Android中measure过程、WRAP_CONTENT详解以及xml布局文件解析流程浅析(上)

在之前一篇博文中<< Android中View绘制流程以及invalidate()等相关方法分析>> ，简单的阐述了 Android View

绘制流程的三个步骤，即：

1、 measure过程 --- 测量过程

2、 layout 过程 --- 布局过程
3、 draw 过程 --- 绘制过程

要想对Android 中View这块深入理解，对这三个步骤地学习是必不可少的。

今天，我着重讲解下如下三个内容：

1、 measure过程

2、WRAP_CONTENT、MATCH_PARENT/FILL_PARENT属性的原理说明

3、xml布局文件解析成View树的流程分析。

希望对大家能有帮助。- - 分析版本基于Android 2.3 。

1、WRAP_CONTENT、MATCH_PARENT/FILL_PARENT

初入Android殿堂的同学们，对这三个属性一定又爱又恨。爱的是使用起来挺爽地---照葫芦画瓢即可，恨的

却是时常混淆这几个属性地意义，需要三思而后行。在带着大家重温下这几个属性的用法吧(希望我没有啰嗦)。

这三个属性都用来适应视图的水平或垂直大小，一个以视图的内容或尺寸为基础的布局比精确地指定视图范围

更加方便。

① fill_parent

设置一个视图的布局为fill_parent将强制性地使视图扩展至父元素大小。

② match_parent

Android 中match_parent和fill_parent意思一样，但match_parent更贴切，于是从2.2开始两个词都可以

用，但2.3版本后建议使用match_parent。

③ wrap_content

自适应大小，强制性地使视图扩展以便显示其全部内容。以TextView和ImageView控件为例，设置为

wrap_content将完整显示其内部的文本和图像。布局元素将根据内容更改大小。

可不要重复造轮子，以上摘自<<Android fill_parent、wrap_content和match_parent的区别>>。

当然，我们可以设置View的确切宽高，而不是由以上属性指定。

[java] view plaincopyprint?

android:layout_weight="wrap_content" //自适应大小
android:layout_weight="match_parent" //与父视图等高
android:layout_weight="fill_parent" //与父视图等高
android:layout_weight="100dip" //精确设置高度值为 100dip

接下来，我们需要转换下视角，看看ViewGroup.LayoutParams类及其派生类。

2、ViewGroup.LayoutParams类及其派生类

2.1、 ViewGroup.LayoutParams类说明

Android API中如下介绍：

LayoutParams are used by views to tell their parents how they want to be laid out.

意思大概是说： View通过LayoutParams类告诉其父视图它想要地大小(即，长度和宽度)。

因此，每个View都包含一个ViewGroup.LayoutParams类或者其派生类，View类依赖于ViewGroup.LayoutParams。

路径：frameworks\base\core\java\android\view\View.java

[java] view plaincopyprint?

public class View implements Drawable.Callback, KeyEvent.Callback, AccessibilityEventSource {
...
/**
* The layout parameters associated with this view and used by the parent
* {@link android.view.ViewGroup} to determine how this view should be
* laid out.
* {@hide}
*/
//该View拥有的 LayoutParams属性，父试图添加该View时，会为其赋值，特别注意，其类型为ViewGroup.LayoutParams。
protected ViewGroup.LayoutParams mLayoutParams;
...
}

2.2、 ViewGroup.LayoutParams源码分析

路径位于：frameworks\base\core\java\android\view\ViewGroup.java

[java] view plaincopyprint?

