Android LayoutInflater 源码解析

备注：本文基于 Android 8.1.0。

1、LayoutInflater 简介
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.
翻译过来就是：LayoutInflater 的作用就是将XML布局文件实例化为相应的 View 对象，需要通过Activity.getLayoutInflater() 或 Context.getSystemService(Class) 来获取与当前Context已经关联且正确配置的标准LayoutInflater。

总共有三种方法来获取 LayoutInflater：

Activity.getLayoutInflater()；
Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ;
LayoutInflater.from(context);
事实上，这三种方法之间是有关联的：

Activity.getLayoutInflater() 最终会调用到 PhoneWindow 的构造方法，实际上最终调用的就是方法三；
而方法三最终会调用到方法二 Context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ；
2、inflate 方法解析
 

image
LayoutInflater 的 inflate 方法总共有四个，属于重载的关系，最终都会调用到 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 方法。

备注：以下源码中有七条备注。

    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 寻找布局的根节点，判断布局的合理性
                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标签，则必须依附于一个RootView，否则抛出异常
                    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 根据节点名来创建View对象 
                    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 如果设置的Root不为null，则根据当前标签的参数生成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 ，则对这个Tag和创建出来的View设置LayoutParams；注意：此处的params只有当被添加到一个Viewz中的时候才会生效；
                            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不为null且是attachToRoot，则添加创建出来的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;
        }
    }

备注：根据以上源码，我们也可以分析出来 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 不同参数值带来的影响：

如果root为null，attachToRoot将失去作用，设置任何值都没有意义；
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；
如果root不为null，attachToRoot设为false，则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；
在不设置attachToRoot参数的情况下，如果root不为null，attachToRoot参数默认为true；
3、rInflate 方法解析
以上代码中我们还有两个方法没有分析：rInflate 和 rInflateChildren ；而 rInflateChildren 实际上是调用了rInflate；

备注：以下源码中有六条备注。

  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 如果这里出现了include标签，就会抛出异常
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, context, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {                
                // ② 同理如果这里出现了merge标签，也会抛出异常
                throw new InflateException("<merge /> must be the root element");
            } else {
                // ③ 最重要的方法在这里，createViewFromTag
                final View view = createViewFromTag(parent, name, context, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                // ④如果当前View是ViewGroup（包裹了别的View）则在此处inflate其所有的子View
                rInflateChildren(parser, view, attrs, true);
                // ⑤添加inflate出来的view到parent中
                viewGroup.addView(view, params);
            }
        }
        if (pendingRequestFocus) {
            parent.restoreDefaultFocus();
        }
        if (finishInflate) {
            // ⑥如果inflate结束，则回调parent的onFinishInflate方法
            parent.onFinishInflate();
        }
    }

总结：

首先进行View的合理性校验，include、merge等标签；
通过 createViewFromTag 创建出 View 对象；
如果是 ViewGroup，则重复以上步骤；
add View 到相应的 parent 中；
4、createViewFromTag 方法解析
备注：以下源码中有六条备注。

  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，则调用mFactory2的onCreateView方法
                view = mFactory2.onCreateView(parent, name, context, attrs);
            } else if (mFactory != null) {
                // ② 有mFactory，则调用mFactory的onCreateView方法
                view = mFactory.onCreateView(name, context, attrs);
            } else {
                view = null;
            }
            if (view == null && mPrivateFactory != null) {
                // ③ 有mPrivateFactory，则调用mPrivateFactory的onCreateView方法
                view = mPrivateFactory.onCreateView(parent, name, context, attrs);
            }
            if (view == null) {
                // ④ 走到这步说明三个Factory都没有，则开始自己创建View
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    if (-1 == name.indexOf('.')) {
                        // ⑤ 如果View的name中不包含 '.' 则说明是系统控件，会在接下来的调用链在name前面加上 'android.view.'
                        view = onCreateView(parent, name, attrs);
                    } else {
                        // ⑥ 如果name中包含 '.' 则直接调用createView方法，onCreateView 后续也是调用了createView
                        view = createView(name, null, attrs);
                    }
                } finally {
                    mConstructorArgs[0] = lastContext;
                }
            }
            return view;
        } catch (InflateException e) {
            throw e;
        } 
    }

总结：

createViewFromTag 方法比较简单，首先尝试通过 Factory 来创建View；
如果没有 Factory 的话则通过 createView 来创建View；
5、createView 方法解析
备注：以下源码中有三条备注。

 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 反射获取这个View的构造器
                constructor = clazz.getConstructor(mConstructorSignature);
                constructor.setAccessible(true);
                // ② 缓存构造器
                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 使用反射创建 View 对象，这样一个 View 就被创建出来了
            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) {
        } 
    }

总结：

createView 方法也比较简单，通过反射来创建的 View 对象；
6、总结
通过本文我们学习到 LayoutInflater 创建 View的过程，也知道了 inflate 方法不同参数的意义，以及开发中遇到的一些异常在源码中的根源。可以看到从布局中 inflate 一个个具体的 View 的过程其实也很简单：

通过 XML 的 Pull 解析方式获取 View 的标签；
通过标签以反射的方式来创建 View 对象；
如果是 ViewGroup 的话则会对子 View 遍历并重复以上步骤，然后 add 到父 View 中；
与之相关的几个方法：inflate ——》 rInflate ——》 createViewFromTag ——》 createView ；
参考
《Android LayoutInflater原理分析，带你一步步深入了解View(一)》
LayoutInflater