LayoutInflater 源码解析
LayoutInfleter 获取
getSystemService()
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| LayoutInflater inflater = (LayoutInflater) MainActivity.this.getSystemService(Context.LAYOUT_INFLATER_SERVICE)
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LayoutInflater.from(Context)
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| LayoutInflater inflater = LayoutInflater.from(context)
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inflate layout 的 2 种方式
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)
参数详解
- 1、 如果 root 为 null，attachToRoot 将失去作用，设置任何值都没有意义。
- 2、 如果 root 不为 null，attachToRoot 设为 true，则会给加载的布局文件的指定一个父布局，即 root，并 generateLayoutParams 在 addView 的时候设置进去。
- 3、 如果 root 不为 null，attachToRoot 设为 false，不会给布局指定一个父布局，会将布局文件最外层的所有 layout 属性进行设置，当该 view 被添加到父 view 当中时，这些 layout 属性会自动生效 setLayoutParams()。
- 4、 使用两个参数的
LayoutInflater.inflate(int resource,ViewGroup root) 或者 View.inflate(resource,root)，在不设置 attachToRoot 参数的情况下，如果 root 不为 null，attachToRoot 参数默认为 true。
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| public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
// ...
View result = root;
final String name = parser.getName();
if (TAG_MERGE.equals(name)) {
// ...
rInflate(parser, root, inflaterContext, attrs, false);
} else {
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
// 如果root不为null，那么会生成LayoutParams
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// 如果attachToRoot为false，那么会为createViewFromTag()生成的根View设置setLayoutParams(params)
temp.setLayoutParams(params);
}
}
// ...
rInflateChildren(parser, temp, attrs, true);
// ...
// 如果root不为null且attachToRoot为true，那么将生成的View添加到root
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果root为null或者attachToRoot为false，那么返回的View为生成的View，否则为root
if (root == null || !attachToRoot) {
result = temp;
}
}
// ...
return result;
}
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public static View inflate(Context context, @LayoutRes int resource, ViewGroup root)
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| public static View inflate(Context context, @LayoutRes int resource, ViewGroup root) {
LayoutInflater factory = LayoutInflater.from(context);
return factory.inflate(resource, root);
}
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小结
merge 相关
- merge 必须作为布局的根标签，如果非 root 标签抛出异常 InflateException
<merge /> must be the root element
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| void rInflate(XmlPullParser parser, View parent, Context context, AttributeSet attrs, boolean finishInflate) {
//...
int type;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
//...
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
//...
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
}
//...
}
}
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疑问：为啥 TAG_MERGE.equals(name) 就一定不为 root 标签呢？
这是因为如果 merge 为根标签，在 inflate(parser, root, attachToRoot)
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| public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
advanceToRootNode(parser); // 这里调用了一次parser.next()
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); // 这里又调用了一次parser.next()
}
}
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- 如果 inflate(resource,root,attachToRoot) 中的 root 为 null 或者 attachToRoot 为 false，那么 resource 布局根标签不能为标签，抛出异常 InflateException
<merge /> can be used only with a valid ViewGroup root and attachToRoot=true
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| // LayoutInflater
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
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);
}
}
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include 相关
- include 不能作为根标签
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| void rInflate(XmlPullParser parser, View parent, Context context, AttributeSet attrs, boolean finishInflate) {
//...
int type;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
//...
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
//...
} else if ((TAG_INCLUDE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
}
//...
