综述
View的绘制流程可以分为三大步，它们分别是measure，layout和draw过程。measure表示View的测量过程，用于测量View的宽度和高度；layout用于确定View在父容器的位置；draw则是负责将View绘制到屏幕中。下面主要来看一下View的Measure过程。
测量过程
View的绘制流程是从ViewRoot的performTraversals方法开始的，ViewRoot对应ViewRootImpl类。ViewRoot在performTraversals中会调用performMeasure方法来进行对根View的测量过程。而在performMeasure方法中又会调用View的measure方法。对于View的measure方法它是一个final类型，也就是说这个measure方法不能被子类重写。但是在measure方法中调用了onMeasure方法。所以View的子类可以重写onMeasure方法来实现各自的Measure过程。在这里也就是主要对onMeasure方法进行分析。
MeasureSpec
MeasureSpec是View类中的一个静态内部类。一个MeasureSpec封装了父布局传递给子布局的布局要求。每个MeasureSpec都代表着一个高度或宽度的要求。每个MesureSpec都是由specSize和specMode组成，它代表着一个32位的int值，其中高2位代表specSize，低30位代表specMode。
MeasureSpec的测量模式有三种，下面介绍一下这三种测量模式：
UNSPECIFIED
父容器对子View没有任何的限制，子View可以是任何的大小。
EXACTLY
父容器为子View大小指定一个具体值，View的最终大小就是specSize。对应View属性match_parent和具体值。
AT_MOST
子View的大小最大只能是specSize，也就是所子View的大小不能超过specSize。对应View属性的wrap_content.
在MeasureSpec中可以通过specSize和specMode并使用makeMeasureSpec方法来创建一个MeasureSpec，还可以通过getMode和getSize来获取MeasureSpec的specMode和specSize。
View的测量过程
在上面已经说到，View的Measure过程是由measure方法来完成的，而measure方法通过调用onMeasure方法来完成View的Measure过程。那么就来看一下onMeasure方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }
在View的onMeasure方法中只是调用了setMeasuredDimension方法，setMeasuredDimension方法的作用就是设置View的高和宽的测量值。对于View测量后宽和高的值是通过getDefaultSize方法来获取的。下面就来一下这个getDefaultSize方法。
public static int getDefaultSize(int size, int measureSpec) { int result = size; int specMode = MeasureSpec.getMode(measureSpec); int specSize = MeasureSpec.getSize(measureSpec); switch (specMode) { case MeasureSpec.UNSPECIFIED: result = size; break; case MeasureSpec.AT_MOST: case MeasureSpec.EXACTLY: result = specSize; break; } return result; }
对于MeasureSpec的AT_MOST和EXACTLY模式下，直接返回的就是MeasureSpec的specSize，也就是说这个specSize就是View测量后的大小。而对于在UNSPECIFIED模式下，View的测量值则为getDefaultSize方法中的第一个参数size。这个size所对应的宽和高是通过getSuggestedMinimumWidth和getSuggestedMinimumHeight两个方法获取的。下面就来看一下这两个方法。
protected int getSuggestedMinimumHeight() { return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight()); } protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); }
在这里可以看到对于View宽和高的取值是根据View是否存在背景进行设置的。在这里以View的宽度来进行说明。若是View没有背景则是View的宽度mMinWidth。对于mMinWidth值得设置可以在XML布局文件中设置minWidth属性，它的默认值为0。也可以通过调用View的setMinimumWidth()方法其赋值。若是View存在背景的话，则取View本身最小宽度mMinWidth和View背景的最小宽度它们中的最大值。
ViewGroup的测量过程
对于ViewGroup的Measure过程，ViewGroup处理Measure自己本身的大小，还需要遍历子View，并调用它们的measure方法，然后各个子元素再去递归执行Measure过程。在ViewGroup中并没有重写onMeasure方法，因为ViewGroup它是一个抽象类，对于不同的具体ViewGroup它的onMeasure方法中所实现的过程不一样。但是在ViewGroup中提供了一个measureChildren方法，对子View进行测量。下面就来看一下这个measureChildren方法。
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); } } }
在这里获取ViewGroup中所有的子View。然后遍历ViewGroup中子View并调用measureChild方法来完成对子View的测量。下面看一下measureChild方法。
protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) { final LayoutParams lp = child.getLayoutParams(); final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width); final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height); child.measure(childWidthMeasureSpec, childHeightMeasureSpec); }
在这段代码中通过getChildMeasureSpec方法获取子View宽和高的MeasureSpec。然后调用子View的measure方法开始对View进行测量。下面就来看一下是如何通过getChildMeasureSpec方法来获取View的MeasureSpec的。
