本篇文章主要分析View和ViewGroup的measure过程， 由于ViewGroup还可以包含子元素， 所以相对于View来说会有几个对子View measure的方法， 下面讲分别分析。
概述
View和ViewGroup的工作流程主要指measure、layout、draw这三大流程， 即测量、布局和绘制。measure流程用来测量View和ViewGroup所占矩形区域的大小， 即测量宽高(最终宽高可能会因为layout过程而改变)。layout流程确定View和ViewGroup的最终宽高和矩形区域在父布局中的位置坐标(父布局再一层层往上， 最终会是确定了在屏幕中的位置坐标)。draw流程则是讲View和ViewGroup绘制在屏幕上(底层调用SurfaceFlinger进行绘制， 这里不进行分析)。
View 的measure过程
首先看一下android 7.0 中View.java中的measure()方法如下，被定义成了final的，View的子类将不能重写此方法。方法文档写到：调用该方法用来查明一个view应该是多大，该view所在的父容器提供宽高参数的约束信息。实际的measure工作在onMeasure方法中进行，因此只有onMeasure方法可以被子类重写。
/**
* <p>
* This is called to find out how big a view should be. The parent
* supplies constraint information in the width and height parameters.
* </p>
* <p>
* The actual measurement work of a view is performed in
* {@link #onMeasure(int, int)}, called by this method. Therefore, only
* {@link #onMeasure(int, int)} can and must be overridden by subclasses.
* </p>
* @param widthMeasureSpec Horizontal space requirements as imposed by the
* parent
* @param heightMeasureSpec Vertical space requirements as imposed by the
* parent
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
// 判断当前view的LayoutMode是否为opticalbounds
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) { // 判断当前view的父容器的LayoutMode是否为opticalbounds
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
// 根据我们传入的widthMeasureSpec和heightMeasureSpec计算key值，我们在mMeasureCache中存储我们view的信息
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
// 如果mMeasureCache为null，则进行new一个对象
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
// mOldWidthMeasureSpec和mOldHeightMeasureSpec分别表示上次对View进行量算时的widthMeasureSpec和heightMeasureSpec
// 执行View的measure方法时，View总是先检查一下是不是真的有必要费很大力气去做真正的量算工作
// mPrivateFlags是一个Int类型的值，其记录了View的各种状态位
// 如果(mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT，
// 那么表示当前View需要强制进行layout（比如执行了View的forceLayout方法），所以这种情况下要尝试进行量算
// 如果新传入的widthMeasureSpec/heightMeasureSpec与上次量算时的mOldWidthMeasureSpec/mOldHeightMeasureSpec不等，
// 那么也就是说该View的父ViewGroup对该View的尺寸的限制情况有变化，这种情况下要尝试进行量算
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// Optimize layout by avoiding an extra EXACTLY pass when the view is
// already measured as the correct size. In API 23 and below, this
// extra pass is required to make LinearLayout re-distribute weight.
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
if (forceLayout || needsLayout) {
// first clears the measured dimension flag
// 通过运算，重置mPrivateFlags值，即View的测量状态
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
// 解决布局中的Rtl问题
resolveRtlPropertiesIfNeeded();
// 判断当前View是否是强制进行测量，如果是则将cacheIndex=-1，反之从mMeasureCache中获取
// 对应的index，即从缓存中读取存储的大小。
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
// 根据cacheIndex的大小判断是否需要重新测量，或者根据布尔变量sIgnoreMeasureCache进行判断。
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
// 重新测量，则调用我们重写的onMeasure()方法进行测量，然后重置View的状态
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
// 通过我们计算的cacheIndex值，从缓存中读取我们的测量值。
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
// 通过setMeasuredDimension()方法设置我们的测量值，然后重置View的状态
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
// 如果View的状态没有改变，则会抛出异常"我们没有调用”setMeasuredDimension()"方法，一般出现在我们重写onMeasure方法，
// 但是没有调用setMeasuredDimension方法导致的。
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
