从源码角度深入理解LayoutInflater

关于LayoutInflater，在开发中经常会遇到，特别是在使用ListView的时候，这个几乎是必不可少。今天我们就一起来探讨LayoutInflater的工作原理。

一般情况下，有两种方式获得一个LayoutInflater实例：

LayoutInflater inflater1, inflater2;
inflater1 = LayoutInflater.from(this);
inflater2 = (LayoutInflater) this
.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

但是当我们查看源码的时候，却发现这两种其实是一种，只不过第一种将第二种封装了一下，我们看看from这个方法的源码：

/**
* Obtains the LayoutInflater from the given 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;
}

获得LayoutInflater对象之后，我们就可以调用inflate来获得View对象了，inflate方法的源码如下：

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}

这里调用了一个inflate的一个重载方法，这个重载方法的最后一个参数和root有关，如果我们的root为空，那么最后一个参数默认为false。我们看看这个重载方法：

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();
}
}

可以看出，先是拿到布局的xml资源，然后，获得一个XmlResourceParser 对象，最后inflate(parser, root, attachToRoot);又是调用一个重载方法：

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 (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, 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) {
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 against its context.
rInflateChildren(parser, temp, attrs, 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 (Exception 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;
}
}

这个方法有点长，前面都是一些简单的判断，一般情况下（特殊情况大家可以按相应的分支走），我们的程序会执行到

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

这行代码，这里创建了一个名为temp的view，如果我们传进来的根布局为null的话，那么这里拿到的就是一个根布局。我们看看这个方法的源代码：

private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}

好啊，这里又调用了一个它的重载方法，那我们就看看这个重载方法，注意最后一个参数恒为false。

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);
ie.initCause(e);
throw ie;

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

仔细分析一下这个重载方法，发现里边的mFactory2和mFactory都为null，那么程序最终其实执行了这个方法里边的这一段：

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;
}
}

前面的判断不用说，后面的判断，如果name中包含

.

，说明我们用的不是普通的view，有可能是自定义View等等，这一条大家可以自行去研究，如果name中不包含

.

,那么程序会执行

view = onCreateView(parent, name, attrs);

，那么我们就去看看这个方法：

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);
}
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(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 = 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);
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 = 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);
}
}
}

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) {
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);
}
}

经过了前两个方法的互相扯皮，最后我们来到了第三个方法上，这是最后创建View的地方，代码虽然很长，但是大家不用怕，这里的代码我们主要分析下面这几行，因为大部分不会被执行到。

public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
.....
.....

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);
.....
.....
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
}

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;

}traceEnd(Trace.TRACE_TAG_VIEW);
}
}

我把这个方法稍微精简一下，可以看到，先是通过Java的反射机制拿到这个name所表示的布局对应的那个Java类，然后是拿到构造方法，最后通过构造方法拿到一个View实例，逻辑还是比较清楚的。

好的，到这里，我们就已经拿到根View了，现在我们再回到上面说的那个inflate(…)方法中，在该方法中创建完temp这个View之后，接着就会执行到

rInflateChildren(parser, temp, attrs, true);

，里边最终会执行到一个递归方法，这个方法是这样的：

void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {

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)) {
parseRequestFocus(parser, parent);
} 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);
}
}

if (finishInflate) {
parent.onFinishInflate();
}
}

这个方法还不算长，进入while循环以后，在if分支里会走到最后一个else里，这里还是先调用我们前文说的那个

createViewFromTag

方法获得一个布局，然后递归，如果获得的这个View是个ViewGroup，那么会把它的子View添加到这个ViewGroup中，如果是普通View，那么接着循环就是了。做完这些之后，下面就和一开始提到的root是否为null有关了，如果root为null，那么attachToRoot这个参数为false，这个时候会执行：

if (root == null || !attachToRoot) {
result = temp;
}

然后我们解析过得到的View就会被返回，如果root不为null,那么attachToRoot这个参数默认为true，那么系统会执行

if (root != null && attachToRoot) {
root.addView(temp, params);
}

也就是会把root套在我们解析得到的View之外，然后返回。

好了，到这里我们的LayoutInflater基本上就分析完了.

如果大家还有什么问题，欢迎留言讨论