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从xml中加载一个View,一般通过以下两个方法：
View#inflate(Context context, @LayoutRes int resource, ViewGroup root)方法和LayoutInflater#inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)方法 -
从
View#inflate方法源码可以看到，它最终也是调用LayoutInflater#inflate方法
<!----------View----------->
public static View inflate(Context context, @LayoutRes int resource, ViewGroup root) {
LayoutInflater factory = LayoutInflater.from(context);
return factory.inflate(resource, root);
}
- 跟着进入
LayoutInflater#inflate方法源码
<!----------LayoutInflater------------>
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
- LayoutInflater两个参数的inflate方法最终又调用了三个参数的inflate方法
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
- 三个参数的方法最终又调用了它的重载方法，真正的返回了View
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 (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) {
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;
}
}
- 为了方便分析取出关键代码如下
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
View result = root;
final String name = parser.getName();
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 {
// 获取xml中的根view
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
// 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)
//如果ViewGroup root不为null并且attachToRoot == false,为xml获取的根view temp设置 ViewGroup.LayoutParams
temp.setLayoutParams(params);
}
}
// 将temp下所有的子view添加到temp中
rInflateChildren(parser, temp, attrs, true);
//如果ViewGroup root不为null并且attachToRoot == true，将temp添加到root中,并设置 ViewGroup.LayoutParams
if (root != null && attachToRoot) {
root.addView(temp, params);
}
// 如果传进来的ViewGroup root为null 或者attachToRoot == false，返回xml中的根view temp，其它情况返回传进来的ViewGroup root
if (root == null || !attachToRoot) {
result = temp;
}
}
return result;
}
- 从最后的方法中可以看到，关键的点在于传入的ViewGroup root的值和boolean attachToRoot的值，这两个值得取值不同，最终影响返回的是传入的ViewGroup root还是获取xml中的根view temp，并且影响是否为temp设置了ViewGroup.LayoutParams
-
ViewGroup root == null && attachToRoot == false====> 返回获取xml中的根view temp，同时temp并没有设置了ViewGroup.LayoutParams（此时如果获取LayoutParams可能为空） -
ViewGroup root == null && attachToRoot == true====> 返回获取xml中的根view temp，同时temp并没有设置了ViewGroup.LayoutParams（此时如果获取LayoutParams可能为空） -
ViewGroup root != null && attachToRoot == false====>返回获取xml中的根view temp,同时设置了ViewGroup.LayoutParams -
ViewGroup root != null && attachToRoot == true====> 返回ViewGroup root，同时为获取到xml中的根view temp设置了ViewGroup.LayoutParams,并添加到ViewGroup root
- 现在再来看
View#inflate(Context context, @LayoutRes int resource, ViewGroup root)方法,它实际上调用了inflate(resource, root, root != null)，也就是说有两种情况：
- 一种是
ViewGroup root == null && attachToRoot == false，此时需要注意的是返回的是xml布局的根View，并且并未为该根View设置ViewGroup.LayoutParams，在这种情况需要获取view的LayoutParams进行操作的需要特别注意 - 另外一种是ViewGroup root != null && attachToRoot == true,此时返回的是传入的ViewGroup root，同时为获取到xml中的根view temp设置了ViewGroup.LayoutParams,并添加到ViewGroup root，这种情况需要注意的是xml中的布局已经被添加到ViewGroup root中，如果需要添加到另外的地方，这种方法是不可行的
- 也就是说对于
View#inflate方法，想要满足ViewGroup root != null && attachToRoot == false是无法满足的。而如果需要使用xml中的根view的ViewGroup.LayoutParams，首先需要满足ViewGroup root不为空(当然在onLayout之后使用是可以的，此时已经有parent了)，另外对于ReclclerView/ListView来说，它会自己在合适的时机将child添加进来，所以attachToRoot必须需要false，否则在渲染View的时候就会首先添加到ViewGroup root中，导致重复添加到parent中报错，因此View#inflate方法是无法满足的，需要使用LayoutInflater#inflate(resource,root,false)方法
-
对于
LayoutInflater#inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot)方法，同样的也会根据传入的值得不同得到不同的结果，在选择使用哪种方法获取View的时候就需要考虑对于接下来要对View进行的操作是否有影响 -
扩展
在学习Fragment的时候,下面的写法应该是熟悉到不能再熟悉了
@Nullable
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
View view = inflater.inflate(layoutRes, container, false);
return view ;
}
-
View view = inflater.inflate(layoutRes, container, false);一直是我们的固定写法，我想一定有同学会跟我一样觉得为什么一定要false，改成true可不可以
public class TestViewInflaterFragment extends Fragment {
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
return inflater.inflate(R.layout.activity_main, container, true);
}
}
- 当把它添加到activity中的时候
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
supportFragmentManager.beginTransaction().add(R.id.fl_container, TestViewInflaterFragment()).commit()
}
}
- 下面的错误一定会教你老老实实做人，大家都是这样说用false是有道理的，但是我们除了记得需要用false，还是需要知道为什么一定要用false，而用true就不行
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.demo/com.test.demo.MainActivity}: java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.
