深入理解LayoutInflater

探究LayoutInflater的实现类##

在android程序员写代码的时候，很多时候会用到LayoutInflater来加载指定的布局：

LayoutInflater inflater = (LayoutInflater)context.getSystemService       (Context.LAYOUT_INFLATER_SERVICE);// 或者通过from(Context context)获取实例LayoutInflater inflater = LayoutInflater.from(context);inflater.inflate(int resource, ViewGroup root);

其实LayoutInflater是一个抽象类：

public abstract class LayoutInflater {    ....}

既然是抽象类，那么一定有它的实现，我们知道系统会在ContextImpl中将所有的系统service，注入到ServiceFetcher中，关于”LAYOUT_INFLATER_SERVICE”有如下实现：

registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() {                public Object createService(ContextImpl ctx) {                    return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext()); }});

可以看到这里实际上是调用了PolicyManager.makeNewLayoutInflater

// Policy的实现类private static final String POLICY_IMPL_CLASS_NAME =       "com.android.internal.policy.impl.Policy";    private static final IPolicy sPolicy;    static {        try {            // 通过反射构造Policy对象            Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);            sPolicy = (IPolicy)policyClass.newInstance();        }         ....}// 该方法会返回一个PhoneWindow对象public static Window makeNewWindow(Context context) {      return sPolicy.makeNewWindow(context);}public static LayoutInflater makeNewLayoutInflater(Context context) {        return sPolicy.makeNewLayoutInflater(context);}

可以看到上面的PolicyManager中的所有操作实际上是一个代理，具体的都是交给Policy类处理，Policy实现了Ipolicy接口

public class Policy implements IPolicy {    private static final String TAG = "PhonePolicy";    private static final String[] preload_classes = {       "com.android.internal.policy.impl.PhoneLayoutInflater",        "com.android.internal.policy.impl.PhoneWindow",        "com.android.internal.policy.impl.PhoneWindow$1",        "com.android.internal.policy.impl.PhoneWindow$DialogMenuCallback",        "com.android.internal.policy.impl.PhoneWindow$DecorView",        "com.android.internal.policy.impl.PhoneWindow$PanelFeatureState",        "com.android.internal.policy.impl.PhoneWindow$PanelFeatureState$SavedState",   };    static {        // For performance reasons, preload some policy specific classes when        // the policy gets loaded.        for (String s : preload_classes) {           try {                Class.forName(s);            } catch (ClassNotFoundException ex) {                Log.e(TAG, "Could not preload class for phone policy: " + s);            }        }    }    // 创建PhoneWindow，也就是Activity中window的具体实现类    public Window makeNewWindow(Context context) {        return new PhoneWindow(context);    }    //  可以看到LayoutInflater的具体实现类就是PhoneLayoutInflater    public LayoutInflater makeNewLayoutInflater(Context context) {        return new PhoneLayoutInflater(context);    }    public WindowManagerPolicy makeNewWindowManager() {        return new PhoneWindowManager();    }    public FallbackEventHandler makeNewFallbackEventHandler(Context context) {       return new PhoneFallbackEventHandler(context);    }}

此时，我们知道LayoutInflater的实现类其实就是PhoneLayoutInflater，下面我们看看PhoneLayoutInflater的源码

public class PhoneLayoutInflater extends LayoutInflater {    private static final String[] sClassPrefixList = {        "android.widget.",        "android.webkit.",        "android.app."    };    ....    @Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {        for (String prefix : sClassPrefixList) {            try {                // 核心代码就是调用LayoutInflater的createView方法，根据传入的控件名称name以及sClassPrefixList的构造对应的控件                // 比如name是Button，则View就是android.widget.Button                View view = createView(name, prefix, attrs);                if (view != null) {                    return view;                }            } catch (ClassNotFoundException e) {            }        }        return super.onCreateView(name, attrs);    }   ....}

具体看下LayoutInflater#createView方法：

// 根据完整路径的类名根据反射构造对应的控件对象    public final View createView(String name, String prefix, AttributeSet attrs)            throws ClassNotFoundException, InflateException {        // 从缓存中获取当前控件的构造方法        Constructor<? extends View> constructor = sConstructorMap.get(name);        Class<? extends View> clazz = null;        try {            // 如果缓存中没有,则获取当前控件全类名对应的Class,并且缓存其构造方法到sConstructorMap集合中            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 {                ......            }            Object[] args = mConstructorArgs;            args[1] = attrs;            // 通过反射构造当前view对象            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]));            }            return view;        } catch (Exception e) {            ....        } finally {            Trace.traceEnd(Trace.TRACE_TAG_VIEW);        }}

LayoutInflater#createView方法比较简单，主要做了下面两件事：
1. 从sConstructorMap集合中获取当前View对应的构造方法，如果没有则根据当前全类名创建构造方法，并且存入sConstructorMap缓存中。
2. 根据构造方法，创建对应的View对象