}
}
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- include 不能用在 View 里面，只能用于 ViewGroup 中，否则报错
"<include /> can only be used inside of a ViewGroup" - include 必须指定 layout 属性，而且 layout 指定的 layout 必须有效
"You must specify a layout in the include tag: <include layout=@layout/layoutID/>" - 如果 include 所在的 ViewGroup 可以 generateLayoutParams() 那么就用这个，如果不能，会默认设置一个 ViewGroup.LayoutParams
inflate() 原理
inflate
这个函数就是我们把 Xml 布局文件实例化为一个 View 对象的入口了；tryInflatePrecompiled() 是编译期通过 xml 生成 view，目前 (2020 年 10 月 26 日) 还不可靠。
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| // LayoutInflater
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); //这段代码其实是必然返回null的，因为当前版本写死了预编译的Enable为false
if (view != null) {
return view;
}
XmlResourceParser parser = res.getLayout(resource); //获取XmlBlock.Parser对象
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
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此处又调用了 inflate(parser, root, attachToRoot) 这个函数，来对 Xml 布局进行解析
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| public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {//XmlPullParser是一个接口
//此函数是真正执行将xml解析为视图view的过程，此处的parser为根据xml布局获取到的parser对象
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final Context inflaterContext = mContext;
final inflateAttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root; //需要返回的view对象
try {
advanceToRootNode(parser); //对START_TAG和END_TAG进行判断和处理
final String name = parser.getName(); //获取当前标签
if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}
if (TAG_MERGE.equals(name)) { //如果使用merge标签
if (root == null || !attachToRoot) { //使用merge标签必须有父布局，且依赖于父布局加载，否则就会抛出异常
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
rInflate(parser, root, inflaterContext, attrs, false);//递归（Recursive）生成布局视图
} else { //如果不使用merge标签，创建tmp 作为临时的根节点，并最终赋值给result返回
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs); ////根据标签名创建一个view
ViewGroup.LayoutParams params = null;
if (root != null) { //如果rootView不为空
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs); //根据rootView生成layoutparams
if (!attachToRoot) { //如果attachToRoot为false
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params); //设置一个临时的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) { //如果root不为空，且attachToRoot为true
root.addView(temp, params); //把temp添加到到rootview中
}
// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) { // 如果root为空且attachToRoot为false
result = temp; //将temp，也就是根结点的View赋值给result
}
}
}
// ...
return result; //返回结果
}
}
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rInflate
不管是 merge 标签，还是非 merge 标签，最终都会调用到 rInflate() 这个函数，这个是用于递归向下遍历 xml 布局，最终调用 createViewFromTag() 函数来反射创建 View 对象
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| 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)) { //如果需REQUEST_FOCUS标签
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) { //如果是“tag”标签
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) { //如果是 Include标签
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs); //对 include标签进行解析
} else if (TAG_MERGE.equals(name)) { //如果是merge标签
throw new InflateException("<merge /> must be the root element"); //直接抛出异常
} else { //其他标签
final View view = createViewFromTag(parent, name, context, attrs); //根据Tag创建view，反射创建View
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true); //递归调用rInflate函数
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
parent.onFinishInflate();
}
}
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createViewFromTag
真正根据解析到的 Tag 标签去反射创建 View 的函数
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| 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); //尝试使用Factory来闯将View对象
if (view == null) { //如果tryCreateView返回null
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
//sample：com.aiwinn.base.widget.CameraSurfaceView
if (-1 == name.indexOf('.')) { //如果当前Tag含有“.”
view = onCreateView(context, parent, name, attrs);
} else {
view = createView(context, name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
}
// ...
}
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tryCreateView() 通过 Factory/Factory2/mPrivateFactory 来创建 View
在这个函数中会首先调用 tryCreateView() 来获取 View 对象
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| 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;
}
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如果标签为 blink 返回 BlinkLayout；根据设置的 Factory2 和 Factory 来创建对象
createView() 没有自定义 Factory，默认创建，反射
如果 tryCreateView() 返回为 null，则进一步调用 createView() 函数来创建 View 对象
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| public final View createView(@NonNull Context viewContext, @NonNull String name,
@Nullable String prefix, @Nullable AttributeSet attrs)
throws ClassNotFoundException, InflateException {
//根据Tag反射创建view
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);//把prefix和name进行拼接，获取到对应的Class对象
if (mFilter != null && clazz != null) {
boolean allowed = mFilter.onLoadClass(clazz);
if (!allowed) {
failNotAllowed(name, prefix, viewContext, attrs);
}
}
constructor = clazz.getConstructor(mConstructorSignature);//获取构构造器对象
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else {
// ...
}
Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = viewContext;
Object[] args = mConstructorArgs;
args[1] = attrs;
try {
final View view = constructor.newInstance(args); //根据获取到的构造器创建一个View的实例化对象
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{
// ...
}
}
}
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彩蛋 BlinkLayout
BlinkLayout 是 LayoutInflater 中的一个内部类，它本身是是 FrameLayout 的子类,如果当前标签为 TAG_1995blink，则创建一个隔 0.5 秒闪烁一次的 BlinkLayout 来承载它的布局内容
源码注释也很有意思，写了 Let’s party like it’s 1995!, 据说是为了庆祝 1995 年的复活节
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| <blink
android:layout_below="@id/iv_qr_code"
android:layout_centerHorizontal="true"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Android研习社"
android:textColor="#157686"
android:textSize="55sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</blink>
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这种效果也适合来做 EditText 中光标的闪烁效果
Ref