Android TextWatcher监控EditText中的输入内容并限制其输入字符个数

布局中EditText在android布局中经常用到，对EditText中输入的内容也经常需要进行限制，我们可以通过TextWatcher去观察输入框中输入的内容，作个笔记。

主布局：

```
java代码：

package com.android.text;

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;
import android.widget.Toast;

public class TextWatcherDemo extends Activity {
private TextView mTextView;
private EditText mEditText;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
mTextView = (TextView)findViewById(R.id.tv);
mEditText = (EditText)findViewById(R.id.ET);
mEditText.addTextChangedListener(mTextWatcher);
}
TextWatcher mTextWatcher = new TextWatcher() {
private CharSequence temp;
private int editStart ;
private int editEnd ;
@Override
public void beforeTextChanged(CharSequence s, int arg1, int arg2,
int arg3) {
temp = s;
}

    @Override
    public void onTextChanged(CharSequence s, int arg1, int arg2,
            int arg3) {
        mTextView.setText(s);
    }
   
    @Override
    public void afterTextChanged(Editable s) {
        editStart = mEditText.getSelectionStart();
        editEnd = mEditText.getSelectionEnd();
        if (temp.length() > 10) {
            Toast.makeText(TextWatcherDemo.this,
                    "你输入的字数已经超过了限制！", Toast.LENGTH_SHORT)
                    .show();
            s.delete(editStart-1, editEnd);
            int tempSelection = editStart;
            mEditText.setText(s);
            mEditText.setSelection(tempSelection);
        }
    }
};

}```