Android LayoutInflater深入分析及应用

LayoutInflater解析

前言：

在Android中，如果是初级玩家，很可能对LayoutInflater不太熟悉，或许只是在Fragment的onCreateView()中模式化的使用过而已。但如果稍微有些工作经验的人就知道，这个类有多么重要，它是连接布局XMl和Java代码的桥梁，我们常常疑惑，为什么Android支持在XML书写布局？

我们想到的必然是Android内部帮我们解析xml文件，LayoutInflater就是帮我们做了这个工作。
首先LayoutInflater是一个系统服务，这个我们可以从from方法看出来

/**
* 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;
}

通常我们拿到LayoutInflater对象之后就会调用其inflate方法进行加载布局，inflate是一个重载方法

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

可以看到，我们调用2个参数的方法时候其默认是添加到父布局中的（父布局一般不为空）

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 {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}

这个方法中，其实是使用Resources将资源ID还原为XMlResoourceParser对象，然后调用inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)方法，解析布局的具体步骤都是在这个方法中实现

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.
//1.循环寻找根节点，其实就是节点指针遍历的过程
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!");
}
//2.得到节点的名字，用于判断该节点
final String name = parser.getName();
if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}
//3.对节点名字进行判断，然后是merge就将其添加到父布局中（依据Merge的特性必须添加到父布局中）
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 {
//4.创建根据节点创建View
// 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
//5.根据attrs生成布局参数
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
//6.如果View不添加到父布局中，那就给其本身设置布局参数
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp
// 7.将该节点下的子View全部加载
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.
//8.如果添加到父布局中，直接addView
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.
//9.如果不添加到父布局，那么将自己返回
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;
}
}

重点的步骤我已经加上注释了，核心

1.找到根布局标签
2.创建根节点对应的View
3.创建其子View

我们从这里面可以看出来，子View的解析其实都是rInflate方法，如果xml中有根布局，那么就调用createViewFromTag创建布局中的根View。我们也可以明白merge的原来，因为它直接调用rInflate添加到父View中，看到rInflate(parser, root, attrs, false, false)和rInflate(parser, temp, attrs, true, true)第二个参数区别我们就明白了。

接下来我们看下rInflate如何创建多个布局

void rInflate(XmlPullParser parser, View parent, final AttributeSet attrs,
boolean finishInflate, boolean inheritContext) throws XmlPullParserException,
IOException {
//获取当前解析器指针所在节点处于布局层次
final int depth = parser.getDepth();
int type;
//进行树的深度优先遍历（如果一个节点有子节点将会再次进入rInflate，否则继续循环）
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();
//如果其中有request_focus标签，那就给这个节点View设置焦点
if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
//如果其中有tag标签，那就给这个节点View设置tag(key,value)
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
//如果其中是include标签，如果include标签
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, parent, attrs, inheritContext);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
//创建该节点代表的View并添加到父view中，此外遍历子节点
final View view = createViewFromTag(parent, name, attrs, inheritContext);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflate(parser, view, attrs, true, true);
viewGroup.addView(view, params);
}
}
//代表着一个节点含其子节点遍历结束
if (finishInflate) parent.onFinishInflate();
}

从上面可以看到，所以创建View都将会交给createViewFromTag(View parent, String name, AttributeSet attrs, boolean inheritContext)中，我们可以看下该方法如何创建View

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;
}
// 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 {
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 {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
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;
}
}

其实很简单，就是4个降级处理

if(factory2!=null){
factory2.onCreateView();
}else if(factory!=null){
factory.onCreateView();
}else if(mPrivateFactory!=null){
mPrivateFactory.onCreateView();
}else{
onCreateView()
}

其他的onCreateView我们不去设置的话为null，我们看下自己的onCreateView(),其实这个方法会调用createView()

public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//从构造器Map（缓存）中获取需要的构造器
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
//如果缓存中没有需要的构造器，那就通过ClassLoader加载需要的类
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);
}
}
//将使用过的构造器缓存
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);
}
}
}
Object[] args = mConstructorArgs;
args[1] = attrs;
constructor.setAccessible(true);
//通过反射获取需要的实例对象
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
final ViewStub viewStub = (ViewStub) view;
//ViewStub将创建一个属于自己的LayoutInflater，因为它需要在不同的时机去inflate
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);
}
}

大体步骤就是，

1.从缓存中获取特定View构造器，如果没有，则加载对应的类，并缓存该构造器，
2.利用构造器反射构造对应的View
3.如果是ViewStub则复制一个LayoutInflater对象传递给它

感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！

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