LayoutInflater源码分析

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前言

LayoutInflater对于做过Android UI开发的人来说再熟悉不过了。本文结合其源码进行分析。系作者原创，转载需注明出处。

LayoutInflater用法

LayoutInflater主要用于将layout xml实例化成对应的view，通过：

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Context#getSystemService(Context.LAYOUT_INFLATER_SERVICE)
// 或者
android.app.Activity#getLayoutInflater()

可以获取到绑定了context的LayoutInflater实例，我们用LayoutInflater.from(context)实际上也是通过getSystemService获取到的LayoutInflater实例。

LayoutInflater提供了限制inflate哪些View的接口：

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/**
 * Hook to allow clients of the LayoutInflater to restrict the set of Views that are allowed
 * to be inflated.
 *
 */
public interface Filter {
    /**
     * Hook to allow clients of the LayoutInflater to restrict the set of Views
     * that are allowed to be inflated.
     *
     * @param clazz The class object for the View that is about to be inflated
     *
     * @return True if this class is allowed to be inflated, or false otherwise
     */
    @SuppressWarnings("unchecked")
    boolean onLoadClass(Class clazz);
}

/**
 * Sets the {@link Filter} to by this LayoutInflater. If a view is attempted to be inflated
 * which is not allowed by the {@link Filter}, the {@link #inflate(int, ViewGroup)} call will
 * throw an {@link InflateException}. This filter will replace any previous filter set on this
 * LayoutInflater.
 *
 * @param filter The Filter which restricts the set of Views that are allowed to be inflated.
 *        This filter will replace any previous filter set on this LayoutInflater.
 */
public void setFilter(Filter filter) {
    mFilter = filter;
    if (filter != null) {
        mFilterMap = new HashMap<String, Boolean>();
    }
}

onLoadClass返回true表示allowed to be inflated，返回false的话该view在inflate的时候会抛出InflateException。
可以看到inflate方法是通过反射创建view实例的，同时会使用一个map缓存constructor。mFilterMap也会缓存每个name对应的view是否允许inflate。然后遇到viewstub的话会使用cloneInContext创建一个相同context的LayoutInflater传入给viewstub实例。

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/**
 * 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 {
    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);
                }
            }
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            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;

        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 (NoSuchMethodException e) {
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;

    } catch (ClassCastException e) {
        // If loaded class is not a View subclass
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } catch (ClassNotFoundException e) {
        // If loadClass fails, we should propagate the exception.
        throw e;
    } catch (Exception e) {
        final InflateException ie = new InflateException(
                attrs.getPositionDescription() + ": Error inflating class "
                        + (clazz == null ? "<unknown>" : clazz.getName()), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

/**
 * Throw an exception because the specified class is not allowed to be inflated.
 */
private void failNotAllowed(String name, String prefix, AttributeSet attrs) {
    throw new InflateException(attrs.getPositionDescription()
            + ": Class not allowed to be inflated "+ (prefix != null ? (prefix + name) : name));
}

同时，inflate的时候我们可以通过给LayoutInflater设置自己的factory来hook inflate过程，从而改变xml中的view对象或者其属性。例如，我们可以在遇到name为”EditText”的节点的时候new 一个Button实例返回，动态的替换UI组件。

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public interface Factory {
    /**
     * Hook you can supply that is called when inflating from a LayoutInflater.
     * You can use this to customize the tag names available in your XML
     * layout files.
     *
     * <p>
     * Note that it is good practice to prefix these custom names with your
     * package (i.e., com.coolcompany.apps) to avoid conflicts with system
     * names.
     *
     * @param name Tag name to be inflated.
     * @param context The context the view is being created in.
     * @param attrs Inflation attributes as specified in XML file.
     *
     * @return View Newly created view. Return null for the default
     *         behavior.
     */
    public View onCreateView(String name, Context context, AttributeSet attrs);
}

public interface Factory2 extends Factory {
    /**
     * Version of {@link #onCreateView(String, Context, AttributeSet)}
     * that also supplies the parent that the view created view will be
     * placed in.
     *
     * @param parent The parent that the created view will be placed
     * in; <em>note that this may be null</em>.
     * @param name Tag name to be inflated.
     * @param context The context the view is being created in.
     * @param attrs Inflation attributes as specified in XML file.
     *
     * @return View Newly created view. Return null for the default
     *         behavior.
     */
    public View onCreateView(View parent, String name, Context context, AttributeSet attrs);
}

