Android中的inflate()原理解析

思来想去，决定还是尝试下写自己的博客，第一次写博客，有什么写得不好的地方请谅解，写得不对的地方请指教。ok！废话少说，直接进入正题！

今天想讲解一下inflate()方法动态加载布局。先说一下Android中的基本用法吧！

LayoutInflater inflater = LayoutInflater.from(context);//获取LayoutInflater对象//加载布局，layoutID为layout文件下的布局文件名;第二个参数表示该布局要添加到哪个父布局中，若无，则置为nullView view = inflater.inflate(layoutId, null);

另外还有一种常用的方法来加载布局：

//加载布局，layoutID为layout文件下的布局文件名;第三个参数表示该布局要添加到哪个父布局中，若无，则置为nullView view = View.inflate(context, layoutId, null);

好了，如此就可以得到从res/layout/的布局view

不过，第二种方法的本质其实还是用到了第一种方法，只是将其封装起来了。

常用的例子是在ListView设置Adapter时，如果要使用自定义的item布局，就会使用到该方法。

说个题外话：以前在自定义item布局写Adapter时，发觉如果将布局中的根节点设为RelativeLayout，然后运行时就发觉报了一堆莫名其妙的错误，后来将其改为LinearLayout，就没事了，不知道是不是Android本身某个地方的bug。总之，在此只是想提醒下各位，如果哪天您在布局里将根节点设为RelativLayout，并将布局inflate()到java代码中，然后有发现运行错误退出了，不妨试下是不是出现了我所说的那种问题。

介绍完动态加载布局inflate()的常用方式后，接下来从源码的角度试着来解析下：

先看下View.inflate()的源码

public static View inflate(Context context, int resource, ViewGroup root) {        LayoutInflater factory = LayoutInflater.from(context);        return factory.inflate(resource, root);    }

从此处可以看出，View.inflate()是将LayoutInflater.inflate()封装起来了。所以最终还是回到LayoutInflater的inflate(),接下来看看它的源码：

public View inflate(int resource, ViewGroup root) {        return inflate(resource, root, root != null);    }

public View inflate(int resource, ViewGroup root, boolean attachToRoot) {        if (DEBUG) System.out.println("INFLATING from resource: " + resource);        XmlResourceParser parser = getContext().getResources().getLayout(resource);        try {            return inflate(parser, root, attachToRoot);        } finally {            parser.close();        }    }

该方法会先获取布局文件resource（总所周知，布局文件是xml格式）的xml的pull解析器parser（至于其具体是怎么获取的，有兴趣的童鞋可以自己去查找相关资料自己学习，在此不多讲）。然后是到了第05行处的方法inflate(parser, root, attachToRoot)，源码如下：

public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot) {        synchronized (mConstructorArgs) {            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");            final AttributeSet attrs = Xml.asAttributeSet(parser);            Context lastContext = (Context)mConstructorArgs[0];            mConstructorArgs[0] = mContext;            View result = root;            try {/***1.start***/                // Look for the root node.//此处parser.next()理论上应该为开始标签，即type==XmlPullParser.START_TAG，//即为根标签                int type;                while ((type = parser.next()) != XmlPullParser.START_TAG &&                        type != XmlPullParser.END_DOCUMENT) {                    // Empty                }//若不是为START_TAG,即为END_DOCUMENT，即使说为空文档                if (type != XmlPullParser.START_TAG) {                    throw new InflateException(parser.getPositionDescription()                            + ": No start tag found!");                }//得到根标签名，即一般为布局文件中的RelativeLayout或LinearLayout等。。。                final String name = parser.getName(); /***1.end***/                               if (DEBUG) {                    System.out.println("**************************");                    System.out.println("Creating root view: "                            + name);                    System.out.println("**************************");                }//若跟标签为merge类型，则必须嵌入父布局中，即root不能空，且attachToRoot需为true//否则抛出异常                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, attrs, false);                } else {                    // Temp is the root view that was found in the xml                    View temp;                    if (TAG_1995.equals(name)) {                        temp = new BlinkLayout(mContext, attrs);                    } else {/***2.start***///此处只说根标签为普通布局的情况下，直接到此处//此处创建根标签名类型的视图view，具体源代码解析稍后在解释                        temp = createViewFromTag(root, name, attrs);/***2.end***/                    }                    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                        params = root.generateLayoutParams(attrs);                        if (!attachToRoot) {                            // Set the layout params for temp if we are not                            // attaching. (If we are, we use addView, below)                            temp.setLayoutParams(params);                        }                    }                    if (DEBUG) {                        System.out.println("-----> start inflating children");                    }/***3.start***/                    // Inflate all children under temp//此处是根据parser解析，然后在根视图temp中循环添加子视图，//一般情况下，得到的temp视图即为我们所要得到的布局视图，具体源代码稍后讲解                    rInflate(parser, temp, attrs, true);/***3.end***/                    if (DEBUG) {                        System.out.println("-----> done inflating children");                    }                    // We are supposed to attach all the views we found (int temp)                    // to root. Do that now.                    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 (XmlPullParserException e) {                InflateException ex = new InflateException(e.getMessage());                ex.initCause(e);                throw ex;            } catch (IOException e) {                InflateException ex = new InflateException(                        parser.getPositionDescription()                        + ": " + e.getMessage());                ex.initCause(e);                throw ex;            } finally {                // Don't retain static reference on context.                mConstructorArgs[0] = lastContext;                mConstructorArgs[1] = null;            }            Trace.traceEnd(Trace.TRACE_TAG_VIEW);            return result;        }    }