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
...
public static class LayoutParams {
/**
* Special value for the height or width requested by a View.
* FILL_PARENT means that the view wants to be as big as its parent,
* minus the parent's padding, if any. This value is deprecated
* starting in API Level 8 and replaced by {@link #MATCH_PARENT}.
*/
@Deprecated
public static final int FILL_PARENT = -1; // 注意值为-1，Android2.2版本不建议使用
/**
* Special value for the height or width requested by a View.
* MATCH_PARENT means that the view wants to be as big as its parent,
* minus the parent's padding, if any. Introduced in API Level 8.
*/
public static final int MATCH_PARENT = -1; // 注意值为-1
/**
* Special value for the height or width requested by a View.
* WRAP_CONTENT means that the view wants to be just large enough to fit
* its own internal content, taking its own padding into account.
*/
public static final int WRAP_CONTENT = -2; // 注意值为-2
/**
* Information about how wide the view wants to be. Can be one of the
* constants FILL_PARENT (replaced by MATCH_PARENT ,
* in API Level 8) or WRAP_CONTENT. or an exact size.
*/
public int width; //该View的宽度，可以为WRAP_CONTENT/MATCH_PARENT 或者一个具体值
/**
* Information about how tall the view wants to be. Can be one of the
* constants FILL_PARENT (replaced by MATCH_PARENT ,
* in API Level 8) or WRAP_CONTENT. or an exact size.
*/
public int height; //该View的高度，可以为WRAP_CONTENT/MATCH_PARENT 或者一个具体值
/**
* Used to animate layouts.
*/
public LayoutAnimationController.AnimationParameters layoutAnimationParameters;
/**
* Creates a new set of layout parameters. The values are extracted from
* the supplied attributes set and context. The XML attributes mapped
* to this set of layout parameters are:、
*/
public LayoutParams(Context c, AttributeSet attrs) {
TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout);
setBaseAttributes(a,
R.styleable.ViewGroup_Layout_layout_width,
R.styleable.ViewGroup_Layout_layout_height);
a.recycle();
}
/**
* Creates a new set of layout parameters with the specified width
* and height.
*/
public LayoutParams(int width, int height) {
this.width = width;
this.height = height;
}
/**
* Copy constructor. Clones the width and height values of the source.
*
* @param source The layout params to copy from.
*/
public LayoutParams(LayoutParams source) {
this.width = source.width;
this.height = source.height;
}
/**
* Used internally by MarginLayoutParams.
* @hide
*/
LayoutParams() {
}
/**
* Extracts the layout parameters from the supplied attributes.
*
* @param a the style attributes to extract the parameters from
* @param widthAttr the identifier of the width attribute
* @param heightAttr the identifier of the height attribute
*/
protected void setBaseAttributes(TypedArray a, int widthAttr, int heightAttr) {
width = a.getLayoutDimension(widthAttr, "layout_width");
height = a.getLayoutDimension(heightAttr, "layout_height");
}
}

我们发现FILL_PARENT/MATCH_PARENT值为 -1 ，WRAP_CONETENT值为-2，是不是有点诧异？将值

设置为负值的目的是为了区别View的具体值(an exact size) 总是大于0的。

ViewGroup子类可以实现自定义LayoutParams，自定义LayoutParams提供了更好地扩展性，例如LinearLayout

就有LinearLayout. LayoutParams自定义类(见下文)。整个LayoutParams类家族还是挺复杂的。

ViewGroup.LayoutParams及其常用派生类的类图(部分类图)如下：

该类图是在太庞大了，大家有兴趣的去看看Android API吧。

前面我们说过，每个View都包含一个ViewGroup.LayoutParams类或者其派生类，下面我们的疑问是Android框架

中时如何为View设置其LayoutParams属性的。

有两种方法会设置View的LayoutParams属性：

1、直接添加子View时，常见于如下几种方法：ViewGroup.java

[java] view plaincopyprint?

//Adds a child view.
void addView(View child, int index)
//Adds a child view with this ViewGroup's default layout parameters
//and the specified width and height.
void addView(View child, int width, int height)
//Adds a child view with the specified layout parameters.
void addView(View child, ViewGroup.LayoutParams params)

三个重载方法的区别只是添加View时构造LayoutParams对象的方式不同而已，稍后我们探寻一下它们的源码。

2、通过xml布局文件指定某个View的属性为：android:layout_heigth=””以及android:layout_weight=”” 时。

总的来说，这两种方式都会设定View的LayoutParams属性值----指定的或者Default值。

方式1流程分析：

直接添加子View时，比较容易理解，我们先来看看这种方式设置LayoutParams的过程：

路径：\frameworks\base\core\java\android\view\ViewGroup.java

[java] view plaincopyprint?