/**
 * Attach a custom Factory interface for creating views while using
 * this LayoutInflater.  This must not be null, and can only be set once;
 * after setting, you can not change the factory.  This is
 * called on each element name as the xml is parsed. If the factory returns
 * a View, that is added to the hierarchy. If it returns null, the next
 * factory default {@link #onCreateView} method is called.
 *
 * <p>If you have an existing
 * LayoutInflater and want to add your own factory to it, use
 * {@link #cloneInContext} to clone the existing instance and then you
 * can use this function (once) on the returned new instance.  This will
 * merge your own factory with whatever factory the original instance is
 * using.
 */
public void setFactory(Factory factory) {
    if (mFactorySet) {
        throw new IllegalStateException("A factory has already been set on this LayoutInflater");
    }
    if (factory == null) {
        throw new NullPointerException("Given factory can not be null");
    }
    mFactorySet = true;
    if (mFactory == null) {
        mFactory = factory;
    } else {
        mFactory = new FactoryMerger(factory, null, mFactory, mFactory2);
    }
}

/**
 * Like {@link #setFactory}, but allows you to set a {@link Factory2}
 * interface.
 */
public void setFactory2(Factory2 factory) {
    if (mFactorySet) {
        throw new IllegalStateException("A factory has already been set on this LayoutInflater");
    }
    if (factory == null) {
        throw new NullPointerException("Given factory can not be null");
    }
    mFactorySet = true;
    if (mFactory == null) {
        mFactory = mFactory2 = factory;
    } else {
        mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
    }
}

在support v4包下有个LayoutInflaterCompat类替我们封装好了setFactory2方法，在LayoutInflater直接调用的基础上做了一些旧版本兼容：

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// from LayoutInflaterCompat
private static void forceSetFactory2(LayoutInflater inflater, Factory2 factory) {
    if (!sCheckedField) {
        try {
            sLayoutInflaterFactory2Field = LayoutInflater.class.getDeclaredField("mFactory2");
            sLayoutInflaterFactory2Field.setAccessible(true);
        } catch (NoSuchFieldException var4) {
            Log.e("LayoutInflaterCompatHC", "forceSetFactory2 Could not find field 'mFactory2' on class " + LayoutInflater.class.getName() + "; inflation may have unexpected results.", var4);
        }

        sCheckedField = true;
    }

    if (sLayoutInflaterFactory2Field != null) {
        try {
            sLayoutInflaterFactory2Field.set(inflater, factory);
        } catch (IllegalAccessException var3) {
            Log.e("LayoutInflaterCompatHC", "forceSetFactory2 could not set the Factory2 on LayoutInflater " + inflater + "; inflation may have unexpected results.", var3);
        }
    }

}

/** @deprecated */
@Deprecated
public static void setFactory(@NonNull LayoutInflater inflater, @NonNull LayoutInflaterFactory factory) {
    if (VERSION.SDK_INT >= 21) {
        inflater.setFactory2(factory != null ? new LayoutInflaterCompat.Factory2Wrapper(factory) : null);
    } else {
        Factory2 factory2 = factory != null ? new LayoutInflaterCompat.Factory2Wrapper(factory) : null;
        inflater.setFactory2(factory2);
        Factory f = inflater.getFactory();
        if (f instanceof Factory2) {
            forceSetFactory2(inflater, (Factory2)f);
        } else {
            forceSetFactory2(inflater, factory2);
        }
    }

}

public static void setFactory2(@NonNull LayoutInflater inflater, @NonNull Factory2 factory) {
    inflater.setFactory2(factory);
    if (VERSION.SDK_INT < 21) {
        Factory f = inflater.getFactory();
        if (f instanceof Factory2) {
            forceSetFactory2(inflater, (Factory2)f);
        } else {
            forceSetFactory2(inflater, factory);
        }
    }

}

在support v7包下的AppCompatActivity实际上也利用了LayoutInflaterCompat类。

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protected void onCreate(@Nullable Bundle savedInstanceState) {
    AppCompatDelegate delegate = this.getDelegate();
    delegate.installViewFactory();
    delegate.onCreate(savedInstanceState);
    if (delegate.applyDayNight() && this.mThemeId != 0) {
        if (VERSION.SDK_INT >= 23) {
            this.onApplyThemeResource(this.getTheme(), this.mThemeId, false);
        } else {
            this.setTheme(this.mThemeId);
        }
    }

    super.onCreate(savedInstanceState);
}

在AppCompatActivity onCreate的时候会调用AppCompatDelegateImpl的installViewFactory方法，给当前context的LayoutInflater赋上一个Factory2,也就是自身。

