android 搜索下拉框

在 Android 中实现搜索下拉框可以使用 AutoCompleteTextView 控件。它是一个文本框，用户可以输入搜索关键字，同时还会显示搜索结果的下拉选项。下面是一个简单的实现示例：

1. 在布局文件中添加 AutoCompleteTextView 控件：

<AutoCompleteTextView
    android:id="@+id/autoCompleteTextView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:hint="Search"
    android:textColorHint="@color/gray"/>

2. 在 Activity 中设置 AutoCompleteTextView 的适配器和监听器：

public class MainActivity extends AppCompatActivity {
    private AutoCompleteTextView autoCompleteTextView;

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

        autoCompleteTextView = findViewById(R.id.autoCompleteTextView);

        // 设置适配器
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_dropdown_item_1line, COUNTRIES);
        autoCompleteTextView.setAdapter(adapter);

        // 设置监听器
        autoCompleteTextView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String selection = (String) parent.getItemAtPosition(position);
                Toast.makeText(MainActivity.this, "You selected: " + selection, Toast.LENGTH_LONG).show();
            }
        });
    }

    private static final String[] COUNTRIES = new String[] {
            "Afghanistan", "Albania", "Algeria", "Andorra", "Angola", "Anguilla", "Antarctica", "Antigua and Barbuda",
            "Argentina", "Armenia", "Aruba", "Australia", "Austria", "Azerbaijan", "Bahamas", "Bahrain", "Bangladesh",
            "Barbados", "Belarus", "Belgium", "Belize", "Benin", "Bermuda", "Bhutan", "Bolivia", "Bosnia and Herzegovina",
            "Botswana", "Bouvet Island", "Brazil", "British Indian Ocean Territory", "Brunei Darussalam", "Bulgaria",
            "Burkina Faso", "Burundi", "Cambodia", "Cameroon", "Canada", "Cape Verde", "Cayman Islands",
            "Central African Republic", "Chad", "Chile", "China", "Christmas Island", "Cocos (Keeling) Islands",
            "Colombia", "Comoros", "Congo", "Congo, the Democratic Republic of the", "Cook Islands", "Costa Rica",
            "Cote d'Ivoire", "Croatia", "Cuba", "Cyprus", "Czech Republic", "Denmark", "Djibouti", "Dominica",
            "Dominican Republic", "Ecuador", "Egypt", "El Salvador", "Equatorial Guinea", "Eritrea", "Estonia",
            "Ethiopia", "Falkland Islands (Malvinas)", "Faroe Islands", "Fiji", "Finland", "France", "French Guiana",
            "French Polynesia", "French Southern Territories", "Gabon", "Gambia", "Georgia", "Germany", "Ghana",
            "Gibraltar", "Greece", "Greenland", "Grenada", "Guadeloupe", "Guam", "Guatemala", "Guinea",
            "Guinea-Bissau", "Guyana", "Haiti", "Heard Island and McDonald Islands", "Holy See (Vatican City State)",
            "Honduras", "Hong Kong", "Hungary", "Iceland", "India", "Indonesia", "Iran, Islamic Republic of",
            "Iraq", "Ireland", "Israel", "Italy", "Jamaica", "Japan", "Jordan", "Kazakhstan", "Kenya", "Kiribati",
            "Korea, Democratic People's Republic of", "Korea, Republic of", "Kuwait", "Kyrgyzstan",
            "Lao People's Democratic Republic", "Latvia", "Lebanon", "Lesotho", "Liberia",
            "Libyan Arab Jamahiriya", "Liechtenstein", "Lithuania", "Luxembourg", "Macao",
            "Macedonia, the former Yugoslav Republic of", "Madagascar", "Malawi", "Malaysia", "Maldives", "Mali",
            "Malta", "Marshall Islands", "Martinique", "Mauritania", "Mauritius", "Mayotte", "Mexico",
            "Micronesia, Federated States of", "Moldova, Republic of", "Monaco", "Mongolia", "Montserrat",
            "Morocco", "Mozambique", "Myanmar", "Namibia", "Nauru", "Nepal", "Netherlands", "Netherlands Antilles",
            "New Caledonia", "New Zealand", "Nicaragua", "Niger", "Nigeria", "Niue", "Norfolk Island",
            "Northern Mariana Islands", "Norway", "Oman", "Pakistan", "Palau", "Palestinian Territory, Occupied",
            "Panama", "Papua New Guinea", "Paraguay", "Peru", "Philippines", "Pitcairn", "Poland", "Portugal",
            "Puerto Rico", "Qatar", "Reunion", "Romania", "Russian Federation", "Rwanda", "Saint Helena",
            "Saint Kitts and Nevis", "Saint Lucia", "Saint Pierre and Miquelon",
            "Saint Vincent and the Grenadines", "Samoa", "San Marino", "Sao Tome and Principe", "Saudi Arabia",
            "Senegal", "Serbia and Montenegro", "Seychelles", "Sierra Leone", "Singapore", "Slovakia", "Slovenia",
            "Solomon Islands", "Somalia", "South Africa", "South Georgia and the South Sandwich Islands",
            "Spain", "Sri Lanka", "Sudan", "Suriname", "Svalbard and Jan Mayen", "Swaziland", "Sweden", "Switzerland",
            "Syrian Arab Republic", "Taiwan, Province of China", "Tajikistan", "Tanzania, United Republic of",
            "Thailand", "Timor-Leste", "Togo", "Tokelau", "Tonga", "Trinidad and Tobago", "Tunisia", "Turkey",
            "Turkmenistan", "Turks and Caicos Islands", "Tuvalu", "Uganda", "Ukraine", "United Arab Emirates",
            "United Kingdom", "United States", "United States Minor Outlying Islands", "Uruguay", "Uzbekistan",
            "Vanuatu", "Venezuela", "Viet Nam", "Virgin Islands, British", "Virgin Islands, U.S.",
            "Wallis and Futuna", "Western Sahara", "Yemen", "Zambia", "Zimbabwe"
    };
}

上面的示例中，我们使用了一个字符串数组作为搜索关键字，你可以根据实际情况替换为你自己的数据源。当用户在 AutoCompleteTextView 中输入搜索关键字时，适配器会根据输入的内容过滤数据源，并显示匹配的搜索结果。

当用户从搜索结果中选择一个选项时，我们在监听器中获取选项的文本内容，并进行相应的操作。在上面的示例中，我们只是简单地显示了一个 Toast 消息，你可以根据实际需求进行其他操作。