// 将最新的widthMeasureSpec和heightMeasureSpec进行存储到mMeasureCache
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
上面的measure方法大致的描述了测量的流程。
- 1 首先判断当前View的layoutMode模式，通过调用isLayoutModeOptical方法进行判断。这个方法就是判断view是否为ViewGroup类型，然后判断layoutMode设定是否为opticalBounds。如果是，则对传入的widthMeasureSpec、heightMeasureSpec进行重新计算封装。
- 2 判断当前view是否强制重新计算，或者传入进来的MeasureSpec是否和上次不同。这两种情况满足一种则进行测量运算。
- 3 如果为强制测量或者忽略缓存，则调用onMeasure()方法进行测量，反之，从mMeasureCache缓存中读取上次的测量数据。
- 4 将最新的widthMeasureSpec和heightMeasureSpec进行存储到mMeasureCache。
上面这段代码主要涉及到3个主要的地方：
- 1 android:layoutMode属性的判断，如果是opticalBounds则需要重新计算MeasureSpec。
- 2 MeasureSpec的概念。
- 3 onMeasure方法。
ViewGroup的android:layoutMode属性
android:layoutMode属性我们很少用的到，设置layoutMode属性有两个常量值，默认是clipBounds，clipBounds 表示了getLeft()、getRight()等返回的原始值组成的边界；opticalBounds描述了视觉上的边界，视觉边界位于Clip bounds中，Clip bounds会更大，因为用来显示一些其他效果比如阴影和闪光等。比如Button，它的clipBounds比opticalBounds大一圈。
button.png
/**
* This constant is a {@link #setLayoutMode(int) layoutMode}.
* Clip bounds are the raw values of {@link #getLeft() left}, {@link #getTop() top},
* {@link #getRight() right} and {@link #getBottom() bottom}.
*/
public static final int LAYOUT_MODE_CLIP_BOUNDS = 0;
/**
* This constant is a {@link #setLayoutMode(int) layoutMode}.
* Optical bounds describe where a widget appears to be. They sit inside the clip
* bounds which need to cover a larger area to allow other effects,
* such as shadows and glows, to be drawn.
*/
public static final int LAYOUT_MODE_OPTICAL_BOUNDS = 1;
layoutMode.png
View.java中的isLayoutModeOptical()方法
/**
* Return true if o is a ViewGroup that is laying out using optical bounds.
* @hide
*/
public static boolean isLayoutModeOptical(Object o) {
return o instanceof ViewGroup && ((ViewGroup) o).isLayoutModeOptical();
}
ViewGroup.java中的isLayoutModeOptical() 方法
/** Return true if this ViewGroup is laying out using optical bounds. */
boolean isLayoutModeOptical() {
return mLayoutMode == LAYOUT_MODE_OPTICAL_BOUNDS;
}
MeasureSpec的概念
measure()方法中传的是MeasureSpec变量，在View测量的时候，系统会将该View的LayoutParams在父容器的约束(父容器的MeasureSpec)下转换成对应的MeasureSpec，然后再根据这个MeasureSpec来确定View的测量宽高。它是View的一个内部类，代表了一个32位int值，高2位代表SpecMode，即测量模式。低30位代表SpecSize，即在该SpecMode下面的规格大小。
SpecMode有三种模式：
- 1 UNSPECIFIED(未指定)，父容器不对子元素施加任何束缚，子元素可以得到任意想要的大小，一般用于系统内部，标识一种测量的状态。
- 2 EXACTLY(精准)，父容器已经检测出子元素所需要的确切大小，这个时候子元素的最终大小就是SpecSize所指定的值。它对应于LayoutParams中的match_parent和具体的数值这两种模式。
- 3 AT_MOST(至多)，父容器指定了一个可用大小，即SpecSize，子元素的大小不能大于这个值，具体是什么要看不同子元素的具体实现，它对应于LayoutParams中的wrap_content。
MeasureSpec的部分源码：
/**
* A MeasureSpec encapsulates the layout requirements passed from parent to child.
* Each MeasureSpec represents a requirement for either the width or the height.
* A MeasureSpec is comprised of a size and a mode.
*/
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/**
* Measure specification mode: The parent has not imposed any constraint
* on the child. It can be whatever size it wants.
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* Measure specification mode: The parent has determined an exact size
* for the child. The child is going to be given those bounds regardless
* of how big it wants to be.
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* Measure specification mode: The child can be as large as it wants up
* to the specified size.