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
...
/**
* Adds a child view. If no layout parameters are already set on the child, the
* default parameters for this ViewGroup are set on the child.
*
* @param child the child view to add
*
* @see #generateDefaultLayoutParams()
*/
public void addView(View child) {
addView(child, -1);
}
/**
* Adds a child view. If no layout parameters are already set on the child, the
* default parameters for this ViewGroup are set on the child.
*
* @param child the child view to add
* @param index the position at which to add the child
*
* @see #generateDefaultLayoutParams()
*/
public void addView(View child, int index) {
LayoutParams params = child.getLayoutParams();
if (params == null) {
params = generateDefaultLayoutParams(); //返回默认地LayoutParams类，作为该View的属性值
if (params == null) {//如果不能获取到LayoutParams对象，则抛出异常。
throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");
}
}
addView(child, index, params);
}
/**
* Adds a child view with this ViewGroup's default layout parameters and the
* specified width and height.
*
* @param child the child view to add
*/
public void addView(View child, int width, int height) {
//返回默认地LayoutParams类，作为该View的属性值
final LayoutParams params = generateDefaultLayoutParams();
params.width = width; //重新设置width值
params.height = height; //重新设置height值
addView(child, -1, params); //这儿，我们有指定width、height的大小了。
}
/**
* Adds a child view with the specified layout parameters.
*
* @param child the child view to add
* @param params the layout parameters to set on the child
*/
public void addView(View child, LayoutParams params) {
addView(child, -1, params);
}
/**
* Adds a child view with the specified layout parameters.
*
* @param child the child view to add
* @param index the position at which to add the child
* @param params the layout parameters to set on the child
*/
public void addView(View child, int index, LayoutParams params) {
...
// addViewInner() will call child.requestLayout() when setting the new LayoutParams
// therefore, we call requestLayout() on ourselves before, so that the child's request
// will be blocked at our level
requestLayout();
invalidate();
addViewInner(child, index, params, false);
}
/**
* Returns a set of default layout parameters. These parameters are requested
* when the View passed to {@link #addView(View)} has no layout parameters
* already set. If null is returned, an exception is thrown from addView.
*
* @return a set of default layout parameters or null
*/
protected LayoutParams generateDefaultLayoutParams() {
//width 为 WRAP_CONTENT大小， height 为WRAP_CONTENT
//ViewGroup的子类可以重写该方法，达到其特定要求。稍后会以LinearLayout类为例说明。
return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
}
private void addViewInner(View child, int index, LayoutParams params,
boolean preventRequestLayout) {
if (!checkLayoutParams(params)) { //params对象是否为null
params = generateLayoutParams(params); //如果params对象是为null，重新构造个LayoutParams对象
}
//preventRequestLayout值为false
if (preventRequestLayout) {
child.mLayoutParams = params; //为View的mLayoutParams属性赋值
} else {
child.setLayoutParams(params);//为View的mLayoutParams属性赋值，但会调用requestLayout()请求重新布局
}
//if else 语句会设置View为mLayoutParams属性赋值
...
}
...
}

主要功能就是在添加子View时为其构建了一个LayoutParams对象。但更重要的是，ViewGroup的子类可以重载

上面的几个方法，返回特定的LayoutParams对象，例如：对于LinearLayout而言，则是LinearLayout.LayoutParams

对象。这么做地目的是，能在其他需要它的地方，可以将其强制转换成LinearLayout.LayoutParams对象。

LinearLayout重写函数地实现为：

[java] view plaincopyprint?

