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(" 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(" 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 {
				//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》