在Activity布局加载流程源码分析(I)、Activity布局加载流程源码分析(II)、DecorView绘制流程源码分析与View绘制三大流程源码分析等四篇文章中，已经很详细分析了Acitivity的布局加载过程及布局的绘制过程。但在Activity布局加载流程源码分析(I) 中，对于setContentView(“资源文件”)怎么转化View的，没有细说，本篇博文主要想梳理一下这块内容（ps:面试的时候，被面试官问到，既然答不上来，所以决定对这部分知识也好好梳理一下）。
在开始分析之前，我们需要了解一些概念，如：
- PhoneWindow： 是Window类具体实现类，Activity中布局加载逻辑主要就是在此类中完成的。
- LayoutInflater： 是布局填充类，主要就是将我们的layout转化为View。
- XmlPullParser： 是XML解析器，主要是解析xml文件也即layout.xml文件。
一、源码分析
从Activity布局加载流程源码分析(I)文中，我们知道，在Activity的onCreate()中setContentView()后，最后也是调用PhoneWindow中的setContentView()方法。源码如下：
//Activity中
public void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);//核心代码
initActionBar();
}//PhoneWindow中
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);//核心代码
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}这里我们主要来看核心代码，也即 mLayoutInflater.inflate(layoutResID, mContentParent)；其中mLayoutInflater的初始化主要是在PhoneWindow中构造方法中初始化的：
public PhoneWindow(Context context) {
super(context);
mLayoutInflater = LayoutInflater.from(context);
}所以，重点我们还是来看一下LayoutInflater类：
public abstract class LayoutInflater {
.......
/**
* Obtains the LayoutInflater from the given context.
*/
public static LayoutInflater from(Context context) {
LayoutInflater LayoutInflater =
(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (LayoutInflater == null) {
throw new AssertionError("LayoutInflater not found.");
}
return LayoutInflater;
}
.......
//填充方法1
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
/**
* 填充方法2
* Inflate a new view hierarchy from the specified xml node. Throws
* {@link InflateException} if there is an error. *
*/
public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
return inflate(parser, root, root != null);
}
/**
* 填充方法3
* Inflate a new view hierarchy from the specified xml resource. Throws
* {@link InflateException} if there is an error.
*/
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
.......
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
/**
* 填充方法4
* Inflate a new view hierarchy from the specified XML node. Throws
* {@link InflateException} if there is an error.
* <p>
*/
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;
try {
.........
final String name = parser.getName();
..........
if (TAG_MERGE.equals(name)) {
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " + root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
if (DEBUG) {
System.out.println("-----> done inflating children");
}
// 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;
}
}
..........
}从上面代码我们知道，mLayoutInflater 的初始化，主要是从context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)中获取的。在LayoutInflater的源码中，还有多个inflate方法中，这里我们先来看看填充方法2：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
.......
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}由代码易知，这里主要是从资源Resources中获取布局xml，通过res.getLayout(resource)得到一个Xml资源解析器XmlResourceParser，然后再调用填充方法4，这里的XmlResourceParser，我们也可以来看一下源码：
public interface XmlResourceParser extends XmlPullParser, AttributeSet, AutoCloseable {
/**
* Close this interface to the resource. Calls on the interface are no
* longer value after this call.
*/
public void close();
}由上易知，XmlResourceParser也是一个接口，这里主要继承了三个类XmlPullParser（XML文件解析器）、AttributeSet（自定义属性值）、AutoCloseable ，主要是了后面解析服务。所以这里我们重点来看看Resource中的getLayout()方法：
public class Resources {
........
public XmlResourceParser getLayout(@LayoutRes int id) throws NotFoundException {
return loadXmlResourceParser(id, "layout");
}
........
/*package*/ XmlResourceParser loadXmlResourceParser(int id, String type)
throws NotFoundException {
synchronized (mAccessLock) {
TypedValue value = mTmpValue;
if (value == null) {
mTmpValue = value = new TypedValue();
}
getValue(id, value, true);
if (value.type == TypedValue.TYPE_STRING) {
return loadXmlResourceParser(value.string.toString(), id,
value.assetCookie, type);
}
throw new NotFoundException(
"Resource ID #0x" + Integer.toHexString(id) + " type #0x"
+ Integer.toHexString(value.type) + " is not valid");
}
}
/*package*/ XmlResourceParser loadXmlResourceParser(String file, int id,
int assetCookie, String type) throws NotFoundException {
if (id != 0) {
try {
// These may be compiled...
synchronized (mCachedXmlBlockIds) {
// First see if this block is in our cache.
final int num = mCachedXmlBlockIds.length;
for (int i=0; i<num; i++) {
if (mCachedXmlBlockIds[i] == id) {
//System.out.println("**** REUSING XML BLOCK! id="
// + id + ", index=" + i);
return mCachedXmlBlocks[i].newParser();
}
}
// Not in the cache, create a new block and put it at
// the next slot in the cache.
