Android LayoutInflater 类分析

作为一名Android开发者，写页面是最普通不过的事情了，在编写页面的时候，系统给提供了两种形式，一种形式是通过XML的方式进行编写，还有一种形式是通过Java代码直接编写

 

我们知道Android Framework是用Java编写，那就是说，所有的交互都是通过Java来完成，那么我们编写的XML文件又是怎样被转换成Java对象的呢？接下来我便为大家解惑，带大家进入Android世界，去了解Android世界内的奥妙！

 

我们事先准备好一个XML布局，如下

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">
    <TextView />
    <LinearLayout />
        <Button />
    </LinearLayout />
</RelativeLayout>

这个XML布局包括了几个元素，RelativeLayout、LinearLayout、TextView、Button，这些元素在我们的 android.widget 包中都有对应的Java类，那么XML是如何转换成Java类对象的呢？我们都知道Android系统给提供了一个API，唤做LayoutInflater，但是具体细节各位了解吗？Follow me！

 

我们在转换XML为Java类对象的时候，通常会这样写，如下

LayoutInflater.from(this).inflate(R.layout.activity_main,null);

只有这么简单的一行代码，首先调用from函数，获取LayoutInflater对象，然后在调用该对象下的inflate方法，我们先看一下from方法，看看LayoutInflater的实现类是哪个

registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
        new CachedServiceFetcher<LayoutInflater>() {
    @Override
    public LayoutInflater createService(ContextImpl ctx) {
        return new PhoneLayoutInflater(ctx.getOuterContext());
    }}
);

我们可以看到，LayoutInflater的实例对象是PhoneLayoutInflater，属于系统提供的一个服务，可以看我先前的文章《Android 系统服务的获取与创建》

接下来我们继续看inflate方法实现，因为继承关系缘故，inflate是在LayoutInflater中实现，后边会用到PhoneLayoutInflater

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

一个很简单的调用，把传入的参数继续向内部传入

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
    final Resources res = getContext().getResources();
    if (DEBUG) {
        Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                + Integer.toHexString(resource) + ")");
    }
 
    final XmlResourceParser parser = res.getLayout(resource);
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

这里边只是获取了一下XML的解析器，然后继续调用重载函数，我们继续往下看，简单看一下就行，我下面会说这个函数做了什么 操作

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;
            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("**************************");
            }
 
            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");
                }
 
                rInflate(parser, root, inflaterContext, attrs, false);
            } else {
                // Temp is the root view that was found in the xml
                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
                    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");
                }
 
                // Inflate all children under temp against its context.
                rInflateChildren(parser, temp, attrs, true);
 
                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) {
            final InflateException ie = new InflateException(e.getMessage(), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } catch (Exception e) {
            final InflateException ie = new InflateException(parser.getPositionDescription()
                    + ": " + e.getMessage(), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } finally {
            // Don't retain static reference on context.
            mConstructorArgs[0] = lastContext;
            mConstructorArgs[1] = null;
 
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
 
        return result;
    }
}

各位读完后，在看看我对这个函数的理解把

解析XML布局文件，获取跟节点名称（RelativeLayout）

然后判断是否是<merge>标签，如果是merge标签就基于root参数进行构建整个XML布局关系

如果不是，便会根据根节点的名称，使用createViewFromTag函数创建一个对应的Layout对象temp，然后使用temp对象作为根节点构建整个XML布局，核心功能是由rInflate，我们稍后在分析这个函数，我们先看一下createViewFromTag

private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
    return createViewFromTag(parent, name, context, attrs, false);
}

这个方法是重载的调用，我们继续往下看

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) {
            view = mFactory2.onCreateView(parent, name, context, attrs);
        } else if (mFactory != null) {
            view = mFactory.onCreateView(name, context, attrs);
        } else {
            view = null;
        }
 
        if (view == null && mPrivateFactory != null) {
            view = mPrivateFactory.onCreateView(parent, name, context, attrs);
        }
 
        if (view == null) {
            final Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = context;
            try {
                if (-1 == name.indexOf('.')) {
                    view = onCreateView(parent, name, attrs);
                } else {
                    view = createView(name, null, attrs);
                }
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        }
 
        return view;
    } catch (InflateException e) {
        throw e;
 
