自定义view 的layout_margin不起作用解决方法

先总结两点

1. 自定义View在onDraw里面需要处理padding的影响，widthMeasureSpec和heightMeasureSpec是包含padding大小的。
2. 子View的margin属性是由ViewGroup处理的，ViewGroup在onMeasure和onLayout时一定要考虑 ViewGroup自己的padding和子View的margin的影响。

你可能遇到过下面这样的错误。

java.lang.ClassCastException: android.view.ViewGroup$LayoutParams cannot be cast to 
android.view.ViewGroup$MarginLayoutParams

找到异常产生的位置，追踪到ViewGroup.addView()方法,

查阅了FrameLayout和LinearLayout等都没有用过这个measureChildren呢，几乎全部都重写了，我们的自定义ViewGroup的measureChildren是不是应该是改成下面这样才对。

final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
    final View child = children[i];
    if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
        // ******************* 注意这里 ********************
        measureChildWithMargins(child, widthMeasureSpec, heightMeasureSpec);
    }
}

LinearLayout:

protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();///4

viewGroup:

public void addView(View child, int index) {
        if (child == null) {
            throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
        }
        LayoutParams params = child.getLayoutParams();
        if (params == null) {
            params = generateDefaultLayoutParams();
            if (params == null) {
                throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
            }
        }
        addView(child, index, params);
    }

private void addViewInner(View child, int index, LayoutParams params,
            boolean preventRequestLayout) {

        if (mTransition != null) {
            // Don't prevent other add transitions from completing, but cancel remove
            // transitions to let them complete the process before we add to the container
            mTransition.cancel(LayoutTransition.DISAPPEARING);
        }

        if (child.getParent() != null) {
            throw new IllegalStateException("The specified child already has a parent. " +
                    "You must call removeView() on the child's parent first.");
        }

        if (mTransition != null) {
            mTransition.addChild(this, child);
        }

        if (!checkLayoutParams(params)) {
            params = generateLayoutParams(params); ///1
        }

        if (preventRequestLayout) {
            child.mLayoutParams = params;
        } else {
            child.setLayoutParams(params); ////2
        }

 public void setLayoutParams(ViewGroup.LayoutParams params) {
        if (params == null) {
            throw new NullPointerException("Layout parameters cannot be null");
        }
        mLayoutParams = params;///3
        resolveLayoutParams();
        if (mParent instanceof ViewGroup) {
            ((ViewGroup) mParent).onSetLayoutParams(this, params);
        }
        requestLayout();
    }

这样原因就清除了

解决:

参考

public class MyViewGroup extends ViewGroup {
    // ..................... 其他代码省略 .....................
    
    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new MyLayoutParams(getContext(), attrs);
    }

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

    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new MyLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }

    public static class MyLayoutParams extends MarginLayoutParams {

        public MyLayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }

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

        public MyLayoutParams(LayoutParams lp) {
            super(lp);
        }
    }
}