废话就不多说了，直接开始。
因为继承AppCompatActivity,部分代码在support里面。
分析版本： Android N 7.1.1 Api Level 25
support版本：25.3.1
先看下我们最熟悉的代码（默认的MainActivity）
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
布局文件R.layout.activity_main到底是怎么加载的？那就去看setContentView方法。
public class AppCompatActivity extends FragmentActivity implements AppCompatCallback, SupportParentable, DelegateProvider {
public void setContentView(View view) {
this.getDelegate().setContentView(view);
}
}
public abstract class AppCompatDelegate {
...
public abstract void setContentView(View var1);
public abstract void setContentView(@LayoutRes int var1);
public abstract void setContentView(View var1, LayoutParams var2);
...
}
发现setContentView是AppCompatDelegate 抽象类的一个抽象方法。
那就去找它的实现类，找到AppCompatDelegateImplV9为其主要实现类
class AppCompatDelegateImplV9 extends AppCompatDelegateImplBase
implements MenuBuilder.Callback, LayoutInflaterFactory {
...
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
...
}
只有五行代码，这里分享一个看源码的技巧，就是只找关键代码。
我们想知道的是布局文件是怎么被加载的，所以这段代码可以看成
class AppCompatDelegateImplV9 extends AppCompatDelegateImplBase
implements MenuBuilder.Callback, LayoutInflaterFactory {
...
@Override
public void setContentView(int resId) {
LayoutInflater.from(mContext).inflate(resId, contentParent);
}
...
}
技巧还有就是，需要有一个心理预期，比如这里应该会有一个读取本地文件的代码。因为xml文件在本地磁盘。
而且还可以大胆一点猜想，加载xml肯定用的不是输入输出流，而是Native方法。
然后继续往下看inflate方法
public abstract class LayoutInflater {
...
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
return inflate(parser, root, root != null);
}
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
if (DEBUG) {
Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
+ Integer.toHexString(resource) + ")");
}
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
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)) {
if (root == null || !attachToRoot) {
throw new InflateException("这里代码就比较长了，毕竟是重要代码之一。
上面的简化一下
public abstract class LayoutInflater {
...
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();
}
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
View result = root;
final String name = parser.getName();
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
if (root != null && attachToRoot) {
root.addView(temp, params);
}
if (root == null || !attachToRoot) {
result = temp;
}
}
return result;
}
}
...
}
把布局文件变成XmlResourceParser 对象，获取到根标签的name,然后调用createViewFromTag()方法根据name转化成View。
可以先不管其他步骤，只看主要步骤，知道有其他步骤就可以了，比如这里xml解析就先不管，直接往下看name是怎么变成View的.
去找 final View temp = createViewFromTag(root, name, inflaterContext, attrs);方法，发现最后调用到是LayoutInflater类的createView()方法
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, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
}
}
（这里有其他加载器mFactory、mFactory2、mPrivateFactory不设置的情况下，它们都为空）,也就是说会走这里的onCreateView方法或者createView方法。其实最终onCreateView方法最终也是调用到了createView方法
protected View onCreateView(View parent, String name, AttributeSet attrs)
throws ClassNotFoundException {
return onCreateView(name, attrs);
}
protected View onCreateView(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);
}
这里可以发现是goole这样设计是为了让大家在写xml布局的时候少写一个前缀android.view.。接下来看看createView方法做了一些什么？
public abstract class LayoutInflater {
...
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//这里的name都是例如android.view.LinearLayout、android.view.Button、android.view.ImageView等
//从Map中获取构造器，用构造器用来创造实例 constructor.newInstance(args);
Constructor constructor = sConstructorMap.get(name);
//如果构造器不为空并且验证类加载器是否和inflater或者context是同一个类加载器，不懂类加载器可以看最底下参考的文章
//这里采用反射的知识获取实例，不懂最底下有参考文章
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class clazz = null;
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
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).asSubclass(View.class);
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
//这里new的是两个参数的构造方法，参数为Context.class, AttributeSet.class
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else {
// If we have a filter, apply it to cached constructor 不知道这是干啥的？看样子是有些类不让你加载，不让加载的类将会抛出异常
if (mFilter != null) {
// Have we seen this name before?
Boolean allowedState = mFilterMap.get(name);
if (allowedState == null) {
// New class -- remember whether it is allowed
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
mFilterMap.put(name, allowed);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, attrs);
}
}
}
//构造参数赋值
Object[] args = mConstructorArgs;
args[1] = attrs;
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
} catch (NoSuchMethodException e) {
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassCastException e) {
// If loaded class is not a View subclass
final InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (ClassNotFoundException e) {
// If loadClass fails, we should propagate the exception.
throw e;
} catch (Exception e) {
final InflateException ie = new InflateException(
attrs.getPositionDescription() + ": Error inflating class "
+ (clazz == null ? "" : clazz.getName()), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
...
