setContentView流程
引言
Activity的oncreate方法应该是Android开发者最熟悉的一个方法了，但是，你知道它是怎么被调用的吗？嘿嘿，可以看一下我之前的activity启动流程那篇文章。今天主要的目的是要来梳理一下另一个很常见且必须的方法----setContentView()。这个方法是怎么做到把Xml文件里的view的描述加载进来的，具体的流程是什么呢？（这个东西在我最初做Android的时候被人问到过，结果我什么也说不上来。。。答案就在setContentView这个方法里其实）
正式进入分析
先上DJ，先上DJ！不好意思，走错片场了。先把代码搞上来看一下。
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
只有两行代码，逻辑很简单，先是window的setcontentview，然后再去实例化ActionBar。这里我们重点关注setcontentview这个方法。getWindow拿到了一个Window的对象，而Window又是一个抽象类，根据源码描述，我们知道它的唯一实现类是PhoneWindow这个类。不信你看注释
/*The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a Window.
*/
(源码的注释还是很重要的，有时候先看一下注释会减少我们好多疑惑或麻烦)
找到具体实现的地方，来看一看
@Override
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);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
我们的主要目的是搞清楚控件是怎么显示的，所以忽略一些次要的东西，直接看一下这行代码
mLayoutInflater.inflate(layoutResID, mContentParent);
很熟悉吧，平时我们想要自己加载一个view的时候也是这样来搞的，对吧。
让我们来看看源码里这个mLayoutInflater是何方神圣，跟我们平时用的一样不一样。
看到没有，跟我们平时用的方式一模一样。不知不觉感觉自己又跟源码靠近了一点。所以，setContentView这个方法到最后其实就是LayoutInflater的inflate的过程。
一通command+左键，我们来到了这个方法里
/**
* 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();
if (DEBUG) {
Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
+ Integer.toHexString(resource) + ")");
}
View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
if (view != null) {
return view;
}
XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
先看注释，大概意思就是，从指定的xml资源中添加新的视图层次结构。如果发生错误，则抛出InflateException。
看这行代码：
View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
根据方法名，尝试从预编译中去inflate一个 view。大概看了一下代码，mUseCompiledView这个boolean值直接判断返回了null，大概这个方法还在重写中吧。那我们接着往下看代码。inflate(parser, root, attachToRoot)，这个就是具体实现了。
/**
* 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, 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 {
advanceToRootNode(parser);
final String name = parser.getName();
if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}
if (TAG_MERGE.equals(name)) {
if (root == null || !attachToRoot) {
throw new InflateException("看注释，从xml节点中inflate一个view。这个方法应该就是从xml文件变成view控件的关键方法了。首先把context付给mConstructorArgs[0],获取attrs，然后开始解析xml文件。advanceToRootNode就是要把xml解析器parser移动到START_TAG这个标签上开始解析。如果解析出的名称是TAG_MERGE，进行了一些判断，当root是null或者attachToRoot为false时，抛出异常提示用户root不能为空且attachToRoot应给为true。如果满足条件则调用 rInflate(parser,root,inflaterContext,attrs,false);这个方法。从这里可以看出我们使用merge这个标签时应该避免root为空且attach这个变量应该为true。
/**
* 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 {
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("
rinflate这个方法就是递归的下降解析xml文件，也就是一层一层解析xml，同时实例化解析出来的view和它们的子view，最后调用root的onFinishInflate()方法告知系统解析完毕，我们已经把所有子view都添加进来了。看代码是根据不同的tag，去做不同的解析操作。同时看到了一些异常，了解了merge应该只能做根view的标签，include不能做根view的标签，否则会抛出相应的异常。解析的方法有parseViewTag,parseInclude等。如果是普通view标签，则调用createViewFromTag()方法。我们来看一下这个方法。
/**
* 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();
}
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(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;
}
}
从attribute set提供的内容里创建一个view。如果不忽略themeatrr，那就根据attr创建一个相应的context。然后调用tryCreateView去尝试创建一个view。
public final View tryCreateView(@Nullable View parent, @NonNull String name,
@NonNull Context context,
@NonNull AttributeSet attrs) {
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(context, attrs);
}
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);
}
return view;
}
发现一个好玩的东西，BlinkLayout，闪烁的viewgroup，继承自framelayout，里面嵌套的view会有闪烁效果。然后如果mFactory2不为空，就用mFactory2去创建view，如果mFactory不为空，则用mFactory去创建view，view=null，然后再用mPrivateFactory去创建view，最后返回。再返回createviewfromtag方法里，如果view不为空，则直接返回view，如果为空，则判断name的路径，如果包含.则调用createview，如果不包含则拼接上”android.view.“再去调用createview方法。
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor constructor = sConstructorMap.get(name);
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 = Class.forName(prefix != null ? (prefix + name) : name, false,
mContext.getClassLoader()).asSubclass(View.class);
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else {
// If we have a filter, apply it to cached constructor
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 = Class.forName(prefix != null ? (prefix + name) : name, false,
mContext.getClassLoader()).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);
}
}
}
Object lastContext = mConstructorArgs[0];
if (mConstructorArgs[0] == null) {
// Fill in the context if not already within inflation.
mConstructorArgs[0] = mContext;
}
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]));
}
mConstructorArgs[0] = lastContext;
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);
}
}
主要流程就是检查当前classLoader是否是正确的加载layoutinflater的classloader，如果是则通过反射调用构造器创建view，同时把constructor放到sConstructorMap里，如果不是，则抛出异常”Class not allowed to be inflated…“。到这里，整个流程就把view和子view都创建了出来，流程结束。