;
}

With this answer:

public class SpinnerInteractionListener
implements AdapterView.OnItemSelectedListener, View.OnTouchListener {


private AdapterView.OnItemSelectedListener onItemSelectedListener;


public SpinnerInteractionListener(AdapterView.OnItemSelectedListener selectedListener) {
this.onItemSelectedListener = selectedListener;
}


boolean userSelect = false;


@Override
public boolean onTouch(View v, MotionEvent event) {
userSelect = true;
return false;
}


@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
if(userSelect) {
onItemSelectedListener.onItemSelected(parent, view, pos, id);
userSelect = false;
}
}


@Override
public void onNothingSelected(AdapterView<?> parent) {
if(userSelect) {
onItemSelectedListener.onNothingSelected(parent);
userSelect = false;
}
}
}

This is a complete implementation of Paul Bourdeaux's idea, namely returning a special initial view (or an empty view) in getView() for position 0.

It works for me and is relatively straightforward. You might consider this approach especially if you already have a custom adapter for your Spinner. (In my case, I was using custom adapter in order to easily customise the layout of the items, each item having a couple of TextViews.)

The adapter would be something along these lines:

public class MySpinnerAdapter extends ArrayAdapter<MyModel> {


public MySpinnerAdapter(Context context, List<MyModel> items) {
super(context, R.layout.my_spinner_row, items);
}


@Override
public View getDropDownView(int position, View convertView, @NonNull ViewGroup parent) {
if (position == 0) {
return initialSelection(true);
}
return getCustomView(position, convertView, parent);
}


@NonNull
@Override
public View getView(int position, View convertView, @NonNull ViewGroup parent) {
if (position == 0) {
return initialSelection(false);
}
return getCustomView(position, convertView, parent);
}




@Override
public int getCount() {
return super.getCount() + 1; // Adjust for initial selection item
}


private View initialSelection(boolean dropdown) {
// Just an example using a simple TextView. Create whatever default view
// to suit your needs, inflating a separate layout if it's cleaner.
TextView view = new TextView(getContext());
view.setText(R.string.select_one);
int spacing = getContext().getResources().getDimensionPixelSize(R.dimen.spacing_smaller);
view.setPadding(0, spacing, 0, spacing);


if (dropdown) { // Hidden when the dropdown is opened
view.setHeight(0);
}


return view;
}


private View getCustomView(int position, View convertView, ViewGroup parent) {
// Distinguish "real" spinner items (that can be reused) from initial selection item
View row = convertView != null && !(convertView instanceof TextView)
? convertView :
LayoutInflater.from(getContext()).inflate(R.layout.my_spinner_row, parent, false);


position = position - 1; // Adjust for initial selection item
MyModel item = getItem(position);


// ... Resolve views & populate with data ...


return row;
}


}

That's it. Note that if you use a OnItemSelectedListener with your Spinner, in onItemSelected() you'd also have to adjust position to take the default item into account, for example:

if (position == 0) {
return;
} else {
position = position - 1;
}
MyModel selected = items.get(position);

Base on @Jonik answer I have created fully functional extension to ArrayAdapter

class SpinnerArrayAdapter<T> : ArrayAdapter<T> {


private val selectText : String
private val resource: Int
private val fieldId : Int
private val inflater : LayoutInflater