at android.view.ViewGroup.addViewInner(ViewGroup.java:4915)
at android.view.ViewGroup.addView(ViewGroup.java:4746)
at android.view.ViewGroup.addView(ViewGroup.java:4686)
at android.view.ViewGroup.addView(ViewGroup.java:4659)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1425)
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809)
at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:799)
at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2580)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367)
at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2229)
at android.support.v4.app.FragmentManagerImpl.dispatchStateChange(FragmentManager.java:3221)
at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:3171)
at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:192)
at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:560)
at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:177)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1333)
at android.app.Activity.performStart(Activity.java:6992)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2780)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
-
仔细看一看错误日志
The specified child already has a parent. You must call removeView() on the child's parent first.,看这句结合前面说过的内容，我想大家肯定已经知道为什么了，当我们写成true的时候，会将xml创建的root view添加到container，然后推测Fragment被加载的时候在某个地方又将xml创建的root view再添加到一个ViewGroup中，所以会导致这个错误，继续看一看推测是否正确 -
先看
Fragment#onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState)在哪里被调用
View performCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
if (mChildFragmentManager != null) {
mChildFragmentManager.noteStateNotSaved();
}
mPerformedCreateView = true;
return onCreateView(inflater, container, savedInstanceState);
}
- 继续看
Fragment#performCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)的调用
//-----------FramgnetManager---------------
void moveToState(Fragment f, int newState, int transit, int transitionStyle,
boolean keepActive) {
//省略部分代码
```
switch (f.mState) {
//省略部分代码
```
case Fragment.CREATED:
// This is outside the if statement below on purpose; we want this to run
// even if we do a moveToState from CREATED => *, CREATED => CREATED, and
// * => CREATED as part of the case fallthrough above.
ensureInflatedFragmentView(f);
if (newState > Fragment.CREATED) {
if (DEBUG) Log.v(TAG, "moveto ACTIVITY_CREATED: " + f);
if (!f.mFromLayout) {
ViewGroup container = null;
if (f.mContainerId != 0) {
if (f.mContainerId == View.NO_ID) {
throwException(new IllegalArgumentException(
"Cannot create fragment "
+ f
+ " for a container view with no id"));
}
//首先根据我们传入的mContainerId（对于本案例来说就是R.id.fl_container）找到container
container = (ViewGroup) mContainer.onFindViewById(f.mContainerId);
if (container == null && !f.mRestored) {
String resName;
try {
resName = f.getResources().getResourceName(f.mContainerId);
} catch (NotFoundException e) {
resName = "unknown";
}
throwException(new IllegalArgumentException(
"No view found for id 0x"
+ Integer.toHexString(f.mContainerId) + " ("
+ resName
+ ") for fragment " + f));
}
}
f.mContainer = container;
//这个f.mView就是我们自己在onCreateView方法中创建返回的View
f.mView = f.performCreateView(f.performGetLayoutInflater(
f.mSavedFragmentState), container, f.mSavedFragmentState);
if (f.mView != null) {
f.mInnerView = f.mView;
f.mView.setSaveFromParentEnabled(false);
if (container != null) {
//关键地方，在这里会将xml创建的view添加到container
container.addView(f.mView);
}
//省略部分代码
```
}
//省略部分代码
```
}
}
-
方法太长，为了便于查看省略了部分代码，看注释已经说明了前面的推测是正确的，当改为true的时候，在创建xml view的时候会将view add到传入的parent（container）中，然后将Fragment加载进来的时候，FragmentManager会再一次将创建的xml view添加到container（也就是我们指定的container id指向的ViewGroup）中，所以导致了重复添加一个view到ViewGroup中的错误。
-
另外再说一个查看源码的小技巧，首先源码太多太复杂，一头扎进去可能就出不来了。首先需要抱着一个目的去查看源码，一般查看源码是为了验证一个东西或者学习源码是怎么样实现某种效果或者某个功能的，这就是这次查看源码的目的，主线。像前面就是为了验证是不是Fragment加载的时候会将创建的View添加到某个ViewGroup中，然后根据方法的调用去找寻可能是我们目的的代码，有时候有些方法的调用是很复杂的，有些方法也特别长，稍微不注意可能就错过了想要的内容。比如之前的
Fragment#performCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)还有另外一个地方呗调用，可能我们会进来了再一直深入，然后很容易就被绕晕了
void ensureInflatedFragmentView(Fragment f) {
if (f.mFromLayout && !f.mPerformedCreateView) {
f.mView = f.performCreateView(f.performGetLayoutInflater(
f.mSavedFragmentState), null, f.mSavedFragmentState);
if (f.mView != null) {
f.mInnerView = f.mView;
f.mView.setSaveFromParentEnabled(false);
if (f.mHidden) f.mView.setVisibility(View.GONE);
f.onViewCreated(f.mView, f.mSavedFragmentState);
dispatchOnFragmentViewCreated(f, f.mView, f.mSavedFragmentState, false);
} else {
f.mInnerView = null;
}
}
}
- 另外
FragmentManager#moveToState(Fragment f, int newState, int transit, int transitionStyle, boolean keepActive)也非常长，怎么样才能提高找到线索的可能性呢? 还记的我们的目的吗?验证是不是Fragment加载的时候会将创建的View添加到某个ViewGroup,添加到ViewGroup，第一时间应该想到ViewGroup#addView方法，然后在对应的方法里面搜索一下addView，如果找到了有，再前后代码查看一下是不是我们的目的，一步一步排查下去最终解决我们的问题。 - 这只是一个查看源码的小技巧，可能并不能每次都能解决所有的问题。但是可以帮我们少走一点弯路。毕竟一入源码深似海。而且最好不要抱着把源码里面的所有东西都看懂，每一个变量代表什么都去纠结（当然如果有这个能力的话也是可以的）。应该抱着学习和解决问题的目的，有针对性的去看源码，沿着一条主线去搞懂并解决我们遇到的问题。
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