Android学习笔记之View（二）

View加载的流程之测量：rootView调用measure（）→onMeasure（）；

measure（）是final方法，表明Android不想让开发者去修改measure的框架，开发者可以onMeasure方法。

来看一下measure的代码:

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
mPrivateFlags &= ~MEASURED_DIMENSION_SET;
if (ViewDebug.TRACE_HIERARCHY) {
ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);
}
onMeasure(widthMeasureSpec, heightMeasureSpec);
if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
throw new IllegalStateException("onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
}

一个界面的展示可能会涉及到很多次发measure，因为一个视图往往包含多个子视图，每个视图都需要经历一次measure过程。ViewGroup中定义了measureChildren（）方法来测量子视图，下面是measureChildren（）：

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
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) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}

View加载的第二步： onLayout，这和measure差不多，都是由rootView调用layout（）→onLayout（）。

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