安卓开发学习之FrameLayout测量流程源码阅读

背景

前两天我分别阅读了下相对布局和线性布局的测量流程的源代码，今天，我就安卓开发中的框架布局(FrameLayout)的测量流程源码的阅读，进行一些记录

分步骤

框架布局的onMeasure()相比相对布局和线性布局的要简单的多，只有一百零几行，步骤也只有四步

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        /**
         * 步骤：
         *  1、初始化变量
         *  2、遍历子view，获取子view中的最大尺寸
         *  3、保存当前布局的尺寸
         *  4、测量match_parent的子view
         */
         ...
}

那就一步一步来看吧

初始化变量

        int count = getChildCount();

        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        // 当前布局宽高，有一个不是精确模式，就要测量match_parent的子view

        mMatchParentChildren.clear();

        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;

没啥好说的

遍历子view，获取最大宽高

       for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                // 子view的第一次测量

                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                // 保存子view的最大尺寸

                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
                // match_parent的子view要再测量一次，因为它们的尺寸也受当前view的影响
            }
        }

        // 加入布局自身的内间距
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // 加上前景图片的宽度
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }

框架布局处理了前景图片，而线性布局和相对布局都没有处理

保存当前布局尺寸

        // 测量当前view，也就是给measuredWidth/Height赋值
        // 故而当前view的尺寸也取决于子view中的最大尺寸
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));

给match_parent的子view重新测量

        // 最后根据当前view的尺寸，给match_parent的子view测量
        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    // 子width = 当前width - 横向内外间距 也就是子view最大的宽度
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                    // 不是match_parent，就具体情况具体分析
                }

                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
                // 故而，match_parent的子view的尺寸，取决于当前view和自己设定的间距
            }
        }

可以看到，框架布局对于子view的match_parent属性处理也很简单，就是给子view最大的尺寸，这一点跟另外两个布局是一样的

结语

可以看到，框架布局的onMeasure()很简单，没有考虑规则、权重、顶点的定位这些东西，所以它在开发过程中，还是很有运用价值的。

接下来，我会在安卓开发学习之FrameLayout的layout过程里，记录框架布局的onLayout()方法的阅读