getLayoutParams()和setLayoutParams()方法源码

今天在写侧滑菜单的时候发现了这个一个问题，初始化的时候在onCreate函数里面设置menu的leftMargin：

menulParams = (LayoutParams) menu.getLayoutParams();
menulParams.width = menuWidth;
menulParams.leftMargin = -1*menuWidth;
content.getLayoutParams().width = screenWidth;

无需调用setLayoutParams函数就可以实现效果，我的解释就是通过getLayoutParams函数得到了LayoutParams的引用，然后修改这个值肯定是会生效的，然而在触摸事件中，我也如法炮制，但是却始终划不出来。

Log.i("debug", "show");
//显示菜单
menulParams.leftMargin = 0;

为什么在onCreate中设置参数的时候get出来设置一下就可以，现在却不可以了呢？于是决定查看一下源代码是怎么回事：

   /**
     * Get the LayoutParams associated with this view. All views should have
     * layout parameters. These supply parameters to the <i>parent</i> of this
     * view specifying how it should be arranged. There are many subclasses of
     * ViewGroup.LayoutParams, and these correspond to the different subclasses
     * of ViewGroup that are responsible for arranging their children.
     * @return The LayoutParams associated with this view
     */
    @ViewDebug.ExportedProperty(deepExport = true, prefix = "layout_")
    public ViewGroup.LayoutParams getLayoutParams() {
        return mLayoutParams;
    }

    /**
     * Set the layout parameters associated with this view. These supply
     * parameters to the <i>parent</i> of this view specifying how it should be
     * arranged. There are many subclasses of ViewGroup.LayoutParams, and these
     * correspond to the different subclasses of ViewGroup that are responsible
     * for arranging their children.
     *
     * @param params the layout parameters for this view
     */
    public void setLayoutParams(ViewGroup.LayoutParams params) {
        if (params == null) {
            throw new NullPointerException("params == null");
        }
        mLayoutParams = params;
        requestLayout();
    }

可以看出：getLayoutParams就是将布局参数的引用返回来，在setLayoutParams函数中除了将布局的形参赋值，还有调用了requestLayout()

恍然大悟，设置参数后是需要对View进行重绘的。在onCreate里面的代码执行的时候View还没有绘制，所以设置完参数后接下来绘制就会生效，而后来的布局参数的改变必须自己调用使View进行重新绘制，所以需要调用setLayoutParams函数！

为了验证，我们不调用setLayoutParams函数，添加上同样OK。

menu.requestLayout();

同样OK。