本篇模拟三个角色：Android 架构师-小福、Android 控件开发工程师-小黑、 Android 开发工程师-小白，下面按照三个角色不同角度分析measure过程。
小福负责分享：
小黑负责分享：
Android 控件开发工程师-小黑的分享
一、布局控件开发中覆写Measure例子
具体可查看之前写的例子《覆写onMeaure进行measure操作》，Android提供的布局FrameLaout、LinearLayout、RelativeLayout都是ViewGroup的子类，可以参考这些类的onMeausre实现，看看Android框架是如何处理的。
二、从遇到的一个异常说起
如果在onMeasure中使用return，并未进行setMeasuredDimension(width, height);类似操作，会出现以下异常
java.lang.IllegalStateException: onMeasure() did not set the measured dimension by calling setMeasuredDimension()
-待填充-
ViewGroup提供的measure方法
measureChildren() – 该函数内使用for()循环调用measureChild()对每一个子视图进行measure操作
measureChild() – 为每一个子视图进行measure操作
measureChildWidthMargins() – 该函数与measureChild的唯一区别在于，measure时考虑把margin及padding也作为子视图大小的一部分
三、什么时候需要覆写onMeaure？
只要是自定义控件并且是ViewGroup都需要覆写onMeasure来指定其测量视图大小规则，但是Androd提供FrameLaout、LinearLayout、RelativeLayout等都是ViewGroup的子类并且都覆写了onMeasure方法，自定义布局的时候可以先考虑先从这些类的子类入手，可能更简单一些。
四、view.getWidth()与view.getMeasuredWidth()区别
从名字上可以看出这两个方法都是为了获取宽度的，相应的也有获取高度的方法，但是问题在于两者的区别是什么？ 下面直接从源码中查看这些值是从哪里来的，从而获知两者的区别。
首先看下View.java中的getMeasuredWidth方法，源码如下：
public class View implements Drawable.Callback, Drawable.Callback2, KeyEvent.Callback,AccessibilityEventSource {/*** Like {@link #getMeasuredWidthAndState()}, but only returns the* raw width component (that is the result is masked by* {@link #MEASURED_SIZE_MASK}).** @return The raw measured width of this view.*/public final int getMeasuredWidth() {// 直接返回mMeasuredWidth与后者相与清理掉其他开关获取真是measure大小return mMeasuredWidth & MEASURED_SIZE_MASK;}/*** <p>This mehod must be called by {@link #onMeasure(int, int)} to store the* measured width and measured height. Failing to do so will trigger an* exception at measurement time.</p>** @param measuredWidth The measured width of this view. May be a complex* bit mask as defined by {@link #MEASURED_SIZE_MASK} and* {@link #MEASURED_STATE_TOO_SMALL}.* @param measuredHeight The measured height of this view. May be a complex* bit mask as defined by {@link #MEASURED_SIZE_MASK} and* {@link #MEASURED_STATE_TOO_SMALL}.*/protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {// 通常在onMeasure中调用，传入测量过的视图宽度与高度mMeasuredWidth = measuredWidth;mMeasuredHeight = measuredHeight;mPrivateFlags |= MEASURED_DIMENSION_SET;}/*** Bits of {@link #getMeasuredWidthAndState()} and* {@link #getMeasuredWidthAndState()} that provide the actual measured size.*/ // MeasureSpec中的Mode或占用int类型中前几位public static final int MEASURED_SIZE_MASK = 0x00ffffff;} 从上面两个方法可以看出getMeasuredWidth的值是从mMeasuredWidth变量获取，而这个变量仅在View.setMeasuredDimension方法中继续初始化，，从setMeasuredDimension方法的注释中就可以看出这个方式是在onMeasure中被调用，也就是getMeasuredWidth获取到的是在视图onMeasure方法中已经获取到视图的大小之后，才能进行赋值。getMeasuredWidth获取的是通过onMeasure测量后的值，在onMeasure执行之前可以调用但是获取到的都是0（int类型的默认初始化值）。
上面已经知道getMeasuredWidth值的含义，接着来看下View.getWidth方法的源码：
public class View implements Drawable.Callback, Drawable.Callback2, KeyEvent.Callback,AccessibilityEventSource {/*** Return the width of the your view.** @return The width of your view, in pixels.*/@ViewDebug.ExportedProperty(category = "layout")public final int getWidth() {// 视图的右侧减去左侧的值return mRight – mLeft;}/*** Assign a size and position to this view.** This is called from layout.** @param left Left position, relative to parent* @param top Top position, relative to parent* @param right Right position, relative to parent* @param bottom Bottom position, relative to parent* @return true if the new size and position are different than the*previous ones* {@hide}*/protected boolean setFrame(int left, int top, int right, int bottom) {boolean changed = false;……// 四个值中任意一个发生改变就行if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {changed = true;……mLeft = left;mTop = top;mRight = right;mBottom = bottom;……}return changed;}public void layout(int l, int t, int r, int b) {……// 当前视图布局时执行，传入当前视图的上下左右边界值boolean changed = setFrame(l, t, r, b);if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {……// 上面执行完成后才会触发onLayoutonLayout(changed, l, t, r, b);……}……mPrivateFlags &= ~FORCE_LAYOUT;}} 从上面的代码可以看出当视图layout操作时，会先调用setFrame方法传入left, top, right, bottom 这些值会在以后layout布局分析是进行详细解释，这些值是其父视图给他当前视图设定的可显示位置与大小（right – left 与 top – bottom获得）。getWidth方法获取的宽度是当前视图可以在屏幕实际上占据的大小。
以诚感人者，人亦诚而应。
