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
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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