public class LinearLayout extends ViewGroup {
...
@Override
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new LinearLayout.LayoutParams(getContext(), attrs);
}
@Override
protected LayoutParams generateDefaultLayoutParams() {
//该LinearLayout是水平方向还是垂直方向
if (mOrientation == HORIZONTAL) {
return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
} else if (mOrientation == VERTICAL) {
return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
}
return null;
}
@Override
protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
return new LayoutParams(p);
}
/**
* Per-child layout information associated with ViewLinearLayout.
*
* @attr ref android.R.styleable#LinearLayout_Layout_layout_weight
* @attr ref android.R.styleable#LinearLayout_Layout_layout_gravity
*/ //自定义的LayoutParams类
public static class LayoutParams extends ViewGroup.MarginLayoutParams {
/**
* Indicates how much of the extra space in the LinearLayout will be
* allocated to the view associated with these LayoutParams. Specify
* 0 if the view should not be stretched. Otherwise the extra pixels
* will be pro-rated among all views whose weight is greater than 0.
*/
@ViewDebug.ExportedProperty(category = "layout")
public float weight; // 见于属性，android:layout_weight="" ;
/**
* Gravity for the view associated with these LayoutParams.
*
* @see android.view.Gravity
*/
public int gravity = -1; // 见于属性， android:layout_gravity="" ;
/**
* {@inheritDoc}
*/
public LayoutParams(Context c, AttributeSet attrs) {
super(c, attrs);
TypedArray a =c.obtainStyledAttributes(attrs, com.android.internal.R.styleable.LinearLayout_Layout);
weight = a.getFloat(com.android.internal.R.styleable.LinearLayout_Layout_layout_weight, 0);
gravity = a.getInt(com.android.internal.R.styleable.LinearLayout_Layout_layout_gravity, -1);
a.recycle();
}
/**
* {@inheritDoc}
*/
public LayoutParams(int width, int height) {
super(width, height);
weight = 0;
}
/**
* Creates a new set of layout parameters with the specified width, height
* and weight.
*
* @param width the width, either {@link #MATCH_PARENT},
* {@link #WRAP_CONTENT} or a fixed size in pixels
* @param height the height, either {@link #MATCH_PARENT},
* {@link #WRAP_CONTENT} or a fixed size in pixels
* @param weight the weight
*/
public LayoutParams(int width, int height, float weight) {
super(width, height);
this.weight = weight;
}
public LayoutParams(ViewGroup.LayoutParams p) {
super(p);
}
public LayoutParams(MarginLayoutParams source) {
super(source);
}
}
...
}

LinearLayout.LayoutParams类继承至ViewGroup.MarginLayoutParams类，添加了对android:layout_weight以及

android:layout_gravity这两个属性的获取和保存。而且它的重写函数返回的都是LinearLayout.LayoutParams

类型。这样，我们可以再对子View进行其他操作时，可以将将其强制转换成LinearLayout.LayoutParams对象进行

使用。

例如，LinearLayout进行measure过程，使用了LinearLayout.LayoutParam对象，有如下代码：

[java] view plaincopyprint?

public class LinearLayout extends ViewGroup {
...
@Override //onMeasure方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//判断是垂直方向还是水平方向，这儿我们假设是VERTICAL垂直方向，
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
/**
* Measures the children when the orientation of this LinearLayout is set
* to {@link #VERTICAL}.
*
* @param widthMeasureSpec Horizontal space requirements as imposed by the parent.
* @param heightMeasureSpec Vertical space requirements as imposed by the parent.
*
* @see #getOrientation()
* @see #setOrientation(int)
* @see #onMeasure(int, int)
*/
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
mTotalLength = 0;
...
// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i); //获得索引处为i的子VIew
...
//注意，我们将类型为 ViewGroup.LayoutParams的实例对象强制转换为了LinearLayout.LayoutParams，
//即父对象转换为了子对象，能这样做的原因就是LinearLayout的所有子View的LayoutParams类型都为
//LinearLayout.LayoutParams
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
...
}
...
}

超类ViewGroup.LayoutParams强制转换为了子类LinearLayout.LayoutParams，因为LinearLayout的每个

”直接“子View的LayoutParams属性都是LinearLayout.LayoutParams类型，因此可以安全转换。

PS : Android 2.3源码Launcher2中也实现了自定义的LayoutParams类，在IDLE界面的每个View至少包含如下

信息：所在X方向的单元格索引和高度、所在Y方向的单元格索引和高度等。

路径: packages\apps\Launcher2\src\com\android\launcher2\CellLayout.java

[java] view plaincopyprint?

