扩展AutoCompleteTextView让其默认显示一组列表。

很多时候， 在做自动下拉框时，默认点上去时需要显示一组默认的下拉数据。但是默认的AutoCompleteTextView是实现不了的， 因为setThreshold方法最小值是1，就算你设的值为0，也会自动改成1的。

    /**
     * <p>Specifies the minimum number of characters the user has to type in the
     * edit box before the drop down list is shown.</p>
     *
     * <p>When <code>threshold</code> is less than or equals 0, a threshold of
     * 1 is applied.</p>
     *
     * @param threshold the number of characters to type before the drop down
     *                  is shown
     *
     * @see #getThreshold()
     *
     * @attr ref android.R.styleable#AutoCompleteTextView_completionThreshold
     */

这时我们可以创建一个类 继承AutoCompleteTextView，覆盖enoughToFilter，让其一直返回true就行。 然后再主动调用showDropDown方法， 就能在不输入任何字符的情况下显示下拉框。

package com.wole.android.pad.view;

import android.content.Context;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.widget.AutoCompleteTextView;

/**
 * Created with IntelliJ IDEA.
 * User: denny
 * Date: 12-12-4
 * Time: 下午2:16
 * To change this template use File | Settings | File Templates.
 */
public class InstantAutoComplete extends AutoCompleteTextView {
    private int myThreshold;

    public InstantAutoComplete(Context context) {
        super(context);
    }

    public InstantAutoComplete(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public InstantAutoComplete(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    public boolean enoughToFilter() {
        return true;
    }

    @Override
    protected void onFocusChanged(boolean focused, int direction,
                                  Rect previouslyFocusedRect) {
        super.onFocusChanged(focused, direction, previouslyFocusedRect);
        if (focused) {
            performFiltering(getText(), 0);
            showDropDown();
        }
    }

    public void setThreshold(int threshold) {
        if (threshold < 0) {
            threshold = 0;
        }
        myThreshold = threshold;
    }

    public int getThreshold() {
        return myThreshold;
    }
}

     searchSuggestionAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, new ArrayList<String>(5));
        search_et.setAdapter(searchSuggestionAdapter);
        search_et.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
            }

 //如果没有输入任何东西 则显示默认列表，否则调用接口，展示下拉列表
            @Override
            public void afterTextChanged(Editable s) {
                if (s.length() >= 1) {
                    if (fetchSearchSuggestionKeywordsAsyncTask != null) {
                        fetchSearchSuggestionKeywordsAsyncTask.cancel(true);
                    }
                    fetchSearchSuggestionKeywordsAsyncTask =new FetchSearchSuggestionKeywordsAsyncTask();
                    fetchSearchSuggestionKeywordsAsyncTask.execute();
                }else{
                    showHotSearchKeywords();
                }

            }
        });

        search_et.setOnItemClickListener(new OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String item = searchSuggestionAdapter.getItem(position);
                search_et.setText(item);
                search_btn.performClick();
            }
        });

         //点击autocompletetextview时，如果没有输入任何东西 则显示默认列表
        search_et.setOnTouchListener(new View.OnTouchListener() {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                if (TextUtils.isEmpty(search_et.getText().toString())) {
                    showHotSearchKeywords();
                }
                return false;
            }

        });

 
//这里发现很奇怪的事情， 需要每次new一个ArrayAdapter，要不然有时调用showDropDown不会有效果
private void showHotSearchKeywords() {
        MiscUtil.prepareHotSearchKeywords(getWoleApplication());
        searchSuggestionAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, getWoleApplication().hotSearchHistoryKeywords);
        search_et.setAdapter(searchSuggestionAdapter);
        searchSuggestionAdapter.notifyDataSetChanged();
        search_et.showDropDown();
    }

    private class FetchSearchSuggestionKeywordsAsyncTask extends AsyncTask<Void, Void, List<String>> {

        @Override
        protected List<String> doInBackground(Void... params) {
            List<String> rt = new ArrayList<String>(5);
            String keyword = search_et.getText().toString();
            if (!TextUtils.isEmpty(keyword)) {
                try {
                    String result = NetworkUtil.doGet(BaseActivity.this, String.format(Global.API_SEARCH_SUGGESTIOIN_KEYWORDS, URLEncoder.encode(keyword, "utf-8")), false);
                    Log.i("FetchSearchSuggestionKeywordsAsyncTask", result);
                    if (!TextUtils.isEmpty(result)) {
                        JSONArray array = new JSONArray(result);
                        for (int i = 0; i < array.length(); i++) {
                            JSONObject jsonObject = array.getJSONObject(i);
                            rt.add(jsonObject.optString("keyword"));
                        }
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            return rt;
        }

        @Override
        protected void onPostExecute(List<String> strings) {
            super.onPostExecute(strings);
            if (!strings.isEmpty()) {
//这里发现很奇怪的事情， 需要每次new一个ArrayAdapter，要不然有时调用showDropDown不会有效果
                searchSuggestionAdapter = new ArrayAdapter<String>(BaseActivity.this, android.R.layout.simple_dropdown_item_1line, strings);
                search_et.setAdapter(searchSuggestionAdapter);
                searchSuggestionAdapter.notifyDataSetChanged();
            }
        }
    }

ref： http://stackoverflow.com/questions/2126717/android-autocompletetextview-show-suggestions-when-no-text-entered