我找到了这个例子http://android-codes-examples.blogspot.com/2011/11/show-or-hide-soft-keyboard-on-opening.html。在alert.show()之前添加以下代码。

InputMethodManager imm = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
imm.toggleSoftInput(InputMethodManager.SHOW_FORCED,0);

我的方法使用了Android 11+的新方法，也支持旧版本:

fun Fragment.showKeyboard() {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
        ViewCompat.getWindowInsetsController(requireView())?.show(WindowInsetsCompat.Type.ime())
    } else {
        val focusedView = view?.findFocus() ?: view?.apply { requestFocus() }
        val imm = (context?.getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager)
        val isShowSucceeded = imm.showSoftInput(focusedView, InputMethodManager.SHOW_IMPLICIT)
        if(!isShowSucceeded) {
            imm.toggleSoftInputFromWindow(
                view?.windowToken, 0, InputMethodManager.HIDE_IMPLICIT_ONLY)
    }
}

}

问题似乎是，因为你输入文本的地方最初是隐藏的(或嵌套或其他)，AlertDialog自动设置标志WindowManager.LayoutParams。FLAG_ALT_FOCUSABLE_IM或WindowManager.LayoutParams。FLAG_NOT_FOCUSABLE，这样就不会触发软输入来显示。

解决这个问题的方法是添加以下内容:

(...)
// Create the dialog and show it
Dialog dialog = builder.create()
dialog.show();

// After show (this is important specially if you have a list, a pager or other view that uses a adapter), clear the flags and set the soft input mode
dialog.getWindow().clearFlags(WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE|WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM);
dialog.getWindow().setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_VISIBLE);

让我指出一些额外的信息，解决yuku，因为我发现很难得到这个工作!我如何从我的AlertDialog. builder获得AlertDialog对象?嗯，这是alert.show()执行的结果:

final AlertDialog.Builder alert = new AlertDialog.Builder(getActivity());
final EditText input = new EditText(getActivity());
alert.setView(input);

// do what you need, like setting positive and negative buttons...

final AlertDialog dialog = alert.show();

input.setOnFocusChangeListener(new OnFocusChangeListener() {
   @Override
   public void onFocusChange(View v, boolean hasFocus) {
      if(hasFocus) {
         dialog.getWindow().setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_VISIBLE);
      }
   }
});

我们可以在对话显示时默认打开键盘

dialog.getWindow().clearFlags(WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM)
dialog.getWindow().setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_VISIBLE)

看看https://stackoverflow.com/a/39144104/2914140，我简化了一点:

// In onCreateView():
view.edit_text.run {
    requestFocus()
    post { showKeyboard(this) }
}

fun showKeyboard(view: View) {
    val imm = view.context.getSystemService(
        Context.INPUT_METHOD_SERVICE) as InputMethodManager?
    imm?.showSoftInput(view, InputMethodManager.SHOW_IMPLICIT)
}

它比https://stackoverflow.com/a/11155404/2914140:好

InputMethodManager imm = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
imm.toggleSoftInput(InputMethodManager.SHOW_FORCED, 0);

因为当你按Home键并移动到主屏幕时，键盘会保持打开状态。