LayoutInflater的认识

LayoutInflater的认识

介绍我们常用的但是却一直忽略的LayoutInflater的这个类，常用来加载布局，生成布局。花了一点时间，撸了一遍的其中的代码。

生成的方式

单例模式

inflate的源码分析

总结

生成的方式

一，public static LayoutInflater from(Context 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;
}

二

LayoutInflater LayoutInflater =
(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

原理都是一样的。

单例模式

根据得到的来源追踪

context中的getSystemService(String name),context 是ComtextImpl的实例的对象，

ComtextImpl的getSystemService(String name)

public Object getSystemService(String name) {
ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
return fetcher == null ? null : fetcher.getService(this);
}

SYSTEM_SERVICE_MAP是一个容器的map

private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP =
new HashMap<String, ServiceFetcher>();

初始化的时候 静态代码块初始化,生成layoutInflater的服务

static {
registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() {
public Object createService(ContextImpl ctx) {
return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());
}});

}

ServiceFetcher中的getService()的方法

public Object getService(ContextImpl ctx) {
ArrayList<Object> cache = ctx.mServiceCache;
Object service;
//  同步
synchronized (cache) {

if (cache.size() == 0) {
for (int i = 0; i < sNextPerContextServiceCacheIndex; i++) {
cache.add(null);
}
} else {
service = cache.get(mContextCacheIndex);
if (service != null) {
return service;
}
}
service = createService(ctx);
cache.set(mContextCacheIndex, service);
return service;
}
}

这是典型的map实现单例模式。

inflate的源码分析

public View inflate(int resource, 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 {
//生成XmlResourceParser的对象，并调用inflate()的方法
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}

inflate(parser, root, attachToRoot)的方法

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");

final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
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 (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("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}

rInflate(parser, root, attrs, false, false);
} else {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, attrs, false);

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
rInflate(parser, temp, attrs, true, 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;
}
}

} catch (XmlPullParserException e) {
InflateException ex = new InflateException(e.getMessage());
ex.initCause(e);
throw ex;
} catch (IOException e) {
InflateException ex = new InflateException(
parser.getPositionDescription()
+ ": " + e.getMessage());
ex.initCause(e);
throw ex;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
}

Trace.traceEnd(Trace.TRACE_TAG_VIEW);

return result;
}
}

拆分来看

//赋值 result ＝ root,mConstructorArgs
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context)mConstructorArgs[0];
mConstructorArgs[0] = mContext;
View result = root;

merge 标签

if (TAG_MERGE.equals(name)) {
rInflate(parser, root, attrs, false, false);//直接加载子view
}

creatview 最大的标签，最外围的标签

final View temp = createViewFromTag(root, name, attrs, false);

createViewFromTag的方法

View createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext) {
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}

Context viewContext;
if (parent != null && inheritContext) {
viewContext = parent.getContext();
} else {
viewContext = mContext;//赋值context
}

// Apply a theme wrapper, if requested.
final TypedArray ta = viewContext.obtainStyledAttributes(attrs, ATTRS_THEME);
final int themeResId = ta.getResourceId(0, 0);
if (themeResId != 0) {
viewContext = new ContextThemeWrapper(viewContext, themeResId);
}
ta.recycle();

if (name.equals(TAG_1995)) {
// Let's party like it's 1995!
return new BlinkLayout(viewContext, attrs);
}

if (DEBUG) System.out.println("******** Creating view: " + name);

try {
//factory build view
View view;
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, viewContext, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, viewContext, attrs);
} else {
view = null;
}

if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, viewContext, attrs);
}

if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
try {
//解析系统的view，并创建（比如TextView）
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
//解析自定义的view，并创建（比如com.xx.xxx.view）
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}

if (DEBUG) System.out.println("Created view is: " + view);
return view;

} catch (InflateException e) {
throw e;

} catch (ClassNotFoundException e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;

} catch (Exception e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class " + name);
ie.initCause(e);
throw ie;
}
}

onCreateView最终回调用的CreateView()

createView(name, "android.view.", attrs);//加上前缀andorid.view.

createView()

public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//map 存储着之前解析的view
Constructor<? extends View> constructor = sConstructorMap.get(name);
Class<? extends View> clazz = null;

try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);

if (constructor == null) {
//生成构造器及参数
// 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);
}
}
constructor = clazz.getConstructor(mConstructorSignature);
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 = 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);
}
}
}
//args的参数的名称为context，和attrs
Object[] args = mConstructorArgs;
args[1] = attrs;

constructor.setAccessible(true);
//生成view(反射)
final View view = constructor.newInstance(args);
// viewstub标签
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
//返回view
return view;

} catch (NoSuchMethodException e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class "
+ (prefix != null ? (prefix + name) : name));
ie.initCause(e);
throw ie;

} catch (ClassCastException e) {
// If loaded class is not a View subclass
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Class is not a View "
+ (prefix != null ? (prefix + name) : name));
ie.initCause(e);
throw ie;
} catch (ClassNotFoundException e) {
// If loadClass fails, we should propagate the exception.
throw e;
} catch (Exception e) {
InflateException ie = new InflateException(attrs.getPositionDescription()
+ ": Error inflating class "
+ (clazz == null ? "<unknown>" : clazz.getName()));
ie.initCause(e);
throw ie;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}

回到inflate的方法里面，inflat剩下的代码的

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
//设置默认的layoutparams(viewgroup 默认是不带margin的layoutparams),所以我们可以重写的这个方法，在自定义viewgroup的时候，让子view能支持margin的标签
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);
}
}

解析子view，rInflate(parser, temp, attrs, true, true);

void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
boolean finishInflate, boolean inheritContext) throws XmlPullParserException,
IOException {
//解析深度，标记的
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();
//focus标签
if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
//tag标签
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
//include 标签
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, parent, attrs, inheritContext);
//merge标签
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
//  创建子view
final View view = createViewFromTag(parent, name, attrs, inheritContext);
// 创建的子view
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
//解析子view的子view
rInflate(parser, view, attrs, true, true);
viewGroup.addView(view, params);
}
}

if (finishInflate) parent.onFinishInflate();
}

最后inflate()剩下的方法

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;

root!=null&&attchToRoot =ture; addView此时返回的result为root;

root＝null || attchToRoot = false ;返回为temp;即生成的布局的view;

总结

1 LayoutInflater是一个默默系统服务

2 LayoutInflater的单例模式实现的

3 系统模式基本上都是单例模式实现的

4 LayoutInflater的inflate（），首先解析布局文件最大的标签，当是merge的标签的时候，最大的标签就会自动放弃。否则生成最大的标签的view的对象，并且调用rInflate的生成的子view的对象。一直迭代下去，知道最里面没有子标签。

5 如果root不为null，并且attchToRoot＝true的时候，返回的是root的view，并自动将生成的view加到root的上面去。root不为null的时候，attchToRoot＝false的时候的，会为生成view加上setLayoutParams();LayoutParams为root的默认生成的LayoutParams的对象，当root为null，就意味啥也没有。

所以推荐使用的inflate(R.layout.xxx, parent, false);