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
/**
* 根据specSize和specMode打包成MeasureSpec
*/
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
/**
* 根据MeasureSpec拆分出specMode
*/
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
/**
* 根据MeasureSpec拆分出specSize
*/
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
...
}
onMeasure方法
onMeasure方法测量View和它的内容来决定测量宽高，该方法被measure方法调用，它应该被子类重写用来提供精准高效的测量。
/**
* <p>
* Measure the view and its content to determine the measured width and the
* measured height. This method is invoked by {@link #measure(int, int)} and
* should be overridden by subclasses to provide accurate and efficient
* measurement of their contents.
* @param widthMeasureSpec horizontal space requirements as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
* @param heightMeasureSpec vertical space requirements as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
*/
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
setMeasuredDimension(）方法用来存储测量后的宽高，调用此方法完成后，测量过程就已经结束了，这之后可以拿到测量后的宽高值。
/**
* <p>This method 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) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
在onMeasure方法调用setMeasuredDimension(）时， setMeasuredDimension(）中传入的是getDefaultSize()的返回值，看一下getDefaultSize()方法：
/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no constraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
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;
}
可以看出getDefaultSize()方法的逻辑很简单，我们只需要看AT_MOST和EXACTLY两种情况，getDefaultSize()返回的就是measureSpec的specSize ，而这传入的measureSpec是在父容器中由父容器的measureSpec和当前子元素的LayoutParams已经共同确定的，所以由measureSpec拆分出来的specSize 就是子元素测量后的大小。针对UNSPECIFIED的情况，一般用于系统内部的测量过程，下面以分析getSuggestedMinimumWidth()为例分析该函数的返回值意义：
/**
* Returns the suggested minimum width that the view should use. This
* returns the maximum of the view's minimum width)
* and the background's minimum width
* ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
* <p>
* When being used in {@link #onMeasure(int, int)}, the caller should still
* ensure the returned width is within the requirements of the parent.
*
* @return The suggested minimum width of the view.
*/
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
可以看出，如果View没有设置背景，那么View的宽度即为mMinWidth ，而mMinWidth 对应于android:mMinWidth 这个属性的值，如果没有设置该属性则mMinWidth 默认为0；如果View指定了背景。则View的宽度为max(mMinWidth, mBackground.getMinimumWidth())，那么mBackground.getMinimumWidth()是什么呢？看一下Drawable的getMinimumWidth()方法：
/**
* Returns the minimum width suggested by this Drawable. If a View uses this
* Drawable as a background, it is suggested that the View use at least this
* value for its width. (There will be some scenarios where this will not be
* possible.) This value should INCLUDE any padding.
*
* @return The minimum width suggested by this Drawable. If this Drawable
* doesn't have a suggested minimum width, 0 is returned.
*/
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
可以看出，getMinimumWidth()返回的就是Drawable的原始宽度，如果没有原始宽度就返回0。总结一下getSuggestedMinimumWidth()的主要工作：如果View没有设置背景就返回android:mMinWidth 这个属性的值，可以人为设置该值，默认为0；如果View设置了背景，则返回mMinWidth和背景最小宽度两者中的最大值。getSuggestedMinimumWidth()的返回值就是View在UNSPECIFIED情况下的测量宽。
直接继承自View的自定义控件如果在xml布局中设置了wrap_content属性，则会实际表现起来相当于match_parent。从getDefaultSize()方法可以看出，当View设置wrap_content属性时，会是MeasureSpec.AT_MOST模式，和match_parent一样，所以需要给改自定义View设置默认的宽高值以表现出wrap_content的效果。TextView针对wrap_content进行了特殊处理，在TextView的onMeasure方法中传入的MeasureSpec中的specSize重新计算了一遍，并没有使用从父容器传来的specSize(该specSize指父容器所能提供的最大空间，和match_parent一样的效果)。
说到最后，可能会有疑问，顶层View--DecorView的MeasureSpec是如何获得的呢？因为子元素的测量需要父容器的MeasureSpec和自身的LayoutParams共同决定(这点在ViewGroup的测量过程中能看出，接下来分析)，所以顶层父容器DecorView的MeasureSpec必须最先获取，然后才能开始遍历子元素的测量工作。
分析Activity的setContentView方法可知，DecorView在PhoneWindow中被创建，在Activity的onResume方法回调的时候继而调用Activity的makeVisible()方法，Activity对应的PhoneWindow对象才被WindowManager添加。此后开始View的三大流程。
// ActivityThread.java
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
...