public class CellLayout extends ViewGroup {
...
public static class LayoutParams extends ViewGroup.MarginLayoutParams {
/**
* Horizontal location of the item in the grid.
*/
public int cellX; //X方向的单元格索引
/**
* Vertical location of the item in the grid.
*/
public int cellY; //Y方向的单元格索引
/**
* Number of cells spanned horizontally by the item.
*/
public int cellHSpan; //水平方向所占高度
/**
* Number of cells spanned vertically by the item.
*/
public int cellVSpan; //垂直方向所占高度
...
public LayoutParams(Context c, AttributeSet attrs) {
super(c, attrs);
cellHSpan = 1; //默认为高度 1
cellVSpan = 1;
}
public LayoutParams(ViewGroup.LayoutParams source) {
super(source); //默认为高度 1
cellHSpan = 1;
cellVSpan = 1;
}
public LayoutParams(int cellX, int cellY, int cellHSpan, int cellVSpan) {
super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
this.cellX = cellX;
this.cellY = cellY;
this.cellHSpan = cellHSpan;
this.cellVSpan = cellVSpan;
}
...
}
...
}

对该自定义CellLayout.LayoutParams类的使用可以参考LinearLayout.LayoutParams类，我也不再赘述了。

方法2流程分析：

使用属性android:layout_heigth=””以及android:layout_weight=”” 时，为某个View设置LayoutParams值。

其实这种赋值方法其实也如同前面那种，只不过它需要一个前期孵化过程---需要利用XML解析将布局文件

解析成一个完整的View树，可别小看它了，所有Xxx.xml的布局文件都需要解析成一个完整的View树。下面，

我们就来仔细走这个过程，重点关注如下两个方面

①、xml布局是如何解析成View树的 ;

②、android:layout_heigth=””和android:layout_weight=””的解析。

PS：一直以来，我都想当然android:layout_heigth以及android:layout_weight这两个属性的解析过程是在

View.java内部完成的，但当我真正去找寻时，却一直没有在View.java类或者ViewGroup.java类找到。直到一位

网友的一次提问，才发现它们的藏身之地。

3、布局文件解析流程分析

解析布局文件时，使用的类为LayoutInflater。关于该类的使用请参考如下博客：

<android中LayoutInflater的使用 >>

主要有如下API方法：

public View inflate (XmlPullParser parser, ViewGroup root, boolean attachToRoot)

public View inflate (int resource, ViewGroup root)

public View inflate (int resource, ViewGroup root, boolean attachToRoot)

这三个类主要迷惑之处在于地三个参数attachToRoot，即是否将该View树添加到root中去。具体可看这篇博客：

<<关于inflate的第3个参数>>

当然还有LayoutInflater的inflate()的其他重载方法，大家可以自行了解下。

我利用下面的例子给大家走走这个流程：

[java] view plaincopyprint?

public class MainActivity extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//1、该方法最终也会调用到 LayoutInflater的inflate()方法中去解析。
setContentView(R.layout.main);
//2、使用常见的API方法去解析xml布局文件，
LayoutInflater layoutInflater = (LayoutInflater)getSystemService();
View root = layoutInflater.inflate(R.layout.main, null);
}
}

Step 1、获得LayoutInflater的引用。

路径：\frameworks\base\core\java\android\app\ContextImpl.java

[java] view plaincopyprint?

/**
* Common implementation of Context API, which provides the base
* context object for Activity and other application components.
*/
class ContextImpl extends Context {
if (WINDOW_SERVICE.equals(name)) {
return WindowManagerImpl.getDefault();
} else if (LAYOUT_INFLATER_SERVICE.equals(name)) {
synchronized (mSync) {
LayoutInflater inflater = mLayoutInflater;
//是否已经赋值，如果是，直接返回引用
if (inflater != null) {
return inflater;
}
//返回一个LayoutInflater对象，getOuterContext()指的是我们的Activity、Service或者Application引用
mLayoutInflater = inflater = PolicyManager.makeNewLayoutInflater(getOuterContext());
return inflater;
}
} else if (ACTIVITY_SERVICE.equals(name)) {
return getActivityManager();
}...
}