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public void installViewFactory() {
    LayoutInflater layoutInflater = LayoutInflater.from(this.mContext);
    if (layoutInflater.getFactory() == null) {
        LayoutInflaterCompat.setFactory2(layoutInflater, this);
    } else if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImpl)) {
        // 假如我们在这之前给LayoutInflater赋了一个factory2，那么AppCompatDelegateImpl就无法继续再赋一个factory2
        Log.i("AppCompatDelegate", "The Activity's LayoutInflater already has a Factory installed so we can not install AppCompat's");
    }

}

而AppCompatActivity自身的onCreateView又委托给了AppCompatViewInflater的createView，将我们的TextView等组件都替换成了对应的AppCompatTextView等组件。

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final View createView(View parent, String name, @NonNull Context context, @NonNull AttributeSet attrs, boolean inheritContext, boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
    Context originalContext = context;
    if (inheritContext && parent != null) {
        context = parent.getContext();
    }

    if (readAndroidTheme || readAppTheme) {
        context = themifyContext(context, attrs, readAndroidTheme, readAppTheme);
    }

    if (wrapContext) {
        context = TintContextWrapper.wrap(context);
    }

    View view = null;
    byte var12 = -1;
    switch(name.hashCode()) {
    case -1946472170:
        if (name.equals("RatingBar")) {
            var12 = 11;
        }
        break;
    case -1455429095:
        if (name.equals("CheckedTextView")) {
            var12 = 8;
        }
        break;
    case -1346021293:
        if (name.equals("MultiAutoCompleteTextView")) {
            var12 = 10;
        }
        break;
    case -938935918:
        if (name.equals("TextView")) {
            var12 = 0;
        }
        break;
    case -937446323:
        if (name.equals("ImageButton")) {
            var12 = 5;
        }
        break;
    case -658531749:
        if (name.equals("SeekBar")) {
            var12 = 12;
        }
        break;
    case -339785223:
        if (name.equals("Spinner")) {
            var12 = 4;
        }
        break;
    case 776382189:
        if (name.equals("RadioButton")) {
            var12 = 7;
        }
        break;
    case 1125864064:
        if (name.equals("ImageView")) {
            var12 = 1;
        }
        break;
    case 1413872058:
        if (name.equals("AutoCompleteTextView")) {
            var12 = 9;
        }
        break;
    case 1601505219:
        if (name.equals("CheckBox")) {
            var12 = 6;
        }
        break;
    case 1666676343:
        if (name.equals("EditText")) {
            var12 = 3;
        }
        break;
    case 2001146706:
        if (name.equals("Button")) {
            var12 = 2;
        }
    }

    switch(var12) {
    case 0:
        view = this.createTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 1:
        view = this.createImageView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 2:
        view = this.createButton(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 3:
        view = this.createEditText(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 4:
        view = this.createSpinner(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 5:
        view = this.createImageButton(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 6:
        view = this.createCheckBox(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 7:
        view = this.createRadioButton(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 8:
        view = this.createCheckedTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 9:
        view = this.createAutoCompleteTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 10:
        view = this.createMultiAutoCompleteTextView(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 11:
        view = this.createRatingBar(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    case 12:
        view = this.createSeekBar(context, attrs);
        this.verifyNotNull((View)view, name);
        break;
    default:
        view = this.createView(context, name, attrs);
    }

    if (view == null && originalContext != context) {
        view = this.createViewFromTag(context, name, attrs);
    }

    if (view != null) {
        this.checkOnClickListener((View)view, attrs);
    }

    return (View)view;
}

@NonNull
protected AppCompatTextView createTextView(Context context, AttributeSet attrs) {
    return new AppCompatTextView(context, attrs);
}
.......

inflate方法解析

inflate我们常用的主要有2个版本：

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/**
* Inflate a new view hierarchy from the specified xml resource. Throws
* {@link InflateException} if there is an error.
* 
* @param resource ID for an XML layout resource to load (e.g.,
*        <code>R.layout.main_page</code>)
* @param root Optional view to be the parent of the generated hierarchy.
* @return The root View of the inflated hierarchy. If root was supplied,
*         this is the root View; otherwise it is the root of the inflated
*         XML file.
*/
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
    return inflate(resource, root, root != null);
}

/**
* Inflate a new view hierarchy from the specified xml resource. Throws
* {@link InflateException} if there is an error.
* 
* @param resource ID for an XML layout resource to load (e.g.,
*        <code>R.layout.main_page</code>)
* @param root Optional view to be the parent of the generated hierarchy (if
*        <em>attachToRoot</em> is true), or else simply an object that
*        provides a set of LayoutParams values for root of the returned
*        hierarchy (if <em>attachToRoot</em> is false.)
* @param attachToRoot Whether the inflated hierarchy should be attached to
*        the root parameter? If false, root is only used to create the
*        correct subclass of LayoutParams for the root view in the XML.
* @return The root View of the inflated hierarchy. If root was supplied and
*         attachToRoot is true, this is root; otherwise it is the root of
*         the inflated XML file.
*/
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();
    }
}

