LayoutInflate源码分析之如何解析视图树

前言：
在前面介绍完LayooutInflater的inflate方法后，这次我们着重来分析下inflate是如何去加载布局的。
我们之前讲过系统获取LayoutInflater的时候，会通过ServiceFetcher的createService方法进行注入到容器
ReceiverRestrictedContext类地址：
https://android.googlesource.com/platform/frameworks/base/+/0e2d281/core/java/android/app/ContextImpl.java

注入代码如下：

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

这里调用了PolicyManager的makeNewLayoutInflater方法：
继续跟踪PolicyManager类：

public final class PolicyManager {    private static final String POLICY_IMPL_CLASS_NAME =        "com.android.internal.policy.impl.Policy";    private static final IPolicy sPolicy;    static {        //通过反射去生成Policy对象        try {            Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);            sPolicy = (IPolicy)policyClass.newInstance();        }....    }    // Cannot instantiate this class    private PolicyManager() {}    // The static methods to spawn new policy-specific objects    public static Window makeNewWindow(Context context) {        return sPolicy.makeNewWindow(context);    }//创建LayoutInflater    public static LayoutInflater makeNewLayoutInflater(Context context) {        return sPolicy.makeNewLayoutInflater(context);    }    public static WindowManagerPolicy makeNewWindowManager() {        return sPolicy.makeNewWindowManager();    }    public static FallbackEventHandler makeNewFallbackEventHandler(Context context) {        return sPolicy.makeNewFallbackEventHandler(context);    }}

在这里它去通过反射构造了里面的policy对象,policy是Ipolicy的具体实现类
跟踪Policy代码：

public class Policy implements IPolicy{public LayoutInflater makeNewLayoutInflater(Context context){return new PhoneLayoutInflater(context);}}

原来LayoutInflater的实现类是PhoneLayoutInflater。
继续查看PhoneLayoutInflater源码：

public class PhoneLayoutInflater extends LayoutInflater {    private static final String[] sClassPrefixList = {//内置view类型的前缀，如textview的完整路径为android.widget.textview        "android.widget.",        "android.webkit."    };           public PhoneLayoutInflater(Context context) {        super(context);    }        protected PhoneLayoutInflater(LayoutInflater original, Context newContext) {        super(original, newContext);    }    //在View名字的前面添加前缀来构造View的完整路径，以便于根据类的完整路径来构造View对象    @Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {        for (String prefix : sClassPrefixList) {            try {                View view = createView(name, prefix, attrs);                if (view != null) {                    return view;                }            } catch (ClassNotFoundException e) {                // In this case we want to let the base class take a crack                // at it.            }        }        return super.onCreateView(name, attrs);    }        public LayoutInflater cloneInContext(Context newContext) {        return new PhoneLayoutInflater(this, newContext);    }}

具体如何我们以setContentView为例：
activity中的setContentView中实际调用的是Window中的setContentView,而window是个抽象类，它的实现类是PhoneWindow
在PhoneWindow中有一个对应的setContentView方法，方法中调用了LayoutInflater的inflate方法，我们都知道所有的inflate
方法最终都会调用含有xml解析器的那个inflate方法：

    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;//存储根视图            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 (DEBUG) {                    System.out.println("**************************");                    System.out.println("Creating root view: "                            + name);                    System.out.println("**************************");                }//如果根视图采用的是merge标签                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");                    }//解析xml布局的merge根标签(第一个标签)，并调用这个方法将其标签下的所有子view直接添加到根标签中                    rInflate(parser, root, inflaterContext, attrs, false);                } else {                    // Temp is the root view that was found in the xml//如果是普通布局，调用createViewTag直接解析出根视图，比如LinearLayout，这个方法是对单个元素进行解析                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);                    ViewGroup.LayoutParams params = null;//这些参数就不解释了，以前博文中解释过                    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) {                            temp.setLayoutParams(params);                        }                    }                    if (DEBUG) {                        System.out.println("-----> start inflating children");                    }                    // Inflate all children under temp against its context.//解析这个根布局下面的所有子view，默认还是递归调用Inflate方法，将根布局下的所有的子view添加到temp中，返回temp                    rInflateChildren(parser, temp, attrs, true);                    if (DEBUG) {                        System.out.println("-----> done inflating children");                    }//如果root不为空，attachToRoot为true,将temp添加到父视图中(前面博文已经讲过)                    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代码            Trace.traceEnd(Trace.TRACE_TAG_VIEW);            return result;        }    }

接下来我们继续从解析单个view的方法去看：

    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,            boolean ignoreThemeAttr) {        if (name.equals("view")) {            name = attrs.getAttributeValue(null, "class");        }...............省略部分代码        try {            View view;            if (view == null) {                final Object lastContext = mConstructorArgs[0];                mConstructorArgs[0] = context;                try {//1.内置View控件解析                    if (-1 == name.indexOf('.')) {                        view = onCreateView(parent, name, attrs);                    } else {//2.自定义控件的解析                        view = createView(name, null, attrs);                    }                } finally {                    mConstructorArgs[0] = lastContext;                }            }            return view;        }.....catch代码    }

上面代码1处是根据tag的名字有没有包含“.”,没有的话就返回-1，直接调用onCreateView进行内置view的解析，否则调用
createView解析自定义控件，那这两个方法又有什么区别呢？还记得之前讲过PhoneLayoutInflate中的OncreateView方法，
它是将“android.widget”前缀传递给createView方法，然后再通过View的完整路径去解析。
createView方法中很简单，就是通过完整的类名反射去构造view对象，最终将view对象返回。
现在单个view解释通了，那整个视图树又是怎么解析的呢？

void rInflate(XmlPullParser parser, View parent, Context context,            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {//得到视图树的深度        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)) {                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 {//根据元素名解析                final View view = createViewFromTag(parent, name, context, attrs);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                //通过递归调用进行解析，深度优先遍历。rInflateChildren(parser, view, attrs, true);//每解析到一个view就会递归调用上面的方法，直到这条路径下的最后一个元素，然后再回溯过来将//每个view元素添加到parent中，解析完成后整个视图树也就构造完成。                viewGroup.addView(view, params);            }        }        if (finishInflate) {            parent.onFinishInflate();        }    }

总结：基本关于LayoutInflater的解析大概就到这里了，以上代码如果有什么不清楚的地方，欢迎指出。

参考书籍：
《Android源码设计模式解析与实战》