继续去PolicyManager查询对应函数，看看内部实现。

路径：frameworks\base\core\java\com\android\internal\policy\PolicyManager.java

[java] view plaincopyprint?

public final class PolicyManager {
private static final String POLICY_IMPL_CLASS_NAME = "com.android.internal.policy.impl.Policy";
private static final IPolicy sPolicy; // 这可不是Binder机制额，这只是是一个接口，别想多啦
static {
// Pull in the actual implementation of the policy at run-time
try {
Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);
sPolicy = (IPolicy)policyClass.newInstance();
}
...
}
...
public static LayoutInflater makeNewLayoutInflater(Context context) {
return sPolicy.makeNewLayoutInflater(context); //继续去实现类中去查找
}
}

IPolicy接口的实现对为Policy类。路径：/frameworks/base/policy/src/com/android/internal/policy/impl/Policy.java

[java] view plaincopyprint?

//Simple implementation of the policy interface that spawns the right
//set of objects
public class Policy implements IPolicy{
...
public PhoneLayoutInflater makeNewLayoutInflater(Context context) {
//实际上返回的是PhoneLayoutInflater类。
return new PhoneLayoutInflater(context);
}
}
//PhoneLayoutInflater继承至LayoutInflater类
public class PhoneLayoutInflater extends LayoutInflater {
...
/**
* Instead of instantiating directly, you should retrieve an instance
* through {@link Context#getSystemService}
*
* @param context The Context in which in which to find resources and other
* application-specific things.
*
* @see Context#getSystemService
*/
public PhoneLayoutInflater(Context context) {
super(context);
}
...
}

LayoutInflater是个抽象类，实际上我们返回的是PhoneLayoutInflater类，但解析过程的操作基本上是在

LayoutInflater中完成地。

Step 2、调用inflate()方法去解析布局文件。

[java] view plaincopyprint?

public abstract class LayoutInflater {
...
public View inflate(int resource, ViewGroup root) {
//继续看下个函数，注意root为null
return inflate(resource, root, root != null);
}
public View inflate(int resource, ViewGroup root, boolean attachToRoot) {
//获取一个XmlResourceParser来解析XML文件---布局文件。
//XmlResourceParser类以及xml是如何解析的，大家自己有兴趣找找。
XmlResourceParser parser = getContext().getResources().getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
}
/**
* The XML parsing interface returned for an XML resource. This is a standard
* XmlPullParser interface, as well as an extended AttributeSet interface and
* an additional close() method on this interface for the client to indicate
* when it is done reading the resource.
*/
public interface XmlResourceParser extends XmlPullParser, AttributeSet {
/**
* Close this interface to the resource. Calls on the interface are no
* longer value after this call.
*/
public void close();
}

我们获得了一个当前应用程序环境的XmlResourceParser对象，该对象的主要作用就是来解析xml布局文件的。

XmlResourceParser类是个接口类，更多关于XML解析的，大家可以参考下面博客：

<<android之XmlResourceParser类使用实例>>

<<android解析xml文件的方式(其一)>>

<<android解析xml文件的方式(其二)>>

<<android解析xml文件的方式(其三)>>

Step 3 、真正地开始解析工作。

[java] view plaincopyprint?