public static int getChildMeasureSpec(int spec, int padding, int childDimension) { int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); int size = Math.max(0, specSize - padding); int resultSize = 0; int resultMode = 0; switch (specMode) { // Parent has imposed an exact size on us case MeasureSpec.EXACTLY: if (childDimension >= 0) { resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size. So be it. resultSize = size; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent has imposed a maximum size on us case MeasureSpec.AT_MOST: if (childDimension >= 0) { // Child wants a specific size... so be it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size, but our size is not fixed. // Constrain child to not be bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size. It can't be // bigger than us. resultSize = size; resultMode = MeasureSpec.AT_MOST; } break; // Parent asked to see how big we want to be case MeasureSpec.UNSPECIFIED: if (childDimension >= 0) { // Child wants a specific size... let him have it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; } else if (childDimension == LayoutParams.MATCH_PARENT) { // Child wants to be our size... find out how big it should // be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } else if (childDimension == LayoutParams.WRAP_CONTENT) { // Child wants to determine its own size.... find out how // big it should be resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size; resultMode = MeasureSpec.UNSPECIFIED; } break; } return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }
在这段代码对于MeasureSpec的获取主要是根据父容器的MeasureSpec和View本身的LayoutParams。下面通过一张表格来看一下它们之间的对应关系。
到这里通过getChildMeasureSpec方法获取到子View的MeasureSpec以后，便调用View的Measure方法，开始对View进行测量。
正如刚才说的那样对于ViewGroup它是一个抽象类，并没有重写View的onMeasure方法。但是到具体的ViewGroup时，例如FrameLayout，LinearLayout，RelativeLayout等，它们通过重写onMeasure方法来来完成自身以及子View的Measure过程。下面以FrameLayout为例，看一下的Measure过程。在FrameLayout中，它的Measure过程也算是比较简单，下面就来看一下FrameLayout中的onMeasure方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int count = getChildCount(); final boolean measureMatchParentChildren = MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY || MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY; mMatchParentChildren.clear(); int maxHeight = 0; int maxWidth = 0; int childState = 0; for (int i = 0; i < count; i++) { final View child = getChildAt(i); if (mMeasureAllChildren || child.getVisibility() != GONE) { measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0); 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); childState = combineMeasuredStates(childState, child.getMeasuredState()); if (measureMatchParentChildren) { if (lp.width == LayoutParams.MATCH_PARENT || lp.height == LayoutParams.MATCH_PARENT) { mMatchParentChildren.add(child); } } } } // Account for padding too maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground(); maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground(); // Check against our minimum height and width maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight()); maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth()); // Check against our foreground's minimum height and width final Drawable drawable = getForeground(); if (drawable != null) { maxHeight = Math.max(maxHeight, drawable.getMinimumHeight()); maxWidth = Math.max(maxWidth, drawable.getMinimumWidth()); } setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState), resolveSizeAndState(maxHeight, heightMeasureSpec, childState << MEASURED_HEIGHT_STATE_SHIFT)); 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); childWidthMeasureSpec = MeasureSpec.makeMeasureSpec( width, MeasureSpec.EXACTLY); } else { childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec, getPaddingLeftWithForeground() + getPaddingRightWithForeground() + lp.leftMargin + lp.rightMargin, lp.width); } 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); } } }
在这部分代码中逻辑也很简单，主要完成了两件事。首先FrameLayout完成自身的测量过程，然后在遍历子View，执行View的measure方法，完成View的Measure过程。在这里代码比较简单就不在进行详细描述。
总结
最后对View和ViewGroup的Measure过程做一下总结。对于View，它的Measure很简单，在获取到View的高和宽的测量值之后，便为其设置高和宽。而对于ViewGroup来说，除了完成自身的Measure过程以外，还需要遍历子View，完成子View的测量过程。