在上篇文章中我們學(xué)習(xí)了setContentView的源碼，還記得其中的LayoutInflater嗎？本篇文章就來(lái)學(xué)習(xí)下LayoutInflater。
@Override
public void setContentView(int resId) {
ensureSubDecor();
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);
mOriginalWindowCallback.onContentChanged();
}
備注：本文基于 Android 8.1.0。
1、LayoutInflater 簡(jiǎn)介
Instantiates a layout XML file into its corresponding View objects. It is never used directly. Instead, use Activity.getLayoutInflater() or Context.getSystemService(Class) to retrieve a standard LayoutInflater instance that is already hooked up to the current context and correctly configured for the device you are running on.
翻譯過(guò)來(lái)就是：LayoutInflater 的作用就是將XML布局文件實(shí)例化為相應(yīng)的 View 對(duì)象，需要通過(guò)Activity.getLayoutInflater() 或 Context.getSystemService(Class) 來(lái)獲取與當(dāng)前Context已經(jīng)關(guān)聯(lián)且正確配置的標(biāo)準(zhǔn)LayoutInflater。
總共有三種方法來(lái)獲取 LayoutInflater：
- Activity.getLayoutInflater()；
- Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ;
- LayoutInflater.from(context);
事實(shí)上，這三種方法之間是有關(guān)聯(lián)的：
- Activity.getLayoutInflater() 最終會(huì)調(diào)用到 PhoneWindow 的構(gòu)造方法，實(shí)際上最終調(diào)用的就是方法三；
- 而方法三最終會(huì)調(diào)用到方法二 Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ；
2、inflate 方法解析
image
LayoutInflater 的 inflate 方法總共有四個(gè)，屬于重載的關(guān)系，最終都會(huì)調(diào)用到 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 方法。
備注：以下源碼中有七條備注。
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;
// ① 尋找布局的根節(jié)點(diǎn)，判斷布局的合理性
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 (TAG_MERGE.equals(name)) {
// ② 如果是Merge標(biāo)簽，則必須依附于一個(gè)RootView，否則拋出異常
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
// ③ 根據(jù)節(jié)點(diǎn)名來(lái)創(chuàng)建View對(duì)象
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
// ④ 如果設(shè)置的Root不為null，則根據(jù)當(dāng)前標(biāo)簽的參數(shù)生成LayoutParams
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
// ⑤ 如果不是attachToRoot ，則對(duì)這個(gè)Tag和創(chuàng)建出來(lái)的View設(shè)置LayoutParams；注意：此處的params只有當(dāng)被添加到一個(gè)Viewz中的時(shí)候才會(huì)生效；
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp against its context.
// ⑥ inflate children tag
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) {
// ⑦ 如果Root不為null且是attachToRoot，則添加創(chuàng)建出來(lái)的View到Root 中
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) {
......
}
return result;
}
}
備注：根據(jù)以上源碼，我們也可以分析出來(lái) inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 不同參數(shù)值帶來(lái)的影響：
- 如果root為null，attachToRoot將失去作用，設(shè)置任何值都沒(méi)有意義；
- 如果root不為null，attachToRoot設(shè)為true，則會(huì)給加載的布局文件的指定一個(gè)父布局，即root；
- 如果root不為null，attachToRoot設(shè)為false，則會(huì)將布局文件最外層的所有l(wèi)ayout屬性進(jìn)行設(shè)置，當(dāng)該view被添加到父view當(dāng)中時(shí)，這些layout屬性會(huì)自動(dòng)生效；
- 在不設(shè)置attachToRoot參數(shù)的情況下，如果root不為null，attachToRoot參數(shù)默認(rèn)為true；
3、rInflate 方法解析
以上代碼中我們還有兩個(gè)方法沒(méi)有分析：rInflate 和 rInflateChildren ；而 rInflateChildren 實(shí)際上是調(diào)用了rInflate；
備注：以下源碼中有六條備注。
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)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
// ① 如果這里出現(xiàn)了include標(biāo)簽，就會(huì)拋出異常