constructor(context: Context?, resource: Int, objects: Array<T>, selectText : String? = null) : super(context, resource, objects) {
this.selectText = selectText ?: context?.getString(R.string.spinner_default_select_text) ?: ""
this.resource = resource
this.fieldId = 0
this.inflater = LayoutInflater.from(context)
}
constructor(context: Context?, resource : Int, objects: List<T>, selectText : String? = null) : super(context, resource, objects) {
this.selectText = selectText ?: context?.getString(R.string.spinner_default_select_text) ?: ""
this.resource = resource
this.fieldId = 0
this.inflater = LayoutInflater.from(context)
}
constructor(context: Context?, resource: Int, textViewResourceId: Int, objects: Array<T>, selectText : String? = null) : super(context, resource, textViewResourceId, objects) {
this.selectText = selectText ?: context?.getString(R.string.spinner_default_select_text) ?: ""
this.resource = resource
this.fieldId = textViewResourceId
this.inflater = LayoutInflater.from(context)
}
constructor(context: Context?, resource : Int, textViewResourceId: Int, objects: List<T>, selectText : String? = null) : super(context, resource, textViewResourceId,  objects) {
this.selectText = selectText ?: context?.getString(R.string.spinner_default_select_text) ?: ""
this.resource = resource
this.fieldId = textViewResourceId
this.inflater = LayoutInflater.from(context)
}


override fun getDropDownView(position: Int, convertView: View?, parent: ViewGroup?): View {
if(position == 0) {
return initialSelection(true)
}


return createViewFromResource(inflater, position -1, convertView, parent, resource)
}


override fun getView(position: Int, convertView: View?, parent: ViewGroup?): View {
if(position == 0) {
return initialSelection(false)
}
return createViewFromResource(inflater, position -1, convertView, parent, resource)
}


override fun getCount(): Int {
return super.getCount() + 1 // adjust for initial selection
}


private fun initialSelection(inDropDown: Boolean) : View {
// Just simple TextView as initial selection.
val view = TextView(context)
view.setText(selectText)
view.setPadding(8, 0, 8, 0)


if(inDropDown) {
// Hide when dropdown is open
view.height = 0
}
return view
}


private fun createViewFromResource(inflater: LayoutInflater, position: Int,
@Nullable convertView: View?, parent: ViewGroup?, resource: Int): View {
val view: View
val text: TextView?


if (convertView == null || (convertView is TextView)) {
view = inflater.inflate(resource, parent, false)
} else {
view = convertView
}


try {
if (fieldId === 0) {
//  If no custom field is assigned, assume the whole resource is a TextView
text = view as TextView
} else {
//  Otherwise, find the TextView field within the layout
text = view.findViewById(fieldId)


if (text == null) {
throw RuntimeException("Failed to find view with ID "
+ context.getResources().getResourceName(fieldId)
+ " in item layout")
}
}
} catch (e: ClassCastException) {
Log.e("ArrayAdapter", "You must supply a resource ID for a TextView")
throw IllegalStateException(
"ArrayAdapter requires the resource ID to be a TextView", e)
}


val item = getItem(position)
if (item is CharSequence) {
text.text = item
} else {
text.text = item!!.toString()
}


return view
}

I assume that you want to have a Spinner with first empty invisible item (that is a strange feature of Spinner that cannot show a list without selecting an item). You should add a class that will contain data:

data class YourData(val id: Int, val name: String?)

This is the adapter.

class YourAdapter(
context: Context,
private val textViewResourceId: Int,
private var items: ArrayList<YourData>
) : ArrayAdapter<YourData>(context, textViewResourceId, items) {


private var inflater: LayoutInflater = context.getSystemService(
Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater


override fun getCount(): Int = items.size + 1


override fun getItem(position: Int): YourData? =
if (position == 0) YourData(0, "") else items[position - 1]


override fun getItemId(position: Int): Long = position.toLong()


override fun getView(position: Int, convertView: View?, parent: ViewGroup): View =
if (position == 0) {
getFirstTextView(convertView)
} else {
getTextView(convertView, parent, position - 1)
}


override fun getDropDownView(position: Int, convertView: View?, parent: ViewGroup): View =
getView(position, convertView, parent)


private fun getFirstTextView(convertView: View?): View {
// Just simple TextView as initial selection.
var textView: TextView? = convertView as? TextView
val holder: FirstViewHolder
if (textView?.tag !is FirstViewHolder) {
textView = TextView(context) // inflater.inflate(R.layout.your_text, parent, false) as TextView
textView.height = 0 // Hide first item.
holder = FirstViewHolder()
holder.textView = textView
textView.tag = holder
}
return textView
}


