布局加载源码分析
书接上文，从View的绘制流程开始：
Window、DecorView、ViewRootImp详解
View的绘制流程-measure、layout、draw
先从Activity.setContentView开始简单回顾下布局是如何被加载的
Activity.java
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
PhoneWindow.java
@Override
public void setContentView(int layoutResID) {
......
if (mContentParent == null) {
//1.初始化DecorView和mContentParent
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 {
//2.加载资源文件，创建View树加载到mContentParent
mLayoutInflater.inflate(layoutResID, mContentParent);
}
......
}
1.加载解析xml文件
来看下LayoutInflater是如何加载布局的
LayoutInflater.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
if (view != null) {
return view;
}
//加载解析XML文件
XmlResourceParser parser = res.getLayout(resource);
try {
//填充View树
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
先看下res对象是如何加载XML的，res是个Resources类的对象。
Resources.java
public XmlResourceParser getLayout(@LayoutRes int id) throws NotFoundException {
return loadXmlResourceParser(id, "layout");
}
XmlResourceParser loadXmlResourceParser(@AnyRes int id, @NonNull String type)
throws NotFoundException {
final TypedValue value = obtainTempTypedValue();
try {
final ResourcesImpl impl = mResourcesImpl;
impl.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");
} finally {
releaseTempTypedValue(value);
}
}
@NonNull
XmlResourceParser loadXmlResourceParser(@NonNull String file, @AnyRes int id, int assetCookie,
@NonNull String type)
throws NotFoundException {
if (id != 0) {
try {
synchronized (mCachedXmlBlocks) {
final int[] cachedXmlBlockCookies = mCachedXmlBlockCookies;
final String[] cachedXmlBlockFiles = mCachedXmlBlockFiles;
final XmlBlock[] cachedXmlBlocks = mCachedXmlBlocks;
// First see if this block is in our cache.
final int num = cachedXmlBlockFiles.length;
for (int i = 0; i < num; i++) {
if (cachedXmlBlockCookies[i] == assetCookie && cachedXmlBlockFiles[i] != null
&& cachedXmlBlockFiles[i].equals(file)) {
return cachedXmlBlocks[i].newParser(id);
}
}
// Not in the cache, create a new block and put it at
// the next slot in the cache.
final XmlBlock block = mAssets.openXmlBlockAsset(assetCookie, file);
if (block != null) {
final int pos = (mLastCachedXmlBlockIndex + 1) % num;
mLastCachedXmlBlockIndex = pos;
final XmlBlock oldBlock = cachedXmlBlocks[pos];
if (oldBlock != null) {
oldBlock.close();
}
cachedXmlBlockCookies[pos] = assetCookie;
cachedXmlBlockFiles[pos] = file;
cachedXmlBlocks[pos] = block;
return block.newParser(id);
}
}
} catch (Exception e) {
final 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));
}
实现细节不用太关注，只需知道这个方法的作用是将XML文件读取到内存中，进行一些数据的解析和封装。
所以从XML加载布局本质上是一个IO操作，是比较耗费性能的。
2.填充View数
LayoutInflater.java
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 {
advanceToRootNode(parser);
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) {
// 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);
}
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
// 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;
}
}
} catch (XmlPullParserException e) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(
getParserStateDescription(inflaterContext, attrs)
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
return result;
}
}
重点关注createViewFromTag这个方法。
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
//解析View标签
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
//如果需要当前标签与主题相关，需要对context进行包装
//将主题信息进入context包装类contextWrapper
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);
//如果此时不存在Factory，无论是Factory还是Factory2还是mPrivateFactory，那么会直接对name进行解析
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
//如果name中包含“.”，即是自定义View，否则为原生控件
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) {
throw e;
} catch (ClassNotFoundException e) {
final InflateException ie = new InflateException(
getParserStateDescription(context, attrs)
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(
getParserStateDescription(context, attrs)
+ ": Error inflating class " + name, e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
}
}
public final View tryCreateView(@Nullable View parent, @NonNull String name,
@NonNull Context context,
@NonNull AttributeSet attrs) {
//处理TAG_1995标签，闪烁的FrameLayout，包裹的内容会一直闪烁，类似QQ提示消息
if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(context, attrs);
}
//设置Factory，来对View做额外的拓展
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;
}
根据源码可以将createViewFromTag分成三个流程：
- 对一些特殊标签做处理，如view、TAG_1995
- 对Factory、Factory2的设置判断，如果设置了就会通过Factory、Factory2进行生成view
- 如果没有设置Factory、Factory2，就会使用LayoutInflater默认的方式，进行View的生成。
createViewFromTag源码分析：
1. 处理view标签
如果标签的名称是view，如
<view
class="RelativeLayout"
android:layout_width="match_parent"
android:layout_height= "match_parent"/>
view标签类似于申明时用父类，构建时用子类，设置了class属性，对应的view就会变成具体的控件。
2.如果该节点与主题相关，则需要特殊处理
如果当前节点与Theme相关，需要将context与theme信息包装进ContextWrapper类
3.处理TAG_1995
这个标签最终会被解析撑BinkLayout，类似于一个FrameLayout，被包裹的内容会一直闪烁。
4.判断是否存在Factory或者Factory2
先对Factory判空，Factory本质上是一种扩展操作，提前解析name，然后直接将解析后的view返回。
public interface Factory {
@Nullable
View onCreateView(@NonNull String name, @NonNull Context context,
@NonNull AttributeSet attrs);
}
public interface Factory2 extends Factory {
@Nullable
View onCreateView(@Nullable View parent, @NonNull String name,
@NonNull Context context, @NonNull AttributeSet attrs);
}
可以看到Factory2和Factory都是一个接口，Factory2继承了Factory，扩展了一个参数。再来看setFactory接口。
public void setFactory(Factory factory) {
if (mFactorySet) {
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;
if (mFactory == null) {
mFactory = factory;
} else {
mFactory = new FactoryMerger(factory, null, mFactory, mFactory2);
}
}
public void setFactory2(Factory2 factory) {
if (mFactorySet) {
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;
if (mFactory == null) {
mFactory = mFactory2 = factory;
} else {
mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
}
}
可以看到两个Factory只能设置一次，并且互斥。
5.createView(Context viewContext, String name,String prefix, AttributeSet attrs)
public final View createView(@NonNull Context viewContext, @NonNull String name,
@Nullable String prefix, @Nullable AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Objects.requireNonNull(viewContext);
Objects.requireNonNull(name);
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> 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, viewContext, 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, viewContext, attrs);
}
} else if (allowedState.equals(Boolean.FALSE)) {
failNotAllowed(name, prefix, viewContext, attrs);
}
}
}
Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
Object[] args = mConstructorArgs;
args[1] = attrs;
try {
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;
} finally {
mConstructorArgs[0] = lastContext;
}
} catch (NoSuchMethodException e) {
final InflateException ie = new InflateException(
getParserStateDescription(viewContext, attrs)
+ ": 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(
getParserStateDescription(viewContext, attrs)
+ ": 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(
getParserStateDescription(viewContext, attrs) + ": Error inflating class "
+ (clazz == null ? "<unknown>" : clazz.getName()), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
具体细节不用太关注，但是很明显的一点，View的实例时通过反射的方式创建的。
总结
经过对布局加载原理的分析，可以看出布局加载的主要性能瓶颈在两个方面
-
加载XML是一个IO过程，如果XML文件很大，就会相对比较耗时。
-
View的实例时通过反射创建的，反射会比直接创建更耗时。
