---------------------------------------------------------------本文基于sdk30----------------------------------------------------------------------
安卓中视图基本上都是通过布局创建的，个人在实际开发中总会产生一些疑问， 如：
1、View的构造方法中各参数究竟起到什么作用？2、自定义View为什么要写全路径，而TextView、ImageView等不需要？3、merge、include标签如何工作的？
下面就通过跟踪具体的源码来分析“从布局到View”的整个流程。
1、LayoutInflater基本用法
LayoutInflater对象可以通过LayoutInflater的from方法获取：
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;
}
从布局文件加载View的方法有两个：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
...
}
其中resource是布局文件的id， root表示父View，attachToRoot表示是否将布局表示的View作为子View，添加到root中，返回值见分析。
2、LayoutInflater的inflate(int, ViewGroup, boolean)方法分析
inflate(int, ViewGroup, boolean)方法源码如下：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
...
XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
首先是获取Context相关联的Resources，然后根据布局文件的id将布局文件加载成XmlResourceParser对象，用于解析布局文件，再调用inflate(XmlPullParser, ViewGroup, boolean)方法：
(去除了部分调试代码)
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;
try {
...
if (TAG_MERGE.equals(name)) {
...
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);
...
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
...
} catch (Exception e) {
...
} finally {
...
}
return result;
}
}
该方法最后返回值为result， 首先赋值为root，当root为null或者attachToRoot为false时，result赋值为temp，也就是布局文件对应的View，意思是root不为null且attachToRoot为true时该方法将返回root，其他情况返回布局文件解析得到的View。
try代码块中节点name为最外层的节点名称，根据name不同共有两个分支：1、根节点name是merge：2、根节点name不是merge，分别从这两种情况分析。
2.1、根节点是否是merge
两个分支最后都调用了rInflate(XmlPullParser, View, Context,AttributeSet, boolean)，该方法分析见2.2节
(1) 根节点是merge时分支代码如下：
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);
当root为null或者attachToRoot为false时，将抛出异常。
rInflate(XmlPullParser, View, Context,AttributeSet, boolean)继续解析xml中布局，以root作为根节点，于是merge标签中的布局被合并到root中去（merge标签中高度、宽度、线性布局的方向等参数被舍弃），所以要求root不为null， attachToRoot为true。
(2) 根节点不是merge时分支主要代码如下:
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
temp.setLayoutParams(params);
}
}
rInflateChildren(parser, temp, attrs, true);
if (root != null && attachToRoot) {
root.addView(temp, params);
}
if (root == null || !attachToRoot) {
result = temp;
}
主要步骤如下：
（1）调用createViewFromTag方法（方法分析见2.3节）根据最外层标签name创建对应的View，即temp；
（2）如果root不为null，调用root的generateLayoutParams生成LayoutParams， 获取temp的layout_width、layout_height、layout_weight等参数（generateLayoutParams方法在LinearLayout、RelativeLayout等ViewGroup中都有具体实现，生成对应的LayoutParams），如果attachToRoot为false，则设置temp的LayoutParams参数；
（3）调用rInflateChildren方法，继续解析子节点，该方法调用rInflate方法，见2.2节；
（4）如果root不为null，布局文件解析成的View，添加到root中。
（5）如果root为null或者attachRoot，整个方法返回值置为temp， 不然就是root。
2.2、rInflate(XmlPullParser, View, Context, AttributeSet, boolean)方法分析
rInflate相关代码如下：
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
boolean pendingRequestFocus = false;
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)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
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);
}
}
......
}
主要看节点为include、merge以及正常节点，主要逻辑如下：1、节点为include，不能为布局根节点；2、节点为merge，则抛出异常，提示merge必须为布局根节点；3、节点为正常节点，则调用createViewFromTag创建对应的View，然后rInflateChildrend方法迭代继续解析节点内的节点，最后将节点对应的View添加到parent中去。
2.3、createViewFromTag(View, String, Context, AttributeSet ,boolean)方法分析
该方法主要代码如下：
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
...
try {
View view = tryCreateView(parent, name, context, attrs);
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
view = onCreateView(context, parent, name, attrs);
} else {
view = createView(context, name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
...
} catch (ClassNotFoundException e) {
...
} catch (Exception e) {
...