r = performResumeActivity(token, clearHide, reason);
if (r != null) {
final Activity a = r.activity;
...
if (r.activity.mVisibleFromClient) {
r.activity.makeVisible();
}
}
}
// Activity.java的makeVisible()方法:
void makeVisible() {
if (!mWindowAdded) {
ViewManager wm = getWindowManager();
wm.addView(mDecor, getWindow().getAttributes());
mWindowAdded = true;
}
mDecor.setVisibility(View.VISIBLE);
}
之后：
WindowManager.addView()->WindowManagerImpl.addView()->WindowManagerGlobal.addView()->创建ViewRootImpl对象->ViewRootImpl.setView(DecorView对象)->ViewRootImpl.requestLayout()->ViewRootImpl.performTraversals()->然后触发performMeasure()、performLayout()、performDraw()。
WindowManagerGlobal.addView()核心代码：
...
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
...
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
...
root.setView(view, wparams, panelParentView);
...
}
先说结论：DecorView的MeasureSpec是由窗口的尺寸(PhoneWindow)和其自身的LayoutParams共同决定的，(对于普通View，MeasureSpec是父容器的MeasureSpec和其自身的LayoutParams共同决定)，MeasureSpec一旦确定，onMeasure中就可以确定View的测量宽高。
对于DecorView，在ViewRootImpl的measureHierarchy方法计算出DecorView的MeasureSpec值，如下：
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
...
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}
其中desiredWindowWidth、desiredWindowHeight分别对应着Window的宽高(屏幕的尺寸)。下面看一下getRootMeasureSpec()方法：
/**
* Figures out the measure spec for the root view in a window based on it's
* layout params.
*
* @param windowSize
* The available width or height of the window
*
* @param rootDimension
* The layout params for one dimension (width or height) of the
* window.
*
* @return The measure spec to use to measure the root view.
*/
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
- 1 WindowManager.LayoutParams.MATCH_PARENT：精准模式，SpecSize就是窗口的大小。
- 2 WindowManager.LayoutParams.WRAP_CONTENT：最大模式，大小不定，但是不能超过窗口的大小。
- 3 固定大小：精准模式，大小为LayoutParams中指定的值。
顶层DecorView的MeasureSpec确定后，开始遍历递归子元素进行测量流程，然后如果遇到了View就是上面分析的测量流程，如果遇到的是ViewGroup则接下来进行分析。
ViewGroup 的measure过程
ViewGroup继承自View，View中的measure方法由于final不能被重新，onMeasure也没有重写。ViewGroup 的measure情况分为两种，一个是measure自己，另一个是measure子元素。但是ViewGroup并没有定义具体的measure自己的过程，其测量过程的onMeasure需要由各个具体的子类去实现，比如LinearLayout、RelativeLayout，因为不同的ViewGroup子类有不同的布局特性。
ViewGroup measure子元素过程有measureChildren()方法如下：
/**
* Ask all of the children of this view to measure themselves, taking into
* account both the MeasureSpec requirements for this view and its padding.
* We skip children that are in the GONE state The heavy lifting is done in
* getChildMeasureSpec.
*
* @param widthMeasureSpec The width requirements for this view
* @param heightMeasureSpec The height requirements for this view
*/
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);
}
}
}
其内部通过循环遍历所有的子元素，对可见的子元素再调用measureChild()方法：
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding.