}
又是一大坨，没办法，重要代码都要仔细认真看下。上面有注释。
可以看到这里使用反射来创建Viewfinal View view = constructor.newInstance(args);。
可以看到上面只是new一个View,就是根View,那它的子View要怎么加载呢，继续看createViewFromTag()后面的rInflateChildren(parser, temp, attrs, true)方法。
public abstract class LayoutInflater {
...
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
/**
* Recursive method used to descend down the xml hierarchy and instantiate
* views, instantiate their children, and then call onFinishInflate().
*
* Note: Default visibility so the BridgeInflater can
* override it.
*/
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
//得到当前的View层级，(第一次进入层级为0)此方法递归调用，所以得到的值会不同。
//不懂getDepth() 最下面有参考文章
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)) {
//处理焦点
parseRequestFocus(parser, parent);
} else if (TAG_TAG.equals(name)) {
//处理tag属性标签
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
//处理include标签（parser.getDepth()获取TAG的高度/层级，0代表第一层，就是说include不能作为根(root)视图）
if (parser.getDepth() == 0) {
throw new InflateException("
用while循环来递归初始化子View, 可以看到的是最后也是调用了createViewFromTag()具体实现初始化子View，然后再添加到根ViewGroup中。
通过rInflate的解析之后，整颗视图数就构建完毕，，当调用了Activity的onResume之后，我们通过setContentView设置的内容就会出现在我们的视野中。
总结一下看源码的技巧
- 确定方向
（带着问题去看源码） - 只看主流程的代码
（主流程看完，视情况看分支代码，不然每个分支都看一下，很容易陷落） - 有个心理预期
（联想代码的实现方式） - 思考问题和代码的关联性
这样就完了吗？支线代码还没看呢！
那具体有这么几个地方
-
this.getDelegate().setContentView(view);这里的this.getDelegate()是怎么生成的？ - 有没有对象设置
mFactory、mFactory2、mPrivateFactory? -
setContentView(int resId)方法中的5行代码，我们只分析了一行。
首先看this.getDelegate()
public class AppCompatActivity extends FragmentActivity implements AppCompatCallback, SupportParentable, DelegateProvider {
@NonNull
public AppCompatDelegate getDelegate() {
if (this.mDelegate == null) {
this.mDelegate = AppCompatDelegate.create(this, this);
}
return this.mDelegate;
}
}
public abstract class AppCompatDelegate {
public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
return create(activity, activity.getWindow(), callback);
}
private static AppCompatDelegate create(Context context, Window window, AppCompatCallback callback) {
int sdk = VERSION.SDK_INT;
if (BuildCompat.isAtLeastN()) {
return new AppCompatDelegateImplN(context, window, callback);
} else if (sdk >= 23) {
return new AppCompatDelegateImplV23(context, window, callback);
} else if (sdk >= 14) {
return new AppCompatDelegateImplV14(context, window, callback);
} else {
return (AppCompatDelegate)(sdk >= 11 ? new AppCompatDelegateImplV11(context, window, callback) : new AppCompatDelegateImplV9(context, window, callback));
}
}
}
因为这里sdk是25,所以调用的是这行代码return new AppCompatDelegateImplN(context, window, callback);其实这里的V23、V14等都是直接继承的(查看继承关系快捷键为Ctrl + H)。
所以最终调用的是V9中的setContentView方法。
public void setContentView(int resId) {
this.ensureSubDecor();
ViewGroup contentParent = (ViewGroup)this.mSubDecor.findViewById(16908290);
contentParent.removeAllViews();
LayoutInflater.from(this.mContext).inflate(resId, contentParent);
this.mOriginalWindowCallback.onContentChanged();
}
再看第二个问题，有没有对象设置mFactory、mFactory2、mPrivateFactory?
我这里没有看到代码，不过听说appCompat会设置mFactory2,将TextView转化成AppCompatTextView等。
再看第三个问题，setContentView(int resId)`方法中的5行代码，我们只分析了一行。
class AppCompatDelegateImplV9 extends AppCompatDelegateImplBase
implements MenuBuilder.Callback, LayoutInflaterFactory {
...
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
...
}
参考：
View绘制体系（三）——AttributeSet与TypedArray详解
关于getClass()，Object.class，getClassLoader的理解
parser.getDepth()讲解