android TextWatcher 学习

1.简介

主要用来监听用户输入，然后剪裁输入。

比如输入框只能输入8个字节的内容，就可以用TextWatcher来实现。

public interface TextWatcher extends NoCopySpan { / * 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. * It is an error to attempt to make changes to s from * this callback. */ public void 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 * have just replaced old text that had length before. * It is an error to attempt to make changes to s from * this callback. */ public void onTextChanged(CharSequence s, int start, int before, int count); / * This method is called to notify you that, somewhere within * s, the text has been changed. * It is legitimate to make further changes to s from * this callback, but be careful not to get yourself into an infinite * loop, because any changes you make will cause this method to be * called again recursively. * (You are not told where the change took place because other * afterTextChanged() methods may already have made other changes * and invalidated the offsets. But if you need to know here, * you can use {@link Spannable#setSpan} in {@link #onTextChanged} * to mark your place and then look up from here where the span * ended up. */ public void afterTextChanged(Editable s); }

想要了解方法的参数什么意思，把注释看一下，然后把参数全部打印一遍就行。

这三个方法很简单，值得注意的地方：

每当输入一次，就会调用一次 before-on-after。每当调用setText(“xxx”)也会如此，那么在on方法中调用setText(“xxx”)就会形成递归，然后就可能死循环。

但毕竟监听这个就是要修改内容的，根据源码注释，可以在after中修改字符。

2.使用

val input1 = findViewById 
  
    (R.id.input1) input1.addTextChangedListener(Watcher1(input1, 8, TAG)) 
  

先简单尝试，限制8个数字或字母

class Watcher1(private val editText: EditText, private val limit: Int, private val TAG: String): TextWatcher { private var suitable = true private val what = "luo" private val editable = editText.editableText override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) { Log.d(TAG, "beforeTextChanged") } override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) { Log.d( MainActivity.TAG, "onTextChanged: s = $s, start = $start, before = $before, count = $count" ) s?.let { suitable = s.length <= limit if (!suitable) { editable.setSpan(what, start, start + count, 1) } } } override fun afterTextChanged(s: Editable?) { Log.d(TAG, "afterTextChanged") if (!suitable) { s?.let { // 注意substring的参数范围 Log.d(TAG, "afterTextChanged: ${editable.getSpanStart(what)}") val start = s.substring(0, editable.getSpanStart(what)) Log.d(TAG, "afterTextChanged: start = $start") val end = s.substring(editable.getSpanEnd(what), s.length) Log.d(TAG, "afterTextChanged: end = $end") val text = start + end // 会继续调用before、on、after editText.setText(text) editText.setSelection(editable.getSpanStart(what)) } } } }

思路：在on中，字符串是已经被修改了的，所以在on中判断，如果字符串不符合要求，就记录这次的输入详情，然后在after中截取字符串。

值得注意的是，如果你使用了after回调的参数 "s"，也许会有bug，因为after中调用setText()，就会形成递归：因为s是第一次的，还未修改，调用setText()修改后会有第二次after，第二次的参数才是真正想要的。所以最好是不用这个参数，而是使用editText的get方法。

混合输入时，限制长度

由于数字、汉字、表情的byte各不相同。如果我们想要保证输入的byte总数不超过定值，那就需要监听输入，然后剪裁。

class Watcher2(private val editText: EditText, private val limit: Int, private val TAG: String) : TextWatcher { private val change = IntArray(2) override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) { Log.d(TAG, "beforeTextChanged") } override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) { Log.d(TAG, "onTextChanged s = $s, start = $start, before = $before, count = $count") if (!suitable(s.toString())) { change[0] = start change[1] = start + count } } override fun afterTextChanged(s: Editable?) { Log.d(TAG, "afterTextChanged") if (!suitable(s.toString())) { s?.let { // 注意substring的参数范围 val start = s.substring(0, change[0]) Log.d(TAG, "afterTextChanged: start = $start") val end = s.substring(change[1], s.length) Log.d(TAG, "afterTextChanged: end = $end") val text = start + end // 会继续调用before、on、after editText.setText(text) editText.setSelection(change[0]) handler.removeCallbacks(runnable) handler.postDelayed(runnable, 100) } } Log.d(TAG, "afterTextChanged: s = $s") } / * Determine whether the current input is legal */ private fun suitable(str: String): Boolean { val size = str.toByteArray().size Log.d(TAG, "suitable: size = $size") return size <= limit } private val handler = object : Handler(Looper.getMainLooper()) {} private val runnable = Runnable { Toast.makeText(editText.context, "限制 $limit byte!", Toast.LENGTH_SHORT).show() } }

在第一个案例中，我使用setSpan来记录输入详情，在这个案例中有点不好使，主要原因是长度不同的。

在after中截取了字符串后记得把输入光标挪个位置。

3.总结

1.弄清楚方法里的参数

2.根据需求，在on中记录输入，在after中截取字符串

3.避免无限递归

代码https://gitee.com/luoccxyz/text-watcher-test