}
}
其中主要流程是：1、首先调用tryCreateView创建View，该方法根据mFactory、mFactory2、mPrivateFactory三个View创建Factory类来创建View，然后根据name是否是类名(是否有 . 分隔)来判断调用onCreateView还是createView，而onCreateView实质上也是调用了createView， 中间加了类名前缀 ”android.view.“，这也是xml布局中部分View类不需要加全路径的原因，如View、ViewStub、SurfaceView类等（这些类实际上被Factory2拦截生成了）。
2.4、LayoutInflater实例的mFactory属性(Factory实例）、mFactory2属性（Factory2实例）：拦截与处理View类的创建
查找LayoutInflater的setFactory2方法使用， LayoutInflater.setFactory2 -> LayoutInflaterCompat.setFactory2 -> AppCompatDelegateImpl.installViewFactory()
于是找到LayoutInflater.Factory2的接口具体实现为AppCompatDelegateImpl类，AppCompatActivity的setContentView也是由该类负责具体实现的。AppCompatDelegateImpl类中Factory类的onCreateView是调用Factory2类的onCreateView实现的，Factory2.onCreateView最后调用createView方法，最终由AppCompatViewInflater类的createView创建View,部分代码如下：
final View createView(View parent, final String name, @NonNull Context context,
@NonNull AttributeSet attrs, boolean inheritContext,
boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
final Context originalContext = context;
// We can emulate Lollipop's android:theme attribute propagating down the view hierarchy
// by using the parent's context
if (inheritContext && parent != null) {
context = parent.getContext();
}
if (readAndroidTheme || readAppTheme) {
// We then apply the theme on the context, if specified
context = themifyContext(context, attrs, readAndroidTheme, readAppTheme);
}
if (wrapContext) {
context = TintContextWrapper.wrap(context);
}
View view = null;
switch (name) {
case "TextView":
view = createTextView(context, attrs);
verifyNotNull(view, name);
break;
...
}
if (view == null && originalContext != context) {
// If the original context does not equal our themed context, then we need to manually
// inflate it using the name so that android:theme takes effect.
view = createViewFromTag(context, name, attrs);
}
...
return view;
}
于是TextView标签解析成AppCompatTextView，ImageView标签解析成AppCompatImageView等等， 其他View由createViewFromTag创建：
private View createViewFromTag(Context context, String name, AttributeSet attrs) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
try {
mConstructorArgs[0] = context;
mConstructorArgs[1] = attrs;
if (-1 == name.indexOf('.')) {
for (int i = 0; i < sClassPrefixList.length; i++) {
final View view = createViewByPrefix(context, name, sClassPrefixList[i]);
if (view != null) {
return view;
}
}
return null;
} else {
return createViewByPrefix(context, name, null);
}
} catch (Exception e) {
// We do not want to catch these, lets return null and let the actual LayoutInflater
// try
return null;
} finally {
// Don't retain references on context.
mConstructorArgs[0] = null;
mConstructorArgs[1] = null;
}
}
createViewFromTag根据View的名称name以及"android.widget."、"android.view."、"android.webkit."这三个前缀尝试生成View，生成View则返回， 如android.widget包名下LinearLayout、RelativeLayout、ListView等，android.view下的View、ViewStub、SurfaceView等（拦截了LayoutInflater中部分View的创建），android.webkit包名下的WebView。
2.5、LayoutInflater类的mPrivateFactory属性
断点调试， 查看LayoutInflater的实例属性，如下：
可以看到mPrivateFactory属性为MainActivity的实例(该属性在Activity的attach方法中设置）， 一步步查找MainActivity的父类，最终在Activity类中看到了LayoutInflater.Factory2的实现，在FragmentActivity中进行了重写。
（1）Activity中逻辑
Activity中Factory2接口实现如下：
public View onCreateView(@Nullable View parent, @NonNull String name,
@NonNull Context context, @NonNull AttributeSet attrs) {
if (!"fragment".equals(name)) {
return onCreateView(name, context, attrs);
}
return mFragments.onCreateView(parent, name, context, attrs);
}
于是，当标签是fragment时交给mFragments进行处理， 最终由FragmentManagerImpl实现fragment标签的创建。
@Override
public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
if (!"fragment".equals(name)) {
return null;
}
String fname = attrs.getAttributeValue(null, "class");
TypedArray a =
context.obtainStyledAttributes(attrs, com.android.internal.R.styleable.Fragment);
if (fname == null) {
fname = a.getString(com.android.internal.R.styleable.Fragment_name);
}
int id = a.getResourceId(com.android.internal.R.styleable.Fragment_id, View.NO_ID);
String tag = a.getString(com.android.internal.R.styleable.Fragment_tag);
a.recycle();
int containerId = parent != null ? parent.getId() : 0;
if (containerId == View.NO_ID && id == View.NO_ID && tag == null) {
throw new IllegalArgumentException(attrs.getPositionDescription()
+ ": Must specify unique android:id, android:tag, or have a parent with"
+ " an id for " + fname);
}
// If we restored from a previous state, we may already have
// instantiated this fragment from the state and should use
// that instance instead of making a new one.
