FrameLayout设置margin无效

FrameLayout设置margin无效

一、问题背景

在FrameLayout中子View设置了android:layout_marginLeft，在4.0以下的机型中，展示效果没有距离左边的间隔；4.0及以上机型则显示正常。

二、问题分析

根据问题的现象，明显是父view在布局子view时出现的问题。因此，需要分析FrameLayout的onLayout方法的源码。

Android2.3.7（api-10）的onLayout方法的源码：

@Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        final int count = getChildCount();

        final int parentLeft = mPaddingLeft + mForegroundPaddingLeft;
        final int parentRight = right - left - mPaddingRight - mForegroundPaddingRight;

        final int parentTop = mPaddingTop + mForegroundPaddingTop;
        final int parentBottom = bottom - top - mPaddingBottom - mForegroundPaddingBottom;

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

                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                int childLeft = parentLeft;
                int childTop = parentTop;

                final int gravity = lp.gravity;

                if (gravity != -1) {
                    final int horizontalGravity = gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                    final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

                    switch (horizontalGravity) {
                        case Gravity.LEFT:
                            childLeft = parentLeft + lp.leftMargin;
                            break;
                        case Gravity.CENTER_HORIZONTAL:
                            childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                                    lp.leftMargin - lp.rightMargin;
                            break;
                        case Gravity.RIGHT:
                            childLeft = parentRight - width - lp.rightMargin;
                            break;
                        default:
                            childLeft = parentLeft + lp.leftMargin;
                    }

                    switch (verticalGravity) {
                        case Gravity.TOP:
                            childTop = parentTop + lp.topMargin;
                            break;
                        case Gravity.CENTER_VERTICAL:
                            childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                                    lp.topMargin - lp.bottomMargin;
                            break;
                        case Gravity.BOTTOM:
                            childTop = parentBottom - height - lp.bottomMargin;
                            break;
                        default:
                            childTop = parentTop + lp.topMargin;
                    }
                }

                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }
           

根据源码，我们可以知道，如果子view没有设置gravity，则不会去获取leftMargin值，因此childLeft没有被改变，导致展示时没有距离左边的间隔。

Android4.0.1（api-14）的onLayout方法的源码：

@Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        final int count = getChildCount();

        final int parentLeft = getPaddingLeftWithForeground();
        final int parentRight = right - left - getPaddingRightWithForeground();

        final int parentTop = getPaddingTopWithForeground();
        final int parentBottom = bottom - top - getPaddingBottomWithForeground();

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

                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                int childLeft;
                int childTop;

                int gravity = lp.gravity;
                if (gravity == -1) {
                    gravity = DEFAULT_CHILD_GRAVITY;
                }

                final int layoutDirection = getResolvedLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.LEFT:
                        childLeft = parentLeft + lp.leftMargin;
                        break;
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                        lp.leftMargin - lp.rightMargin;
                        break;
                    case Gravity.RIGHT:
                        childLeft = parentRight - width - lp.rightMargin;
                        break;
                    default:
                        childLeft = parentLeft + lp.leftMargin;
                }

                switch (verticalGravity) {
                    case Gravity.TOP:
                        childTop = parentTop + lp.topMargin;
                        break;
                    case Gravity.CENTER_VERTICAL:
                        childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                        lp.topMargin - lp.bottomMargin;
                        break;
                    case Gravity.BOTTOM:
                        childTop = parentBottom - height - lp.bottomMargin;
                        break;
                    default:
                        childTop = parentTop + lp.topMargin;
                }

                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }
           

根据源码，在4.0及以上版本中，如果子view没有设置gravity，会设置一个默认值，从而一步获取leftMargin值并修改childLeft，因此展示时就有了距离左边的间隔。

三、问题总结

在FrameLayout中布局子view时，若需要设置layout_margin值，需要设置子view的layout_gravity，默认设置Gravity.TOP| Gravity.LEFT。