我们可以使用动画和布局参数来实现EditText的平滑扩展和收缩。我们需要设置EditText的maxLines为Integer.MAX_VALUE以允许多行输入。然后，我们可以使用TextChangedListener来检测当用户输入时EditText的高度是否需要扩展或收缩，并使用动画逐步更改高度值。以下是一个代码示例：

private int maxEditTextHeight;
private ObjectAnimator growAnimator, shrinkAnimator;

//在OnCreate（）方法中初始化EditText的布局参数和动画
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    final EditText editText = (EditText) findViewById(R.id.edit_text);
    maxEditTextHeight = editText.getLayoutParams().height;
    editText.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {}

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {}

        @Override
        public void afterTextChanged(Editable s) {
            Layout layout = editText.getLayout();
            int height = editText.getHeight();
            int newHeight = height;
            if (layout != null){
                int newLine = editText.getLineCount();
                if (newLine > 1 && newHeight <= maxEditTextHeight) {
                    newHeight = layout.getHeight() + editText.getPaddingTop() + editText.getPaddingBottom();
                    growAnimator = ObjectAnimator.ofInt(editText.getParent(), "height", editText.getParent().getHeight(), newHeight);
                    growAnimator.setDuration(200).setInterpolator(new AccelerateDecelerateInterpolator());
                    growAnimator.start();
                } else if (newLine == 1 && newHeight > maxEditTextHeight) {
                    newHeight = maxEditTextHeight;
                    shrinkAnimator = ObjectAnimator.ofInt(editText.getParent(), "height", editText.getParent().getHeight(), newHeight);
                    shrinkAnimator.setDuration(200).setInterpolator(new AccelerateDecelerateInterpolator());
                    shrinkAnimator.start();
                }
            }
        }
    });
}

在上面的代码中，“maxEditTextHeight”是初始EditText高度，我们使用TextChangedListener来检