可知当使用两个参数的inflate的时候，root为null，attachToRoot就为false，root不为null，attachToRoot就为true，从attachToRoot命名也能知道这是判断是否将inflate得到的view添加为root的子view的依据之一。再来看inflate具体是怎样parse xml的：

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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;
    }
}

首先 <merge>标签必须要有一个不为null的root和值为true的attachToRoot，否则抛出异常：InflateException。如果满足条件则调用rInflate进行渲染：

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/**
 * Recursive method used to descend down the xml hierarchy and instantiate
 * views, instantiate their children, and then call onFinishInflate().
 * <p>
 * <strong>Note:</strong> Default visibility so the BridgeInflater can
 * override it.
 */
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) {
                throw new InflateException("<include /> cannot be the root element");
            }
            parseInclude(parser, context, parent, attrs);
        } else if (TAG_MERGE.equals(name)) {
            throw new InflateException("<merge /> must be the root element");
        } else {
            final View view = createViewFromTag(parent, name, context, attrs);
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            rInflateChildren(parser, view, attrs, true);
            viewGroup.addView(view, params);
        }
    }

    if (pendingRequestFocus) {
        parent.restoreDefaultFocus();
    }

    if (finishInflate) {
        parent.onFinishInflate();
    }
}

可以看到rInflate会递归地对<merge>标签下的节点实例化成view，并根据attrs生成LayoutParams，添加到root上。这也是为什么<merge>能降低view层级的原因，因为下面的标签是直接add到root上的，不必多一层layout。同时需要注意到<merge>解析的时候调rInflate传入的finishInflate为false，在rInflate中又调用了rInflateChildren, rInflateChildren又调了一次rInflate，只不过这次传入的finishInflate为true。这个设计还是很巧妙的，当<merge>下的节点全部inflate完成之后，每个view都回调了onFinishInflate，不过由于root还没inflate完成，所以root的onFinishInflate还没调用。同时也可以注意到，当parse到<requestFocus>标签时，会调用父容器view的restoreDefaultFocus()，进而调用requestFocus()获取焦点。而且不一样的是，<include>标签必须不在最外层，<merge>标签必须在最外层。

接着往下看，假如不是<merge>标签，就会给最外层标签生成对应的viewgroup实例temp，如果root不为null的话，会利用root生成一个layoutparams，然后看看是否attachToRoot, 如果是，那就root.addView(temp, params);；否，那就temp.setLayoutParams(params);。假如root为null的话，可以知道生成的temp没有设置任何layoutparams，所以这时候我们最外层的xml属性部分会失效，比如宽高，gravity。

至此，LayoutInflater的源码差不多就分析到这里了。

what about ViewStub

既然提到了ViewStub那么就顺便也来看看ViewStub的工作原理：

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/**
 * Inflates the layout resource identified by {@link #getLayoutResource()}
 * and replaces this StubbedView in its parent by the inflated layout resource.
 *
 * @return The inflated layout resource.
 *
 */
public View inflate() {
    final ViewParent viewParent = getParent();

    if (viewParent != null && viewParent instanceof ViewGroup) {
        if (mLayoutResource != 0) {
            final ViewGroup parent = (ViewGroup) viewParent;
            final View view = inflateViewNoAdd(parent);
            replaceSelfWithView(view, parent);

            mInflatedViewRef = new WeakReference<>(view);
            if (mInflateListener != null) {
                mInflateListener.onInflate(this, view);
            }

            return view;
        } else {
            throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
        }
    } else {
        throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
    }
}

private View inflateViewNoAdd(ViewGroup parent) {
    final LayoutInflater factory;
    if (mInflater != null) {
        factory = mInflater;
    } else {
        factory = LayoutInflater.from(mContext);
    }
    final View view = factory.inflate(mLayoutResource, parent, false);

    if (mInflatedId != NO_ID) {
        view.setId(mInflatedId);
    }
    return view;
}

private void replaceSelfWithView(View view, ViewGroup parent) {
    final int index = parent.indexOfChild(this);
    parent.removeViewInLayout(this);

    final ViewGroup.LayoutParams layoutParams = getLayoutParams();
    if (layoutParams != null) {
        parent.addView(view, index, layoutParams);
    } else {
        parent.addView(view, index);
    }
}

可以看到ViewStub首先拿到了自己在parent view里的index，然后remove自己，将inflate出来的view在原index处插入，layoutparams都是根据view的属性生成的，和viewStub无关。

完结，撒花🎉🎉🎉