Fragment fragment = id != View.NO_ID ? findFragmentById(id) : null;
if (fragment == null && tag != null) {
fragment = findFragmentByTag(tag);
}
if (fragment == null && containerId != View.NO_ID) {
fragment = findFragmentById(containerId);
}
if (FragmentManagerImpl.DEBUG) Log.v(TAG, "onCreateView: id=0x"
+ Integer.toHexString(id) + " fname=" + fname
+ " existing=" + fragment);
if (fragment == null) {
fragment = mContainer.instantiate(context, fname, null);
fragment.mFromLayout = true;
fragment.mFragmentId = id != 0 ? id : containerId;
fragment.mContainerId = containerId;
fragment.mTag = tag;
fragment.mInLayout = true;
fragment.mFragmentManager = this;
fragment.mHost = mHost;
fragment.onInflate(mHost.getContext(), attrs, fragment.mSavedFragmentState);
addFragment(fragment, true);
} else if (fragment.mInLayout) {
// A fragment already exists and it is not one we restored from
// previous state.
throw new IllegalArgumentException(attrs.getPositionDescription()
+ ": Duplicate id 0x" + Integer.toHexString(id)
+ ", tag " + tag + ", or parent id 0x" + Integer.toHexString(containerId)
+ " with another fragment for " + fname);
} else {
// This fragment was retained from a previous instance; get it
// going now.
fragment.mInLayout = true;
fragment.mHost = mHost;
// If this fragment is newly instantiated (either right now, or
// from last saved state), then give it the attributes to
// initialize itself.
if (!fragment.mRetaining) {
fragment.onInflate(mHost.getContext(), attrs, fragment.mSavedFragmentState);
}
}
// If we haven't finished entering the CREATED state ourselves yet,
// push the inflated child fragment along. This will ensureInflatedFragmentView
// at the right phase of the lifecycle so that we will have mView populated
// for compliant fragments below.
if (mCurState < Fragment.CREATED && fragment.mFromLayout) {
moveToState(fragment, Fragment.CREATED, 0, 0, false);
} else {
moveToState(fragment);
}
if (fragment.mView == null) {
throw new IllegalStateException("Fragment " + fname
+ " did not create a view.");
}
if (id != 0) {
fragment.mView.setId(id);
}
if (fragment.mView.getTag() == null) {
fragment.mView.setTag(tag);
}
return fragment.mView;
}
主要逻辑是： 依次根据fragment的id、tag、containerId来获取已经存在的fragment，没有就创建，然后设置xml中配置的id、tag、containerId，调用addFragment将fragment添加到Activity中，最后返回fragment的View。
（2）FragmentActivity中
LayoutInflater.Factory2的两个方法均由FragmentLayoutInflaterFactory来负责具体View的创建：
@Override
public View onCreateView(@NonNull String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
return onCreateView(null, name, context, attrs);
}
@Nullable
@Override
public View onCreateView(@Nullable View parent, @NonNull String name, @NonNull Context context,
@NonNull AttributeSet attrs) {
if (FragmentContainerView.class.getName().equals(name)) {
return new FragmentContainerView(context, attrs, mFragmentManager);
}
if (!"fragment".equals(name)) {
return null;
}
...
return fragment.mView;
}
对fragment标签的处理与Activity中流程大体相同，拓展了对androidx.fragment.app.FragmentContainerView的处理。
3、总结
LayoutInflater中加载xml为布局，首先通过mFactory2、mFactory属性创建TextView、Button等常用控件，可以借此对这些常用的控件进行拓展（如MaterialComponentsViewInflater创建这些控件的拓展类MaterialTextView、MaterialRadioButton等）， mPrivateFactory(Activity、FragmentActivity中实现）解析fragment的标签，最后由LayoutInflater的onCreateView和createView来负责可能遗漏的View创建。
附： 项目依赖
由于appcompat相关包更新较为频繁， 这里给出本文分析相关的依赖包：
implementation 'androidx.appcompat:appcompat:1.2.0'
implementation 'com.google.android.material:material:1.2.1'
implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
implementation 'androidx.navigation:navigation-fragment:2.3.2'
implementation 'androidx.navigation:navigation-ui:2.3.2'
implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.2.0'
implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'