1.簡介

主要用來監聽用戶輸入，然后剪裁輸入。

比如輸入框只能輸入8個字節的內容，就可以用TextWatcher來實現。

public interface TextWatcher extends NoCopySpan {/*** This method is called to notify you that, within <code>s</code>,* the <code>count</code> characters beginning at <code>start</code>* are about to be replaced by new text with length <code>after</code>.* It is an error to attempt to make changes to <code>s</code> from* this callback.*/public void beforeTextChanged(CharSequence s, int start,int count, int after);/*** This method is called to notify you that, within <code>s</code>,* the <code>count</code> characters beginning at <code>start</code>* have just replaced old text that had length <code>before</code>.* It is an error to attempt to make changes to <code>s</code> from* this callback.*/public void onTextChanged(CharSequence s, int start, int before, int count);/*** This method is called to notify you that, somewhere within* <code>s</code>, the text has been changed.* It is legitimate to make further changes to <code>s</code> 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); }

想要了解方法的參數什么意思，把注釋看一下，然后把參數全部打印一遍就行。

這三個方法很簡單，值得注意的地方：

每當輸入一次，就會調用一次 before-on-after。每當調用setText("xxx")也會如此，那么在on方法中調用setText("xxx")就會形成遞歸，然后就可能死循環。

但畢竟監聽這個就是要修改內容的，根據源碼注釋，可以在after中修改字符。

2.使用

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

先簡單嘗試，限制8個數字或字母

class Watcher1(private val editText: EditText, private val limit: Int, private val TAG: String): TextWatcher {private var suitable = trueprivate val what = "luo"private val editable = editText.editableTextoverride 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 <= limitif (!suitable) {editable.setSpan(what, start, start + count, 1)}}}override fun afterTextChanged(s: Editable?) {Log.d(TAG, "afterTextChanged")if (!suitable) {s?.let {// 注意substring的參數范圍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// 會繼續調用before、on、aftereditText.setText(text)editText.setSelection(editable.getSpanStart(what))}}} }

思路：在on中，字符串是已經被修改了的，所以在on中判斷，如果字符串不符合要求，就記錄這次的輸入詳情，然后在after中截取字符串。

值得注意的是，如果你使用了after回調的參數 "s"，也許會有bug，因為after中調用setText()，就會形成遞歸：因為s是第一次的，還未修改，調用setText()修改后會有第二次after，第二次的參數才是真正想要的。所以最好是不用這個參數，而是使用editText的get方法。

混合輸入時，限制長度

由于數字、漢字、表情的byte各不相同。如果我們想要保證輸入的byte總數不超過定值，那就需要監聽輸入，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] = startchange[1] = start + count}}override fun afterTextChanged(s: Editable?) {Log.d(TAG, "afterTextChanged")if (!suitable(s.toString())) {s?.let {// 注意substring的參數范圍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// 會繼續調用before、on、aftereditText.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().sizeLog.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() } }

在第一個案例中，我使用setSpan來記錄輸入詳情，在這個案例中有點不好使，主要原因是長度不同的。

在after中截取了字符串后記得把輸入光標挪個位置。

3.總結

1.弄清楚方法里的參數

2.根據需求，在on中記錄輸入，在after中截取字符串

3.避免無限遞歸

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

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