    } catch (ClassNotFoundException e) {
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class " + name, e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
 
    } catch (Exception e) {
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class " + name, e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    }
}

先判断mFactory2和mFactory对象，这两个对象是外部设置对象，不设置不会进入判断，如果设置后并创建了view，那后边便不会执行了

咱们最普通的用法都是用系统的加载方式

if (view == null) {
    final Object lastContext = mConstructorArgs[0];
    mConstructorArgs[0] = context;
    try {
        if (-1 == name.indexOf('.')) {
            view = onCreateView(parent, name, attrs);
        } else {
            view = createView(name, null, attrs);
        }
    } finally {
        mConstructorArgs[0] = lastContext;
    }
}

就是上面这一段代码。这段代码是调用onCreateView和createView来创建

我们先看一下onCreateView

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 {
            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);
}

我们的LayoutInflater的实现类是PhoneLayoutInflater，PhoneLayoutInflater只实现了onCreateView方法，所以会先调用子类的onCreateView方法

这个子类的onCreateView方法主要功能是通过循环sClassPrefixList这个数组并调用createView尝试创建view，如果可以创建成功便返回，如果循环结束后仍然没有创建成功便调用LayoutInflater的ch

protected View onCreateView(String name, AttributeSet attrs)
        throws ClassNotFoundException {
    return createView(name, "android.view.", attrs);
}

这个方法也是调用了onCreateView，但是在前缀参数上传递了android.view.参数，代表是系统view

我们来往下看createView把

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);
                }
            }
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            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;
 
        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 (NoSuchMethodException e) {
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
 
    } catch (ClassCastException e) {
        // If loaded class is not a View subclass
        final InflateException ie = new InflateException(attrs.getPositionDescription()
                + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } catch (ClassNotFoundException e) {
        // If loadClass fails, we should propagate the exception.
        throw e;
    } catch (Exception e) {
        final InflateException ie = new InflateException(
                attrs.getPositionDescription() + ": Error inflating class "
                        + (clazz == null ? "<unknown>" : clazz.getName()), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

这个这个函数便是先把前缀prefix和name进行了一个拼接，然后使用反射技术通过拼接出来的包名创建对应的view对象，最终返回创建出来的view，到这里就弄清楚了view是怎么创建出来的。这快可以告一段落了，我们往下继续看是怎么把一个庞大的XML布局转换成一个ViewGroup对象的

我们看一下rInflateChildren函数的具体实现把

final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
        boolean finishInflate) throws XmlPullParserException, IOException {
    rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}

rInflateChildren函数的作用是根据一个根节点组合一个完整的View，也就是我们实例中的RelativeLayout和他下边的两个节点：TextView、LInearLayout

rInflate函数的作用是把这个ViewGroup下的所有子View都添加进来，形成一个完整的View Tree

我们分析一下rInflate把，看看逻辑是如何实现的

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) {
                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 {
            final View view = createViewFromTag(parent, name, context, attrs);
            final ViewGroup viewGroup = (ViewGroup) parent;
            final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
            rInflateChildren(parser, view, attrs, true);
            viewGroup.addView(view, params);
        }
    }
 
    if (pendingRequestFocus) {
        parent.restoreDefaultFocus();
    }
 
    if (finishInflate) {
        parent.onFinishInflate();
    }
}

这个方法用while循环的形式来解析XML布局，然后得到节点名称name字段，并使用name属性去创建对应的view对象

然后递归调用直到整个XML解析完成

 

最后执行完成后，方法退回到inflate方法内，执行以下代码，来返回解析结果，返回后，inflate方法彻底执行完成，至此，View对象也是生成完毕

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
    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");
}
 
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
 
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;
}

上边这段代码里，temp对象便是通过rInflateChindren以及层层递归得到的最终对象，处理最后赋值给result对象，并返回结果