private fun getTextView(
convertView: View?,
parent: ViewGroup,
position: Int
): TextView {
var textView: TextView? = convertView as? TextView
val holder: ViewHolder
if (textView?.tag is ViewHolder) {
holder = textView.tag as ViewHolder
} else {
textView = inflater.inflate(textViewResourceId, parent, false) as TextView
holder = ViewHolder()
holder.textView = textView
textView.tag = holder
}
holder.textView.text = items[position].name


return textView
}


private class FirstViewHolder {
lateinit var textView: TextView
}


private class ViewHolder {
lateinit var textView: TextView
}
}

To create:

YourAdapter(context!!, R.layout.text_item, ArrayList())

To add items:

private fun fill(items: List<YourData>, adapter: YourAdapter) {
adapter.run {
clear()
addAll(items)
notifyDataSetChanged()
}
}

When you load items to your Spinner with that fill() command, you should know, that indices are also incremented. So if you wish to select 3rd item, you should now select 4th: spinner?.setSelection(index + 1)

Explanation- We can send a Dummy Value in first Parameter of our arrayList to spinner Adapter. Then send position(0 or any in between of arrayList) of Our dummy value in arrayList to SpinnerAdapter constructor .

public void addDataToList(LstQRTrackingSummary qrTrackingSummary) {
SpinnerMenu mSpinnerMenuzero = new SpinnerMenu();
mSpinnerMenuzero.setName("Select Action");
spinner_list.add(mSpinnerMenuzero);


if (qrTrackingSummary.getOQRTrackingButtons().getBtnToDispatch()) {
SpinnerMenu mSpinnerMenu = new SpinnerMenu();
mSpinnerMenu.setName("Dispatch");
mSpinnerMenu.setmDrawable(ResourcesCompat.getDrawable(context.getResources(),R.drawable.ic_dispatch,null));
spinner_list.add(mSpinnerMenu);
}


if (qrTrackingSummary.getOQRTrackingButtons().getBtnForwardToDispatch()) {
SpinnerMenu mSpinnerMenu = new SpinnerMenu();
mSpinnerMenu.setName("Forward To Dispatch");
mSpinnerMenu.setmDrawable(ResourcesCompat.getDrawable(context.getResources(), R.drawable.ic_forward_to_dispatch, null));
spinner_list.add(mSpinnerMenu);
}
}

then in spinner adapter class hide it in same way as i did and its done.

public class SpinnerAdapter extends ArrayAdapter<SpinnerMenu> {


private Context context;
private ArrayList<SpinnerMenu> menuList;
private LayoutInflater inflater;
private SpinnerMenu spinnerMenu;
public Integer positionToHide ;


public SpinnerAdapter(Context context, Integer positionToHide,ArrayList<SpinnerMenu> menuList) {
super(context, 0, menuList);
this.context = context;
this.positionToHide = positionToHide ;
inflater = LayoutInflater.from(context);
this.menuList = menuList;
}


@NonNull
@Override
public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
return getCustomView(position, convertView, parent);
}


@Override
public View getDropDownView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
return getCustomView(position, convertView, parent);
}


private View getCustomView(int position, View convertView , ViewGroup parent) {
View view = null ;
if (view == null) {
view = inflater.inflate(R.layout.spinner_baseview, parent, false);
}


ImageView spin_image = view.findViewById(R.id.spinner_image);
TextView spin_text = view.findViewById(R.id.spinner_text);
spinnerMenu = menuList.get(position);


if (position == positionToHide) {
TextView mytext = new TextView(context);
mytext.setVisibility(View.GONE);
mytext.setHeight(0);
view = mytext;
view.setVisibility(View.GONE);
} else {
spin_image.setImageDrawable(spinnerMenu.getmDrawable());
spin_text.setText(spinnerMenu.getName());
}
return view;
}

}