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
// ② 同理如果這里出現(xiàn)了merge標(biāo)簽，也會(huì)拋出異常
throw new InflateException("<merge /> must be the root element");
} else {
// ③ 最重要的方法在這里，createViewFromTag
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
// ④如果當(dāng)前View是ViewGroup（包裹了別的View）則在此處inflate其所有的子View
rInflateChildren(parser, view, attrs, true);
// ⑤添加inflate出來(lái)的view到parent中
viewGroup.addView(view, params);
}
}
if (pendingRequestFocus) {
parent.restoreDefaultFocus();
}
if (finishInflate) {
// ⑥如果inflate結(jié)束，則回調(diào)parent的onFinishInflate方法
parent.onFinishInflate();
}
}
總結(jié)：
- 首先進(jìn)行View的合理性校驗(yàn)，include、merge等標(biāo)簽；
- 通過(guò) createViewFromTag 創(chuàng)建出 View 對(duì)象；
- 如果是 ViewGroup，則重復(fù)以上步驟；
- add View 到相應(yīng)的 parent 中；
4、createViewFromTag 方法解析
備注：以下源碼中有六條備注。
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) {
// ① 有mFactory2，則調(diào)用mFactory2的onCreateView方法
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
// ② 有mFactory，則調(diào)用mFactory的onCreateView方法
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
// ③ 有mPrivateFactory，則調(diào)用mPrivateFactory的onCreateView方法
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
if (view == null) {
// ④ 走到這步說(shuō)明三個(gè)Factory都沒(méi)有，則開(kāi)始自己創(chuàng)建View
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
// ⑤ 如果View的name中不包含 '.' 則說(shuō)明是系統(tǒng)控件，會(huì)在接下來(lái)的調(diào)用鏈在name前面加上 'android.view.'
view = onCreateView(parent, name, attrs);
} else {
// ⑥ 如果name中包含 '.' 則直接調(diào)用createView方法，onCreateView 后續(xù)也是調(diào)用了createView
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
throw e;
}
}
總結(jié)：
- createViewFromTag 方法比較簡(jiǎn)單，首先嘗試通過(guò) Factory 來(lái)創(chuàng)建View；
- 如果沒(méi)有 Factory 的話則通過(guò) createView 來(lái)創(chuàng)建View；
5、createView 方法解析
備注：以下源碼中有三條備注。
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
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 = 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);
}
}
// ① 反射獲取這個(gè)View的構(gòu)造器
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
// ② 緩存構(gòu)造器
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 lastContext = mConstructorArgs[0];
if (mConstructorArgs[0] == null) {
// Fill in the context if not already within inflation.
mConstructorArgs[0] = mContext;
}
Object[] args = mConstructorArgs;
args[1] = attrs;
// ③ 使用反射創(chuàng)建 View 對(duì)象，這樣一個(gè) View 就被創(chuàng)建出來(lái)了
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]));
}
mConstructorArgs[0] = lastContext;
return view;
} catch (ClassCastException e) {
}
}
總結(jié)：
- createView 方法也比較簡(jiǎn)單，通過(guò)反射來(lái)創(chuàng)建的 View 對(duì)象；
6、總結(jié)
通過(guò)本文我們學(xué)習(xí)到 LayoutInflater 創(chuàng)建 View的過(guò)程，也知道了 inflate 方法不同參數(shù)的意義，以及開(kāi)發(fā)中遇到的一些異常在源碼中的根源?？梢钥吹綇牟季种?inflate 一個(gè)個(gè)具體的 View 的過(guò)程其實(shí)也很簡(jiǎn)單：
- 通過(guò) XML 的 Pull 解析方式獲取 View 的標(biāo)簽；
- 通過(guò)標(biāo)簽以反射的方式來(lái)創(chuàng)建 View 對(duì)象；
- 如果是 ViewGroup 的話則會(huì)對(duì)子 View 遍歷并重復(fù)以上步驟，然后 add 到父 View 中；
- 與之相關(guān)的幾個(gè)方法：inflate ——》 rInflate ——》 createViewFromTag ——》 createView ；
參考
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