Android Button.getWidth()为0的问题

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