Android android.text.TextWatcher详解

When an object of a type is attached to an Editable, its methods will be called when the text is changed.

当一个可编辑的对象的文本改变的时候，它的方法就会被调用。

此接口有三个方法

afterTextChanged(Editable s)

This method is called to notify you that, somewhere within s, the text has been changed.

当s的某个位置的文本已经改变，这个方法会被调用

beforeTextChanged(CharSequence s, int start, int count, int after)

This method is called to notify you that, within s, the count characters beginning at start are about to be replaced by new text with length after.

在s中，当从start开始的count个字符将被长度为after的新文本替换时，这个方法会被调用

onTextChanged(CharSequence s, int start, int before, int count)

This method is called to notify you that, within s, the count characters beginning at start have just replaced old text that had length before.

在s中，当从start开始有count个字符替换长度为before的旧文本时，这个方法会被调用

下面是一个限制输入"."个数的例子

import android.app.Activity;import android.os.Bundle;import android.text.Editable;import android.text.TextWatcher;import android.util.Log;import android.widget.EditText;import android.widget.TextView;public class MainActivity extends Activity {    EditText et_content;    TextView tv_numOfChar;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        et_content = (EditText)findViewById(R.id.et_content);        tv_numOfChar = (TextView)findViewById(R.id.tv_numOfChar);        et_content.addTextChangedListener(new TextWatcher() {            @Override            public void beforeTextChanged(CharSequence s, int start, int count, int after) {                Log.i("log","beforeTextChanged:" + s + "------" + start + "------" + after + "------"+ count);            }            @Override            public void onTextChanged(CharSequence s, int start, int before, int count) {                Log.i("log","onTextChanged:" + s + "------" + start + "------"+ before+ "------" + count);                if (et_content.getText().toString().indexOf(".") >= 0) {                    tv_numOfChar.setText("已经输入\".\"不能重复输入");                        if (et_content.getText().toString().indexOf(".", et_content.getText().toString().indexOf(".") + 1) > 0) {                            et_content.setText(et_content.getText().toString().substring(0, et_content.getText().toString().length() - 1));                            et_content.setSelection(et_content.getText().toString().length());                        }                }            }            @Override            public void afterTextChanged(Editable s) {                Log.i("log","afterTextChanged:" + s);            }        });    }}

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