XmlBlock block = mAssets.openXmlBlockAsset(
assetCookie, file);
if (block != null) {
int pos = mLastCachedXmlBlockIndex+1;
if (pos >= num) pos = 0;
mLastCachedXmlBlockIndex = pos;
XmlBlock oldBlock = mCachedXmlBlocks[pos];
if (oldBlock != null) {
oldBlock.close();
}
mCachedXmlBlockIds[pos] = id;
mCachedXmlBlocks[pos] = block;
//System.out.println("**** CACHING NEW XML BLOCK! id="
// + id + ", index=" + pos);
return block.newParser();
}
}
} catch (Exception e) {
NotFoundException rnf = new NotFoundException(
"File " + file + " from xml type " + type + " resource ID #0x"
+ Integer.toHexString(id));
rnf.initCause(e);
throw rnf;
}
}
throw new NotFoundException(
"File " + file + " from xml type " + type + " resource ID #0x"
+ Integer.toHexString(id));
}
........
}通过上面代码我们知道，最后通过XmlBlock.newParser()生成一个xml解析器，也即是XmlResourceParser，从以上我们知道Android中布局文件用的xml解析方法就是PULL解析方式。 下面我们具体来看填充方法4：
/**
* 填充方法4
* Inflate a new view hierarchy from the specified XML node. Throws
* {@link InflateException} if there is an error.
* <p>
*/
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;
try {
// Look for the root node.
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
+ ": No start tag found!");
}
final String name = parser.getName();
..........
if (TAG_MERGE.equals(name)) {//核心代码1
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
rInflate(parser, root, inflaterContext, attrs, false);//核心代码2
} else {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);//核心代码3
ViewGroup.LayoutParams params = null;
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " + root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);//核心代码4
if (DEBUG) {
System.out.println("-----> done inflating children");
}
// 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;
}
}
..........
}我们在写布局文件的时候，经常也会考虑到一些布局优化的问题，所以难免会用到include、merge、ViewStub标签，所以在XML解析的时候，需要针对此类标签作特别处理。
从上面代码中，我们来看核心代码1, TAG_MERGE.equals(name)，其中 TAG_MERGE就为merge，这里主要就是针对merge标签处理。我们先来看核心代码3和4，
核心代码3： createViewFromTag(root, name, inflaterContext, attrs),具体我们也来看看源码：
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}
/**
* Creates a view from a tag name using the supplied attribute set.
*/
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
// Apply a theme wrapper, if allowed and one is specified.
if (!ignoreThemeAttr) {
final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
final int themeResId = ta.getResourceId(0, 0);
if (themeResId != 0) {
context = new ContextThemeWrapper(context, themeResId);
}
ta.recycle();
}
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(context, attrs);
}
try {
View view;
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
throw e;
} catch (ClassNotFoundException e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
}
}我们也来看看核心代码4：rInflateChildren(parser, temp, attrs, true)，我们具体来看看相关代码：
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}通过上面发现，最后也还是调用了核心代码2，rInflate(parser, root, inflaterContext, attrs, false)，下面我们具体来看看核心代码2：
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) 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)) {
parseRequestFocus(parser, parent);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {//代码1
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {//代码2
throw new InflateException("<merge /> must be the root element");
} else {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}这里我们看到，主要就是通过PULL解析布局文件的方式解析出相关的View结构，具体PULL解析方式这里就不介绍了，小伙伴可以自行百度。在这里PULL解析的过程中，还对标签include,merge做了特别处理，是不是发现貌似代码中没有对标签ViewStub进行处理，其实是有的，主要是在createView()方法中，这里就不介绍了，小伙伴可以自行查一下源码。
好了，到这里，就差不多分析完了。
注：源码采用android-6.0.1_r1版本，建议下载源码然后自己走一遍流程，这样更能加深理解。
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