Android LayoutInflater 源码解析

时间 2019-11-08

标签 android layoutinflater 源码解析栏目 Android 繁體版

原文原文链接

在上篇文章中咱们学习了setContentView的源码，还记得其中的LayoutInflater吗？本篇文章就来学习下LayoutInflater。node

@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();
    }
复制代码

备注：本文基于 Android 8.1.0。android

一、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.缓存

翻译过来就是：LayoutInflater 的做用就是将XML布局文件实例化为相应的 View 对象，须要经过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。bash

总共有三种方法来获取 LayoutInflater：微信

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：app

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) ；

二、inflate 方法解析

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。ide

备注：如下源码中有七条备注。布局

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 若是是Merge标签，则必须依附于一个RootView，不然抛出异常
                    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 根据节点名来建立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 若是设置的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 若是不是attachToRoot ，则对这个Tag和建立出来的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 若是Root不为null且是attachToRoot，则添加建立出来的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;
        }
    }
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备注：根据以上源码，咱们也能够分析出来 inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) 不一样参数值带来的影响：学习

若是root为null，attachToRoot将失去做用，设置任何值都没有意义；
若是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；
若是root不为null，attachToRoot设为false，则会将布局文件最外层的全部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参数的状况下，若是root不为null，attachToRoot参数默认为true；

三、rInflate 方法解析

以上代码中咱们还有两个方法没有分析：rInflate 和 rInflateChildren ；而 rInflateChildren 其实是调用了rInflate；ui

备注：如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 若是这里出现了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 同理若是这里出现了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若是当前View是ViewGroup（包裹了别的View）则在此处inflate其全部的子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若是inflate结束，则回调parent的onFinishInflate方法
            parent.onFinishInflate();
        }
    }
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总结：

首先进行View的合理性校验，include、merge等标签；
经过 createViewFromTag 建立出 View 对象；
若是是 ViewGroup，则重复以上步骤；
add View 到相应的 parent 中；

四、createViewFromTag 方法解析

备注：如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，则开始本身建立View
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    if (-1 == name.indexOf('.')) {
                        // ⑤ 若是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 若是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;
        } 
    }
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总结：

createViewFromTag 方法比较简单，首先尝试经过 Factory 来建立View；
若是没有 Factory 的话则经过 createView 来建立View；

五、createView 方法解析

备注：如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 使用反射建立 View 对象，这样一个 View 就被建立出来了 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) { } } 复制代码

总结：

createView 方法也比较简单，经过反射来建立的 View 对象；

六、总结

经过本文咱们学习到 LayoutInflater 建立 View的过程，也知道了 inflate 方法不一样参数的意义，以及开发中遇到的一些异常在源码中的根源。能够看到从布局中 inflate 一个个具体的 View 的过程其实也很简单：

经过 XML 的 Pull 解析方式获取 View 的标签；
经过标签以反射的方式来建立 View 对象；
若是是 ViewGroup 的话则会对子 View 遍历并重复以上步骤，而后 add 到父 View 中；
与之相关的几个方法：inflate ——》 rInflate ——》 createViewFromTag ——》 createView ；

参考

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