getLayoutParams()方法和setLayoutParams()方法

参考来源：http://blog.csdn.net/liuhaomatou/article/details/22899925#comments
1. getLayoutParams()和setLayoutParams()方法的解析
a. getLayoutParams():

/**
     * Get the LayoutParams associated with this view. All views should have
     * layout parameters. These supply parameters to the parent 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.
     *
     * This method may return null if this View is not attached to a parent
     * ViewGroup or {@link #setLayoutParams(android.view.ViewGroup.LayoutParams)}
     * was not invoked successfully. When a View is attached to a parent
     * ViewGroup, this method must not return null.
     *
     * @return The LayoutParams associated with this view, or null if no
     *         parameters have been set yet
     */

  public ViewGroup.LayoutParams getLayoutParams() {
    return mLayoutParams;
}

大意就是getLayoutParams（）方法的调用必须要有父控件。否则返回的就是null对象。该方法返回的是该控件的LayoutParams 对象。

b. setLayoutParams():

/**
     * Set the layout parameters associated with this view. These supply
     * parameters to the parent 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, cannot be null
     */
    public void setLayoutParams(ViewGroup.LayoutParams params) {
        if (params == null) {
            throw new NullPointerException("Layout parameters cannot be null");
        }
        mLayoutParams = params;
        resolveLayoutParams();
        if (mParent instanceof ViewGroup) {
            ((ViewGroup) mParent).onSetLayoutParams(this, params);
        }
        requestLayout();
    }

设置该控件的布局参数，也就是将修改好的参数信息重新赋值给该控件。

如下的示例代码：

 mImageViewTabline = (ImageView) findViewById(R.id.iv_tabline);
        //
        Display display = getWindow().getWindowManager().getDefaultDisplay();
        DisplayMetrics displayMetrics = new DisplayMetrics();
        display.getMetrics(displayMetrics);
        //  获取到屏幕的的三分之一
        mScreen1_3 = displayMetrics.widthPixels / 3;
        // 获取到tabline控件的LayoutParams
        LayoutParams params = mImageViewTabline.getLayoutParams();
        // 设置tabline的宽度
        params.width = mScreen1_3;
        // 将修改好的的宽度设置给tabline控件
        mImageViewTabline.setLayoutParams(params);

1. 关于setLayoutParams报错
   　　在继承BaseAdapter的时候，用getView返回View的时候，用代码控制布局，需要用到View.setLayoutParams，但是报错了，报的是类型转换错误，经过研究，发现，这里不能使用ViewGroup.LayoutParams而必须使用对应父View的LayoutParams类型。如：某View被LinearLayout包含，则该View的setLayoutParams参数类型必须是LinearLayout.LayoutParams。原因在于LinearLayout（或其他继承自ViewGroup的layout，如：RelativeLayout）在进行递归布局的时候，LinearLayout会获取子View的LayoutParams，并强制转换成LinearLayout.LayoutParams，如LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();

或者是如下定义：
LayoutParams lp = (LayoutParams) child.getLayoutParams();

以转换成内部类型LinearLayout.LayoutParams。

自己测试运行的时候报空指针，原因为child.getLayoutParams();这里没有获得到子控件所在的布局，查看代码发现parent.addView(child);应该写在上面，之后child才能getLayoutParams();

1. LayoutParams类的解析
   LayoutParams继承于Android.View.ViewGroup.LayoutParams.
   LayoutParams相当于一个Layout的信息包，它封装了Layout的位置、高、宽等信息。假设在屏幕上一块区域是由一个Layout占领的，如果将一个View添加到一个Layout中，最好告诉Layout用户期望的布局方式，也就是将一个认可的layoutParams传递进去。
   可以这样去形容LayoutParams，在象棋的棋盘上，每个棋子都占据一个位置，也就是每个棋子都有一个位置的信息，如这个棋子在4行4列，这里的“4行4列”就是棋子的LayoutParams。
   但LayoutParams类也只是简单的描述了宽高，宽和高都可以设置成三种值：
   1，一个确定的值；
   2，FILL_PARENT，即填满（和父容器一样大小）；
   3，WRAP_CONTENT，即包裹住组件就好。
   LayoutParams类是用于child view（子视图） 向 parent view（父视图）传达自己的意愿的一个东西（孩子想变成什么样向其父亲说明）其实子视图父视图可以简单理解成一个LinearLayout 和 这个LinearLayout里边一个 TextView 的关系 。TextView 就是LinearLayout的子视图 child view 。需要注意的是LayoutParams只是ViewGroup的一个内部类，这里边这个也就是ViewGroup里边这个LayoutParams类是 base class 基类实际上每个不同的ViewGroup都有自己的LayoutParams子类。
   也就是说在LinearLayout类中有LayoutParams子类，在ViewGroup里面同样有LayoutParams子类。感兴趣的可以去翻看源码就知道了。

示例代码：

public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
                Log.e("tag", position + "," + positionOffset + "," + positionOffsetPixels);
                // 获取到父控件的参数
                LinearLayout.LayoutParams layoutParams = (LinearLayout.LayoutParams) mImageViewTabline.getLayoutParams();
                // 0--->1
                if (mCurrentPageIndex == 0 && position == 0) {
                    layoutParams.leftMargin = (int) (positionOffset * mScreen1_3 + mCurrentPageIndex * mScreen1_3);
                } else if (mCurrentPageIndex == 1 && position == 0) {
                    //1---->0
                    layoutParams.leftMargin = (int) (mCurrentPageIndex * mScreen1_3 + (positionOffset - 1) * mScreen1_3);
                } else if (mCurrentPageIndex == 1 && position == 2) {
                    // 1--->2
                    layoutParams.leftMargin = (int) (mCurrentPageIndex * mScreen1_3 + positionOffset * mScreen1_3);
                } else if (mCurrentPageIndex == 2 && position == 1) {
                    // 2--->1
                    layoutParams.leftMargin = (int) (mCurrentPageIndex * mScreen1_3 + (positionOffset - 1) * mScreen1_3);
                }

                // 设置宽度给tabline
                mImageViewTabline.setLayoutParams(layoutParams);

            }