android - LayoutParams 返回 null

Question

这是我的代码，但不知道为什么返回 null？但是我可以在这里进行空检查，但是有什么问题吗？

    TextView descriptiontv = (TextView) findViewById(R.id.descriptiontv);
    TextView tc = new TextView(c);
    final PopupWindow windowPopUp = new PopupWindow(tc,LayoutParams.MATCH_PARENT,LayoutParams.WRAP_CONTENT ,false);

    tc.setBackgroundResource(R.drawable.bg_training_baloon_hc_2x);
    tc.setText("this is a demo test to check how it looks, i m just wanting to test whether it works or not");
    tc.setPadding(20, 20, 20, 20);
    LinearLayout.LayoutParams params = new android.widget.LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT);
    params.setMargins(30, 0, 30, 0);
    tc.setLayoutParams(params);

Answer 1

根据我的经验，你必须先将 View 添加到它的父 View ，然后才能为其设置布局参数，如下所示:

TextView calendarYear = new TextView(getActivity());
calendarYear.setText(calendar.getYear() + "");
calendarYear.setTextSize(20);
calendarYear.setTypeface(null, Typeface.BOLD);
calendarYear.setGravity(Gravity.CENTER);
calendarYear.setTextColor(resources.getColor(android.R.color.black));
calendarItemsLinearLayout.addView(calendarYear);

// We have to add the TextView to the layout first before we
// can set the layout margins for it
ViewGroup.LayoutParams layoutParams = calendarYear.getLayoutParams();
((LinearLayout.LayoutParams)layoutParams).setMargins(0, (int)(16 * density), 0, 0);
calendarYear.setLayoutParams(layoutParams);