public abstract class LayoutInflater {
...
/**
* Inflate a new view hierarchy from the specified XML node. Throws
* {@link InflateException} if there is an error.
*/
//我们传递过来的参数如下： root 为null ， attachToRoot为false 。
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext; //该mConstructorArgs属性最后会作为参数传递给View的构造函数
View result = root; //根View
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
...
final String name = parser.getName(); //节点名，即API中的控件或者自定义View完整限定名。
if (TAG_MERGE.equals(name)) { // 处理<merge />标签
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
//将<merge />标签的View树添加至root中，该函数稍后讲到。
rInflate(parser, root, attrs);
} else {
// Temp is the root view that was found in the xml
//创建该xml布局文件所对应的根View。
View temp = createViewFromTag(name, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
// Create layout params that match root, if supplied
//根据AttributeSet属性获得一个LayoutParams实例，记住调用者为root。
params = root.generateLayoutParams(attrs);
if (!attachToRoot) { //重新设置temp的LayoutParams
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
// Inflate all children under temp
//添加所有其子节点，即添加所有字View
rInflate(parser, temp, attrs);
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) {
result = temp;
}
}
}
...
return result;
}
}
/*
* default visibility so the BridgeInflater can override it.
*/
View createViewFromTag(String name, AttributeSet attrs) {
//节点是否为View，如果是将其重新赋值，形如 <View class="com.qin.xxxView"></View>
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
try {
View view = (mFactory == null) ? null : mFactory.onCreateView(name,
mContext, attrs); //没有设置工厂方法
if (view == null) {
//通过这个判断是Android API的View，还是自定义View
if (-1 == name.indexOf('.')) {
view = onCreateView(name, attrs); //创建Android API的View实例
} else {
view = createView(name, null, attrs);//创建一个自定义View实例
}
}
return view;
}
...
}
//获得具体视图的实例对象
public final View createView(String name, String prefix, AttributeSet attrs) {
Constructor constructor = sConstructorMap.get(name);
Class clazz = null;
//以下功能主要是获取如下三个类对象：
//1、类加载器 ClassLoader
//2、Class对象
//3、类的构造方法句柄 Constructor
try {
if (constructor == null) {
// Class not found in the cache, see if it's real, and try to add it
clazz = mContext.getClassLoader().loadClass(prefix != null ? (prefix + name) : name);
...
constructor = clazz.getConstructor(mConstructorSignature);
sConstructorMap.put(name, constructor);
} else {
// If we have a filter, apply it to cached constructor
if (mFilter != null) {
...
}
}
//传递参数获得该View实例对象
Object[] args = mConstructorArgs;
args[1] = attrs;
return (View) constructor.newInstance(args);
}
...
}
}

这段代码的作用是获取xml布局文件的root View，做了如下两件事情

1、获取xml布局的View实例，通过createViewFromTag()方法获取，该方法会判断节点名是API 控件

还是自定义控件，继而调用合适的方法去实例化View。

2、判断root以及attachToRoot参数，重新设置root View值以及temp变量的LayoutParams值。

如果仔细看着段代码，不知大家心里有没有疑惑：当root为null时，我们的temp变量的LayoutParams值是为

null的，即它不会被赋值？有个View的LayoutParams值为空，那么，在系统中不会报异常吗？见下面部分

代码：

[java] view plaincopyprint?

//我们传递过来的参数如下： root 为null ， attachToRoot为false 。
public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
...
try {
...
if (TAG_MERGE.equals(name)) { // 处理<merge />标签
...
} else {
// Temp is the root view that was found in the xml
//创建该xml布局文件所对应的根View。
View temp = createViewFromTag(name, attrs);
ViewGroup.LayoutParams params = null;
//注意！！！ root为null时，temp变量的LayoutParams属性不会被赋值的。
if (root != null) {
// Create layout params that match root, if supplied
//根据AttributeSet属性获得一个LayoutParams实例，记住调用者为root。
params = root.generateLayoutParams(attrs);
if (!attachToRoot) { //重新设置temp的LayoutParams
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
...
}
}
...
}
}

关于这个问题的详细答案，我会在后面讲到。这儿我简单说下，任何View树的顶层View被添加至窗口时，

一般调用WindowManager.addView()添加至窗口时，在这个方法中去做进一步处理。即使，LayoutParams

值为空，UI框架每次measure()时都忽略该View的LayoutParams值，而是直接传递MeasureSpec值至View树。

接下来，我们关注另外一个函数，rInflate()，该方法会递归调用每个View下的子节点，以当前View作为根View

形成一个View树。

[java] view plaincopyprint?