首先时获取到根标签（1.start---->1.end处的代码），然后根据根标签的类型做出不同的行为，一般直接到2.start--->2.end处的temp = createViewFromTag(root, name, attrs);创建根标签的视图temp，最后在该temp视图上继续解析parser，在temp视图中递归创建添加子视图（3.start--->3.end处的rInflate(parser, temp, attrs, true);）

先说明下是如何根据标签名创建出一个视图来，createViewFromTag()源码如下：

View createViewFromTag(View parent, String name, AttributeSet attrs) {        if (name.equals("view")) {            name = attrs.getAttributeValue(null, "class");        }        if (DEBUG) System.out.println("******** Creating view: " + name);        try {            View view;            if (mFactory2 != null) view = mFactory2.onCreateView(parent, name, mContext, attrs);            else if (mFactory != null) view = mFactory.onCreateView(name, mContext, attrs);            else view = null;            if (view == null && mPrivateFactory != null) {                view = mPrivateFactory.onCreateView(parent, name, mContext, attrs);            }                        if (view == null) {                if (-1 == name.indexOf('.')) {                    view = onCreateView(parent, name, attrs);                } else {                    view = createView(name, null, attrs);                }            }            if (DEBUG) System.out.println("Created view is: " + view);            return view;        } catch (InflateException e) {            throw e;        } catch (ClassNotFoundException e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class " + name);            ie.initCause(e);            throw ie;        } catch (Exception e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class " + name);            ie.initCause(e);            throw ie;        }    }

此处的代码不多，会根据不同的工厂类和标签名来的情况来创建视图，但不管是何处的onCreateView，观看其源码，最终还是会调用22行出的createView()来创建视图，

其源代码如下：

public final View createView(String name, String prefix, AttributeSet attrs)            throws ClassNotFoundException, InflateException {/***1.start**///首先从内存缓冲sConstructorMap中获取类名为name的构造器        Constructor<? extends View> constructor = sConstructorMap.get(name);        Class<? extends View> clazz = null;        try {            Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);/**若构造器不存在，即还没加载过该类型的视图，就通过反射的技术获取到class对象，* 然后获取其构造器，并存到缓存sConstructorMap中*/            if (constructor == null) {                // Class not found in the cache, see if it's real, and try to add it//反射加载类对象                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);                    }                }//获取构造器                constructor = clazz.getConstructor(mConstructorSignature);//存放到缓存中                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);                    }                }            }/***1.end***/Object[] args = mConstructorArgs;            args[1] = attrs;/***2.start***/            final View view = constructor.newInstance(args);            if (view instanceof ViewStub) {                // always use ourselves when inflating ViewStub later                final ViewStub viewStub = (ViewStub) view;                viewStub.setLayoutInflater(this);            }            return view;/***2.end***/        } catch (NoSuchMethodException e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class "                    + (prefix != null ? (prefix + name) : name));            ie.initCause(e);            throw ie;        } catch (ClassCastException e) {            // If loaded class is not a View subclass            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Class is not a View "                    + (prefix != null ? (prefix + name) : name));            ie.initCause(e);            throw ie;        } catch (ClassNotFoundException e) {            // If loadClass fails, we should propagate the exception.            throw e;        } catch (Exception e) {            InflateException ie = new InflateException(attrs.getPositionDescription()                    + ": Error inflating class "                    + (clazz == null ? "" : clazz.getName()));            ie.initCause(e);            throw ie;        } finally {            Trace.traceEnd(Trace.TRACE_TAG_VIEW);        }    }

此处的代码不难理解，先在1.start--->1.end处获取类的构造器，然后在2.start--->2.end出创建出实例对象并返回，即可得到根据标签名创建的视图.
好了，回到前面inflate()方法中的3.stat--->3.end处的rInflate()方法中，其源码如下：

void rInflate(XmlPullParser parser, View parent, final 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;            }/***1.start***///获取到下一个标签名            final String name = parser.getName();/***1.end***/            if (TAG_REQUEST_FOCUS.equals(name)) {                parseRequestFocus(parser, parent);            } else if (TAG_INCLUDE.equals(name)) {                if (parser.getDepth() == 0) {                    throw new InflateException(" cannot be the root element");                }                parseInclude(parser, parent, attrs);            } else if (TAG_MERGE.equals(name)) {                throw new InflateException(" must be the root element");            } else if (TAG_1995.equals(name)) {                final View view = new BlinkLayout(mContext, attrs);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                rInflate(parser, view, attrs, true);                viewGroup.addView(view, params);                            } else {/***2.stat***/                final View view = createViewFromTag(parent, name, attrs);                final ViewGroup viewGroup = (ViewGroup) parent;                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);                rInflate(parser, view, attrs, true);                viewGroup.addView(view, params);/***2.end***/            }        }//执行到此处，说明可以退出递归，即其符视图parent已创建完毕，结束其加载        if (finishInflate) parent.onFinishInflate();    }

先看到1.start--->1.end出，是获取方法中parser的子标签名，然后根据该子标签名到2.start--->2.end处，还是根据方法createViewFromTag创建出新的子视图view，再递归rInflate创建子视图view的子视图，最后添加到父视图parent中。由于此处用来递归，可能难以理解，需要各位仁兄慢慢品味其逻辑。

到此，整个布局的加载已经完毕！

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