Android android.text.TextWatcher詳解

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

當一個可編輯的對象的文本改變的時候，它的方法就會被調用。

此接口有三個方法

afterTextChanged(Editable s)

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

當s的某個位置的文本已經改變，這個方法會被調用

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，當從start開始的count個字符將被長度爲after的新文本替換時，這個方法會被調用

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，當從start開始有count個字符替換長度爲before的舊文本時，這個方法會被調用

下面是一個限制輸入"."個數的例子

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("已經輸入\".\"不能重複輸入");
                        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);
            }
        });
    }

}