无论是出于项目功能需求还是出于界面美观，或者是因为项目中对控件进行封装等，自定义View在我们开发中是非常常见的，
 Android官方DEMO中也提供了自定义View的例子，今天学习的是自定义ViewGroup.

public class CustomLayout extends ViewGroup {
    /** 子控件离左边的距离 */
    private int mLeftWidth;
    /** 子控件离左边的距离 */
    private int mRightWidth;

    /** 计算子控件容器的范围 */
    private final Rect mTmpContainerRect = new Rect();
    /** 计算子控件的范围 */
    private final Rect mTmpChildRect = new Rect();

    /*
     * View的三个构造函数
     */
    public CustomLayout(Context context) {
        super(context);
    }

    public CustomLayout(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public CustomLayout(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    /**
     * 任何不滚动的布局管理器都应该像这样重写该方法
     */
    @Override
    public boolean shouldDelayChildPressedState() {
        return false;
    }

    /**
     * 测量（子）控件大小
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // 子控件数量
        int count = getChildCount();
        mLeftWidth = 0;
        mRightWidth = 0;

        // 设置最大高度，最大宽度及状态
        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;
        // 遍历所有子控件，分别进行测量
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                // 测量子控件：某一个子view宽及高,
                // 内部加上了viewGroup的padding值、margin值和传入的宽高widthUsed、heightUsed
                measureChildWithMargins(child, widthMeasureSpec, 0,
                        heightMeasureSpec, 0);

                // 通过LayoutParams来设置大小
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (lp.position == LayoutParams.POSITION_LEFT) {
                    mLeftWidth += Math.max(maxWidth, child.getMeasuredWidth()
                            + lp.leftMargin + lp.rightMargin);
                } else if (lp.position == LayoutParams.POSITION_RIGHT) {
                    mRightWidth += Math.max(maxWidth, child.getMeasuredWidth()
                            + lp.leftMargin + lp.rightMargin);
                } else {
                    maxWidth = Math.max(maxWidth, child.getMeasuredWidth()
                            + lp.leftMargin + lp.rightMargin);
                }
                maxHeight = Math.max(maxHeight, child.getMeasuredHeight()
                        + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState,
                        child.getMeasuredState());
            }
        }

        // 总宽度是内部所有子视图的宽度加上其他约束的宽度
        maxWidth += mLeftWidth + mRightWidth;

        // 检查默认的最小高度和最小宽度，取最大值
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // 设置最终的尺寸
        setMeasuredDimension(
                resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
    }

    /**
     * 设置子控件在ViewGroup中的位置
     */
    @Override
    protected void onLayout(boolean changed, int left, int top, int right,
            int bottom) {
        final int count = getChildCount();

        // 视图在布局中放置的左右外边
        int leftPos = getPaddingLeft();
        int rightPos = right - left - getPaddingRight();

        // 视图流内部的中心区域
        final int middleLeft = leftPos + mLeftWidth;
        final int middleRight = rightPos - mRightWidth;

        // 所操作的布局的顶边和底边
        final int parentTop = getPaddingTop();
        final int parentBottom = bottom - top - getPaddingBottom();

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                // 测量宽度和测量高度
                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                // 计算正在放置子视图的窗体
                if (lp.position == LayoutParams.POSITION_LEFT) {
                    mTmpContainerRect.left = leftPos + lp.leftMargin;
                    mTmpContainerRect.right = leftPos + width + lp.rightMargin;
                    leftPos = mTmpContainerRect.right;
                } else if (lp.position == LayoutParams.POSITION_RIGHT) {
                    mTmpContainerRect.right = rightPos - lp.rightMargin;
                    mTmpContainerRect.left = rightPos - width - lp.leftMargin;
                    rightPos = mTmpContainerRect.left;
                } else {
                    mTmpContainerRect.left = middleLeft + lp.leftMargin;
                    mTmpContainerRect.right = middleRight - lp.rightMargin;
                }
                mTmpContainerRect.top = parentTop + lp.topMargin;
                mTmpContainerRect.bottom = parentBottom - lp.bottomMargin;
                // //通过子视图的重心值和尺寸决定其在容器内的最终布局
                Gravity.apply(lp.gravity, width, height, mTmpContainerRect,
                        mTmpChildRect);

                // 放置子视图
                child.layout(mTmpChildRect.left, mTmpChildRect.top,
                        mTmpChildRect.right, mTmpChildRect.bottom);
            }
        }
    }

    /**
     * 下面的实现部分是针对每个子视图的布局参数的，如果你不需要这些（比如说你写了一个布局管理器）不需要混合放置子视图，那么你可以删除这部分
     */
    // ----------------------------------------------------------------------

    /**
     * 要自定义ViewGroup支持子控件的layout_margin参数，
     * 则自定义的ViewGroup类必须重载generateLayoutParams
     * ()函数，并且在该函数中返回一个ViewGroup.MarginLayoutParams派生类对象，这样才能使用margin参数。
     */
    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new CustomLayout.LayoutParams(getContext(), attrs);
    }

    // 返回一组默认布局参数。没有设置布局参数执行addview（View）时，这些参数被请求时返回null，并抛出一个异常。
    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams(LayoutParams.MATCH_PARENT,
                LayoutParams.MATCH_PARENT);
    }

    @Override
    protected ViewGroup.LayoutParams generateLayoutParams(
            ViewGroup.LayoutParams p) {
        return new LayoutParams(p);
    }

    @Override
    protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams;
    }

    /**
     * 自定义子控件的LayoutParams
     */
    public static class LayoutParams extends MarginLayoutParams {
        // 设置居中方式
        public int gravity = Gravity.TOP | Gravity.START;

        public static int POSITION_MIDDLE = 0;
        public static int POSITION_LEFT = 1;
        public static int POSITION_RIGHT = 2;

        public int position = POSITION_MIDDLE;

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);

            // 获取定义在XML中的自定义属性
            TypedArray a = c.obtainStyledAttributes(attrs,
                    R.styleable.CustomLayoutLP);
            gravity = a.getInt(
                    R.styleable.CustomLayoutLP_android_layout_gravity, gravity);
            position = a.getInt(R.styleable.CustomLayoutLP_layout_position,
                    position);
            a.recycle();
        }

        public LayoutParams(int width, int height) {
            super(width, height);
        }

        public LayoutParams(ViewGroup.LayoutParams source) {
            super(source);
        }
    }
}

 把定义好的ViewGroup当作我们通常使用的诸如LinearLayout等控件一样使用就可以啦。

更多API代码：https://github.com/ldm520/ANDROID_API_DEMOS等Android官方API开源代码。