* The heavy lifting is done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param parentHeightMeasureSpec The height requirements for this view
*/
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);
}
measureChildWithMargins()方法也类似，在调用getChildMeasureSpec()方法获取子元素的MeasureSpec中增加了子元素的Margin值。measureChild()函数内部清晰的表达了子元素 MeasureSpec的产生过程：先得到子元素的LayoutParams，然后调用getChildMeasureSpec()方法将父容器的parentMeasureSpec与padding值、子元素的LayoutParams一起合成了子元素的MeasureSpec。getChildMeasureSpec()方法如下：
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
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;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
上面代码的主要逻辑是：如果子元素的LayoutParams中宽高是具体精准数值时，不管父容器的 MeasureSpec是什么，子元素的MeasureSpec都是精准模式切大小遵循LayoutParams中的具体数值宽高；如果子元素的LayoutParams中宽高是match_parent，当父容器是精准模式时，子元素也是精准模式并且大小不会超过父容器剩余空间大小，当父容器是最大模式时，子元素也是最大模式并且其大小不会超过父容器剩余空间；如果子元素的LayoutParams中宽高是wrap_content时，不管父容器是精准模式还是最大化，子元素的模式总是最大化且不能超过父容器的剩余空间。
以LinearLayout分析ViewGroup的measure自己过程
LinearLayout的onMeasure()方法：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
看得到有垂直测量和水平测量两种，下面分析 measureVertical()方法的大致逻辑：
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
if (hasDividerBeforeChildAt(i)) {
mTotalLength += mDividerHeight;
}
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
totalWeight += lp.weight;
...
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,heightMeasureSpec, usedHeight);
...
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
...
从上面这段代码可以看出，会遍历子元素并对每个子元素执行measureChildBeforeLayout()方法，这个方法内部会调用子元素的measure()方法，然后各个子元素开始一次进入measure过程，系统通过mTotalLength来存储LinearLayout在竖直方向的初步高度。每测量一个元素，mTotalLength就会增加，增加的部分包括子元素的高度以及子元素在竖直方向的margin等，当子元素测量完毕后，LinearLayout会测量自己的大小：
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
...
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState), heightSizeAndState);
如果LinearLayout在布局中指定的高度是wrap_content，那么LinearLayout的高度是等有所子元素测量完毕后子元素所占用的高度总和，但是仍然不能超过LinearLayout的父容器的剩余空间，还需要考虑竖直方向的pading。resolveSizeAndState()函数主要是完成最后的测量。
获取View宽高的时机
- 1 Activity/View的onWindowFocusChanged.View已经初始化完毕，宽高准备好了，onWindowFocusChanged会被调用多次，当Activity的窗口得到焦点和失去焦点的时候均会被调用，对应Activity的onResume和onPause方法。
@Override
public void onWindowFocusChanged(boolean hasFocus)
{
super.onWindowFocusChanged(hasFocus);
if (hasFocus)
{
System.out.println("onWindowFocusChanged width=" + mTextView.getWidth() + " height=" + mTextView.getHeight());
}
}
- 2 View.post(runnable).通过post可以将一个runnable投递到消息队列的尾部，然后等待Looper调用此runnable的时候，View已经初始化好了。
view.post(new Runnable(){
@Override
public void run() {
int width = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
});
- 3 ViewTreeObserver. 使用ViewTreeObserver的很多回调方法可以获取View的宽高，比如OnGlobalLayoutListener和OnPreDrawListener。当View树的状态发生改变或者View树内部的View可见性发生改变时，OnGlobalLayoutListener会被回调，伴随着View树的状态改变等，addOnGlobalLayoutListener方法会被回调多次。
// MineFragment中，显示"圈子，新人必读"引导页的PopupWindow中使用
private void showPopWindow() {
layoutAbovePopWindow.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
changeWindowAlpha(0.7f);
popupWindow.showAsDropDown(layoutAbovePopWindow, 0, Tools.getPixelByDip(getActivity(), 10));
popupWindow.update();
layoutAbovePopWindow.getViewTreeObserver().removeGlobalOnLayoutListener(this);
}
});
}
// 开户上传身份证照片UploadPhotoActivity.java中使用
private void initUI() {
getPresenter().initUploadLayout(mCurrentPartnerId);
mUploadPhotoView.setClearListener(this);
mUploadPhotoView.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
@Override
public boolean onPreDraw() {
int photoLayoutHeight = mUploadPhotoView.getMeasuredHeight();
if (photoLayoutHeight > 0) {
mUploadPhotoView.setPhotoLayout(photoLayoutHeight);
}
mUploadPhotoView.getViewTreeObserver().removeOnPreDrawListener(this);
return false;
}
});
}
- 4 手动对View进行measure来得到View的宽高，该方法不常用。
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