首先，onChange事件处理程序的参数不是输入的新值，而是一个事件对象。

onChange(event) {
    let text = event.target.value;
    ...

我试着那样做但是

export default function AppTextInput({icon, placeholder,onChangeText, ...otherProps}) {

  const  onChanged =(text) =>{
      let newText = '';
      let numbers = '0123456789';
      for (var i=0; i < text.length; i++) {

              if (numbers.indexOf(text[i]) > -1) {
                  newText = newText + text[i];
              } else {
                  alert("please enter integer numbers only");

              }
      }

    }

    return (

        <View style={styles.container}>
            {icon &&
                <MaterialCommunityIcons style={{marginRight: 10}} name={icon} color={colors.grayMedium} size={20}/>}
            <TextInput style={defaultStyles.text} placeholder={placeholder}
                       onChangeText={(text)=> onChanged(text)}  maxLength={3} {...otherProps}
            ></TextInput>
        </View>
    )
}

当我输入时，警报工作，但是onChangeText={（text）=〉setNumRounds（text）}和其他人看不到我输入，为什么？

<View style={{top: -80}}>
                    <AppTextInput icon="timer-sand"  placeholder={"Prep Time"} keyboardType='numeric' onChangeText={(text) => setPrepTime(text)}/>
                    <AppTextInput icon="timer" placeholder={"Round Duration"} keyboardType='number-pad' onChangeText={(text) => setRoundDuration(text)}/>
                    <AppTextInput icon="timer" placeholder={"Break Duration"} keyboardType='number-pad' onChangeText={(text) => setBreakDuration(text)}/>
                    <AppTextInput icon="repeat" placeholder={"Number of Rounds"} keyboardType='number-pad'  onChangeText={(text) => setNumRounds(text)}/>
                    <AppTextInput icon="format-list-numbered" placeholder={"Number of Sets"} keyboardType='number-pad' onChangeText={(text) => setNumSets(text)}/>
                    {exerciseInputEles}
                </View>

下面是您可以使用的代码：
1.我将onChanged函数移到另一个文件中，并使用Regex将非数字字符替换为空字符串。
除此之外，我将onChangeText的输入值从(text)=> onChanged(text)更改为onChangeText（这是 prop ），因为onChanged不再存在。

export default function AppTextInput({
  icon,
  placeholder,
  onChangeText,
  ...otherProps
}) {
  return (
    <View style={styles.container}>
      {icon && (
        <MaterialCommunityIcons
          style={{ marginRight: 10 }}
          name={icon}
          color={colors.grayMedium}
          size={20}
        />
      )}
      <TextInput
        style={defaultStyles.text}
        placeholder={placeholder}
        onChangeText={onChangeText}
        maxLength={3}
        {...otherProps}
      ></TextInput>
    </View>
  );
}

1.这里我添加了一个名为replaceNonNumeric的函数，该函数用于将任何非数字的数字替换为空字符串，这样就只有数字在里面了。
1.我在您更改状态之前调用了replaceNonNumeric，并将新值传递给setState。

...
 const replaceNonNumeric = (text) => {
    return text.replace(/[^0-9]/g, '');
  };

  return (
    <View style={{ top: -80 }}>
      <AppTextInput
        icon="timer-sand"
        placeholder={'Prep Time'}
        keyboardType="numeric"
        onChangeText={(text) => {
          const newText = replaceNonNumeric(text);
          setPrepTime(newText);
        }}
      />
      <AppTextInput
        icon="timer"
        placeholder={'Round Duration'}
        keyboardType="number-pad"
        onChangeText={(text) => {
          const newText = replaceNonNumeric(text);
          setRoundDuration(newText);
        }}
      />
      <AppTextInput
        icon="timer"
        placeholder={'Break Duration'}
        keyboardType="number-pad"
        onChangeText={(text) => {
          const newText = replaceNonNumeric(text);
          setBreakDuration(newText);
        }}
      />
      <AppTextInput
        icon="repeat"
        placeholder={'Number of Rounds'}
        keyboardType="number-pad"
        onChangeText={(text) => {
          const newText = replaceNonNumeric(text);
          setNumRounds(newText);
        }}
      />
      <AppTextInput
        icon="format-list-numbered"
        placeholder={'Number of Sets'}
        keyboardType="number-pad"
        onChangeText={(text) => {
          const newText = replaceNonNumeric(text);
          setNumSets(text);
        }}
      />
      {exerciseInputEles}
    </View>
...