View在onCreate的时候，没有渲染组件，所以获取到的宽度和高度为0，
需要添加一个观察者，在layout渲染后再去取宽高。代码如下：
private Button btn_icon;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_image_button);
btn_icon = findViewById(R.id.btn_icon);
ViewTreeObserver vto = btn_icon.getViewTreeObserver();
vto.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
btn_icon.getViewTreeObserver().removeGlobalOnLayoutListener(this);
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) btn_icon.getLayoutParams();
Context ctx = btn_icon.getContext();
int marginLen = (DisplayUtil.getScreenWidth(ctx) - btn_icon.getWidth()) / 2;
lp.setMargins(marginLen, 20, marginLen, 20);
btn_icon.setLayoutParams(lp);
}
});
}
参考：https://blog.csdn.net/zheng5229875/article/details/39314159