/**
* Recursive method used to descend down the xml hierarchy and instantiate
* views, instantiate their children, and then call onFinishInflate().
*/
//递归调用每个字节点
private void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs)
throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) { //处理<requestFocus />标签
parseRequestFocus(parser, parent);
} else if (TAG_INCLUDE.equals(name)) { //处理<include />标签
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, parent, attrs);//解析<include />节点
} else if (TAG_MERGE.equals(name)) { //处理<merge />标签
throw new InflateException("<merge /> must be the root element");
} else {
//根据节点名构建一个View实例对象
final View view = createViewFromTag(name, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
//调用generateLayoutParams()方法返回一个LayoutParams实例对象，
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs); //继续递归调用
viewGroup.addView(view, params); //OK，将该View以特定LayoutParams值添加至父View中
}
}
parent.onFinishInflate(); //完成了解析过程，通知....
}

值得注意的是，每次addView前都调用了viewGroup.generateLayoutParams(attrs)去构建一个LayoutParams

实例，然后在addView()方法中为其赋值。参见如下代码：ViewGroup.java

[java] view plaincopyprint?

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
...
public LayoutParams generateLayoutParams(AttributeSet attrs) {
return new LayoutParams(getContext(), attrs);
}
public static class LayoutParams {
... //会调用这个构造函数
public LayoutParams(Context c, AttributeSet attrs) {
TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout);
setBaseAttributes(a,
R.styleable.ViewGroup_Layout_layout_width,
R.styleable.ViewGroup_Layout_layout_height);
a.recycle();
}
protected void setBaseAttributes(TypedArray a, int widthAttr, int heightAttr) {
width = a.getLayoutDimension(widthAttr, "layout_width");
height = a.getLayoutDimension(heightAttr, "layout_height");
}
}

好吧 ~~ 我们还是探寻根底，去TypeArray类的getLayoutDimension()看看。

路径：/frameworks/base/core/java/android/content/res/TypedArray.java

[java] view plaincopyprint?

public class TypedArray {
...
/**
* Special version of {@link #getDimensionPixelSize} for retrieving
* {@link android.view.ViewGroup}'s layout_width and layout_height
* attributes. This is only here for performance reasons; applications
* should use {@link #getDimensionPixelSize}.
*
* @param index Index of the attribute to retrieve.
* @param name Textual name of attribute for error reporting.
*
* @return Attribute dimension value multiplied by the appropriate
* metric and truncated to integer pixels.
*/
public int getLayoutDimension(int index, String name) {
index *= AssetManager.STYLE_NUM_ENTRIES;
final int[] data = mData;
//获得属性对应的标识符， Identifies，目前还没有仔细研究相关类。
final int type = data[index+AssetManager.STYLE_TYPE];
if (type >= TypedValue.TYPE_FIRST_INT
&& type <= TypedValue.TYPE_LAST_INT) {
return data[index+AssetManager.STYLE_DATA];
} else if (type == TypedValue.TYPE_DIMENSION) { //类型为dimension类型
return TypedValue.complexToDimensionPixelSize(
data[index+AssetManager.STYLE_DATA], mResources.mMetrics);
}
//没有提供layout_weight和layout_height会来到此处，这儿会报异常！
//因此布局文件中的View包括自定义View必须加上属性layout_weight和layout_height。
throw new RuntimeException(getPositionDescription()
+ ": You must supply a " + name + " attribute.");
}
...
}

从上面得知，我们将View的AttributeSet属性传递给generateLayoutParams()方法，让其构建合适地

LayoutParams对象，并且初始化属性值weight和height。同时我们也得知布局文件中的View包括自定义View

必须加上属性layout_weight和layout_height，否则会报异常。

Step 3 主要做了如下事情：
首先，获得了了布局文件地root View，即布局文件中最顶层的View。

其次，通过递归调用，我们形成了整个View树以及设置了每个View的LayoutParams对象。

总结：通过对布局文件的解析流程的学习，也就是转换为View树的过程，我们明白了解析过程的个中奥妙，以及

设置ViewLayoutParams对象的过程。但是，我们这儿只是简单的浮光掠影，更深层次的内容希望大家能深入学习。

本来是准备接下去往下写的，但无奈贴出来的代码太多，文章有点长而且自己也有点凌乱了，因此决定做两篇

博客发表吧。下篇内容包括如下方面：

1、MeasureSpec类说明 ;

2、measure过程中如何正确设置每个View的长宽 ;

3、UI框架正确设置顶层View的LayoutParams对象，对Activity而言，顶层View则是DecorView，

其他的皆是普通View了。