探究LayoutInflater.inflate流程

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {        final Resources res = getContext().getResources();        ....        // 通过传递的布局id，创建一个XmlResourceParser对象        final XmlResourceParser parser = res.getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }}   /**     *     * @param parser xml解析器     * @param root   需要解析布局的父视图     * @param attachToRoot  是否将解析的视图添加到父视图     * @return     */    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {        synchronized (mConstructorArgs) {            final Context inflaterContext = mContext;            final AttributeSet attrs = Xml.asAttributeSet(parser);            Context lastContext = (Context) mConstructorArgs[0];            // Context对象            mConstructorArgs[0] = inflaterContext;            // 存储当前父视图            View result = root;            try {                ......                final String name = parser.getName();                // 1.解析merge标签                if (TAG_MERGE.equals(name)) {                    if (root == null || !attachToRoot) {                        throw new InflateException(" can be used only with a valid "                                + "ViewGroup root and attachToRoot=true");                    }                    rInflate(parser, root, inflaterContext, attrs, false);                } else {                    // 2.通过xml的tag解析layout的根视图,比如LinearLayout                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);                    ViewGroup.LayoutParams params = null;                    if (root != null) {                        // 生成布局参数                        params = root.generateLayoutParams(attrs);                        // 3. 如果attachToRoot是false,表示不添加当前视图到父视图中,那么将params设置到自己的布局参数中                        if (!attachToRoot) {                            temp.setLayoutParams(params);                        }                    }                    // 4. 解析temp视图中的所有子view                    rInflateChildren(parser, temp, attrs, true);                    // 如果root不是null,并且attachToRoot是true,那么将temp添加到父视图中,并设置对应的布局参数                    if (root != null && attachToRoot) {                        root.addView(temp, params);                    }                    // 如果root是null,并且attachToRoot是false,那么返回的结果就是temp                    if (root == null || !attachToRoot) {                        result = temp;                    }                }            }            ....            return result;}

上述inflate方法主要做了下面的操作：
1. 单独解析merge标签，rInflate会将merge标签下的所有子View直接添加到根标签中
2. 通过createViewFromTag方法解析普通元素
3. 根据root和attachToRoot的状态，决定是否添加当前View对象到父视图中
4. 解析temp视图中的所有子view

createViewFromTag方法解析普通元素

可以看到，通过inflate加载视图中，解析单个元素的createViewFromTag是很常用的，下面先看看createViewFromTag方法：

private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {        return createViewFromTag(parent, name, context, attrs, false);}View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,            boolean ignoreThemeAttr) {        if (name.equals("view")) {            name = attrs.getAttributeValue(null, "class");        }        try {            View view;            // 用户可以通过设置LayoutInflater的factory自行解析,如果没有设置则默认为null,所以这可以忽略这段            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) {                final Object lastContext = mConstructorArgs[0];                mConstructorArgs[0] = context;                try {                    if (-1 == name.indexOf('.')) {                        // 这里是android内置的View控件,由于android自带的View控件,我们在使用的时候不需要全类名,所以这里是-1                        view = onCreateView(parent, name, attrs);                    } else {                        // 自定义View控件的解析,自定义View必须写View的完整类名，比如""                        view = createView(name, null, attrs);                    }                } finally {                    mConstructorArgs[0] = lastContext;                }            }            return view;        }        //省略catch代码}

我们知道对于系统自带的View会走到onCreateView方法创建，前面的分析已经知道当我们使用LayoutInflater的时候，其实是使用其实现类PhoneLayoutInflater，它复写了onCreateView方法，在该方法里同样会通过createView这样的方法创建对应的View对象，并且传入”android.widget.”这样的包名，这就是为什么我们使用系统自带的View控件时候，不需要写全类名的原因。

rInflateChildren方法解析所有子元素

上面的分析，我们已经知道在LayoutInflater#inflate方法中，当解析完根视图以后，会通过rInflateChildren解析当前根视图下的所有子视图

final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,            boolean finishInflate) throws XmlPullParserException, IOException {        rInflate(parser, parent, parent.getContext(), attrs, finishInflate);}void rInflate(XmlPullParser parser, View parent, Context context,            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {        final int depth = parser.getDepth();        int type;        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)) {                parseRequestFocus(parser, parent);            } else if (TAG_TAG.equals(name)) {                parseViewTag(parser, parent, attrs);            } else if (TAG_INCLUDE.equals(name)) { // 解析include标签                if (parser.getDepth() == 0) {                    throw new InflateException(" cannot be the root element");                }                parseInclude(parser, context, parent, attrs);            } else if (TAG_MERGE.equals(name)) {                throw new InflateException(" 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);                // 递归调用进行解析,并且将解析出的View添加到其父视图中                rInflateChildren(parser, view, attrs, true);                viewGroup.addView(view, params);            }        }        if (finishInflate) {            parent.onFinishInflate();        }}

可以看到rInflate中，每次解析到一个View元素就会递归调用，知道该路径下的最后一个元素，然后在回朔回来将每个View元素添加到他们对应的parent中
通过rInflate解析完成以后，整棵View结构树就构建完成了。

关于LayoutInflater加载布局的解析过程到这里就完毕了。

探究LayoutInflater的实现类##

探究LayoutInflater.inflate流程

createViewFromTag方法解析普通元素

rInflateChildren方法解析所有子元素

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