前言
最近又遇到RecyclerView的item最外層佈局參數失效的問題，之前都沒有去了解真正的原因，現在正好有空探尋一下這個問題，就從瞭解源碼開始吧。
View的inflate()
平時我經常使用View.inflate()，它是View的一個靜態方法，看到源碼：
public static View inflate(Context context, @LayoutRes int resource, ViewGroup root) {
LayoutInflater factory = LayoutInflater.from(context);
return factory.inflate(resource, root);
}其中調用了LayoutInflater的靜態方法from()：
/**
* 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;
}這裏根據上下文，從系統獲取了一個LayoutInfalter的實例對象。
LayoutInflater的infalte()
然後接着上面，用這個獲取的對象去調用LayoutInfalter的inflate()：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
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();
}
}其中可以看到獲取了一個xml解析器，獲取解析器的時候把要填充的xml的ID作爲了參數，然後又去調用了一個inflate()的重載：
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;
// 假設該方法返回的結果爲根View, 即方法參數root
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)) {
// 如果要填充的xml根佈局爲merge標籤
if (root == null || !attachToRoot) {
// 如果root爲空或者不需要將創建出來的View添加到root中直接會拋異常
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
// 創建並填充子View
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
// 根據xml，創建對應的View，叫temp
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
// 如果傳進來的root不爲空
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
// 那麼根據xml中配置的屬性，去獲取佈局參數
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
// 如果temp不需要被添加到root中，就給他設置佈局參數
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp against its context.
// 創建並填充子View
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不爲空，並且temp需要被添加到root中
// 那就把temp添加到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) {
// root爲空或者temp不需要被添加到root中
// 那麼該方法返回的View就是temp
// 否則返回root
result = temp;
}
}
} catch (XmlPullParserException e) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(parser.getPositionDescription()
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
return result;
}
}以上的這個方法信息量很大，我寫了很多中文註釋在代碼中。
關於方法返回值
我們可以知道，如果root爲空或者我當前要創建的View不需要添加到root中，那麼返回的就是我要創建的View，反之返回的是root。
外層佈局失效的問題
我寫的代碼是這樣的View view = View.inflate(context, R.layout.item_pic, null);，然後根據函數調用鏈：
// 第一步：
View view = View.inflate(context, R.layout.item_pic, null);
// 第二步
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
// 就變成了infalte(R.layout.item_pic, null, false);
return inflate(resource, root, root != null);
}
……
// 最後就進入了上面的方法
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot){
……
}因爲我最初傳入的root就是空啊，所以我最後只是創建了View，返回了View 但是沒有給他設置佈局參數，所以出現外層佈局失效的問題。
那我創建RecyclerView的item時，直接把RecyclerView作爲參數root傳進去可以嗎？答案是不行的，因爲這樣的話調用inflate()的時候是這樣的：inflate(resource, recyclerView, true)。然後你創建了View，並且把它添加到了RecyclerView。然而RecyclerView接收到你返回的View之後，還會自動把它添加到本身，這樣又會出現重複添加的問題：
java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child’s parent first
最佳解決方案就是直接使用LayoutInflater：View view = LayoutInflater.from(context).inflate(R.layout.item_pic, recyclerView, false);
這樣的話，回去看下inflate()中的邏輯，發現view會被設置佈局參數，並且不會直接被添加到recyclerView中。
LayoutInflater的createViewFromTag()
在inflate()中使用createViewFromTag()去創建xml對應的View，以下是主要代碼：
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
……
try {
View view;
// 可以設置LayoutInflater的Factory來自行解析View，默認這些Factory都爲空
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) {
// 沒有Factory的情況下，通過LayoutInflater的onCreateView()或者createView()去創建View
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
// 創建原生的View
// 會在onCreateView()的函數調用鏈中補充前綴"android.view."
// 最終也會調用createView()
view = onCreateView(parent, name, attrs);
} else {
// 創建自定義View
// 我們在xml裏寫的都是全類名，標籤中包含"."
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
}
……
}以上說到的Factory，我之前在TintContextWrapper強轉Activity失敗原因深度探索分析到過，當我們使用AppCompatActivity時，會設置Factory2爲AppCompatViewInflater，然後View都會走AppCompatViewInflater的createView()。
LayoutInflater的createView()：
/**
* Low-level function for instantiating a view by name. This attempts to
* instantiate a view class of the given <var>name</var> found in this
* LayoutInflater's ClassLoader.
*
* <p>
* There are two things that can happen in an error case: either the
* exception describing the error will be thrown, or a null will be
* returned. You must deal with both possibilities -- the former will happen
* the first time createView() is called for a class of a particular name,
* the latter every time there-after for that class name.
*
* @param name The full name of the class to be instantiated.
* @param attrs The XML attributes supplied for this instance.
*
* @return View The newly instantiated view, or null.
*/
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
// 從緩存中獲取View的構造函數
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) {
// 如果緩存中沒有需要的構造函數
// Class not found in the cache, see if it's real, and try to add it
// 如果prefix不爲空，那麼構造完整View路徑，並且加載該類
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);
}
}
// 從Class對象中獲取構造函數
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
// 將構造函數存入緩存中
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;
// 通過反射構造View
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// Use the same context when inflating ViewStub later.
// 如果是ViewStub，延遲加載
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
}
……
}對於創建View使用反射其實我比較疑惑，方法註釋說的是這個createView()是低版本上使用的方法，用來從類加載器中加載給定的View。後來我百度了下，可能是爲了實現全局換膚這種類似的操作。
LayoutInflater的rInflate()
通過createViewFromTag()創建了View之後，只是創建了根佈局的View，那其中的子View呢，回顧inflate()中的代碼，我們會發現rInflate()和rInflateChildren()這兩個方法，rInflateChildren()調用的還是rInflate()：
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
// 獲取樹的深度
final int depth = parser.getDepth();
int type;
// 深度優先遍歷
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();
// 解析標籤
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");
}
// 解析inclued標籤
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
// merge標籤只能是根佈局
// 到了這裏說明不是根佈局，拋出異常
throw new InflateException("<merge /> must be the root element");
} else {
// 創建View
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
// 創建佈局參數
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
// 遞歸地去遍歷子樹
rInflateChildren(parser, view, attrs, true);
// 將創建的View加入父佈局
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}佈局優化總結
從以上的源碼分析，我們其實可以得出如下結論：
用merge標籤作爲填充的xml的根佈局時，必須指定一個父元素並且設置attachToRoot屬性爲true。
我們通常使用ViewStub來做預加載處理，來改善頁面加載速度和提高流暢性。
include標籤用來複用佈局。
在分析源碼到用反射創建View的時候，我發現了一篇比較不錯的文章Android 中LayoutInflater（佈局加載器）系列博文說明。