Android开发——深入理解LayoutInflater

LayoutInflater类可以从XML文件中实例化View对象，其中涉及到XML解析，使用的是PULL解析；从标签实例化View，使用的是反射。

如何获取LayoutInflater？

为了获取LayoutInflater对象，一般不会调用其构造方法，而是调用getLayoutInflater()或getSystemService(Class)方法来获取已经和当前Context绑定的LayoutInflater对象，另外也可以使用LayoutInflater的静态方法from(Context)调用，不过这些调用方法归根结底都是调用了getSystemService(Class)获取的LayoutInflater。
首先看LayoutInflater.from(Context)方法的实现：

 public static LayoutInflater from(Context context) {
        LayoutInflater LayoutInflater =
                (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        if (LayoutInflater == null) {
            throw new AssertionError("LayoutInflater not found.");
        }
        return LayoutInflater;
    }

从上面可以看出，使用了getSystemService方法获取LayoutInflater。再看Activity的getLayoutInflater方法：

 public LayoutInflater getLayoutInflater() {
        return getWindow().getLayoutInflater();
    }

我们知道Activity的getWindow()方法会返回PhoneWindow对象，接下来看PhoneWindow的getLayoutInflater方法：

 public LayoutInflater getLayoutInflater() {
        return mLayoutInflater;
    }

从上面的代码可以看出，直接返回了mLayoutInflater，而LayoutInflater的初始化是在PhoneWindow的构造方法中：

public PhoneWindow(Context context) {
        super(context);
        mLayoutInflater = LayoutInflater.from(context);
    }

可以看出这儿使用的from方法构造的，但是从上面我们知道from也是通过getSystemService创建的，所以最终都是通过getSystemService()方法创建LayoutInflater对象，所以这也就是必须得提供Context参数的原因。

如何使用LayoutInflater加载View？

得到LayoutInflater对象之后，就可以调用其inflater方法加载View，inflater有好几个重载方法，可以根据需要调用不同的inflater方法。

从源码角度分析inflater方法

虽然inflater方法有多个重载方法，但是最终都会进入一个inflater方法，具体如下：

 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;
        }
    }

首先官方文档中是这样解析三个参数以及返回值的，parser是XML的解析器；root参数是生成View层次的可选父视图(如果attachToRoot为true)，或者只是简单地为返回的根布局提供LayoutParams值；attachToRoot表示加载的视图是否应该被添加到root参数；返回的View表示加载层次的根布局。如果root提供了并且attachToRoot为true，那么就是root；否则就是XML的根结点对应的View。
下面，我们就源码给出解释。首先将result置为root，然后进行XML解析，主要看try块内的代码，前面是一些范围判断，接下去首先解析第一个标签，即XML的根结点，如果是merge标签，那么按照merge的属性，必须外面有布局，所以如果root为null或者attachToRoot为false，直接抛出异常，否则调用rInflate方法，这个方法下面再看，首先看根标签不是merge的情况。
当根标签不是merge的时候，那么就进入了else语句块。首先调用createViewFromTag生成根View，下面看一下createViewFromTag的实现，代码如下:

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

从上面可以看出，又调用了createViewFromTag的一个重载方法，如下：

/**
     * Creates a view from a tag name using the supplied attribute set.
     * <p>
     * <strong>Note:</strong> Default visibility so the BridgeInflater can
     * override it.
     *
     * @param parent the parent view, used to inflate layout params
     * @param name the name of the XML tag used to define the view
     * @param context the inflation context for the view, typically the
     *                {@code parent} or base layout inflater context
     * @param attrs the attribute set for the XML tag used to define the view
     * @param ignoreThemeAttr {@code true} to ignore the {@code android:theme}
     *                        attribute (if set) for the view being inflated,
     *                        {@code false} otherwise
     */
    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;
        }
    }

createViewFromTag方法使用提供的AttributeSet和标签名创建一个View对象。主要看try块中的代码，首先尝试使用LayoutInflater的Factory2和Factory创建视图，一般情况下这两个参数均为null，最后会进入到第204行，如果View标签是一个自定义View或者Support库中的View，那么在XML中定义时就会出现”xxx.xxx.xxx”的样式，所以就会包含”.”，就会调用createView方法，否则onCreateView方法。先看onCreateView方法，

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

可以看到对于自带的View，createView中第二个参数为”android.view”，如果再加上标签名，就可以构成View的完整路径了；而对于含有”.”的标签名，最终也是调用createView方法，但是第二个参数为null，因为第一个参数name已经是完整路径了，比如”android.support.v4.view.ViewPager”。下面是createView的实现：

 /**
     * Low-level function for instantiating a view by name. This attempts to
     * instantiate a view class of the given <var>name</var> found in this
     * LayoutInflater's ClassLoader.
     * 
     * <p>
     * There are two things that can happen in an error case: either the
     * exception describing the error will be thrown, or a null will be
     * returned. You must deal with both possibilities -- the former will happen
     * the first time createView() is called for a class of a particular name,
     * the latter every time there-after for that class name.
     * 
     * @param name The full name of the class to be instantiated.
     * @param attrs The XML attributes supplied for this instance.
     * 
     * @return View The newly instantiated view, or null.
     */
    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[] 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]));
            }
            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);
        }
    }

createView方法主要就是使用反射创建View对象。至此，XML的根节点的View被创建好了，接下来继续看inflater中的代码。首先如果root不为null，那么就调用generateLayoutParams方法创建LayoutParams对象，此时如果attachToRoot为false，那么就意味着不会把根结点temp添加给root，只是使用root生成的LayoutParams，所以只是调用temp的setLayoutParams方法。然后调用rInflateChildren方法加载temp的子视图，接下来主要代码：

// 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;
                    }

从上面的代码可以看出，如果root不为null且attachToRoot为true，那么就将temp添加到root中；如果root为null或者attachToRoot为false，那么result就是temp，即XML的根结点。那么下面根据root和attachToRoot总结一下各种情况下的返回值：
1. 如果root为null，那么返回值是XML的根结点对应的View
2. 如果root不为null，attachToRoot为false，那么返回值是XML的根结点对应的View，但是View的LayoutParams是root生成的
3. 如果root不为null，且attachToRoot为true，那么返回值是root，XML的根结点对应的View被添加到root中
再了解了上面规则之后，先看inflate的其他几个方法：

View inflate(@LayoutRes int resource, @Nullable ViewGroup root)
View inflate(XmlPullParser parser, @Nullable ViewGroup root)

上述方法中，如果root不为null，那么其调用的最终inflate中attachToRoot为true，否则为false。
在了解了具体什么情况返回的是什么View之后，再看一下rInflateChildren方法：

/**
     * Recursive method used to inflate internal (non-root) children. This
     * method calls through to {@link #rInflate} using the parent context as
     * the inflation context.
     * <strong>Note:</strong> Default visibility so the BridgeInflater can
     * call it.
     */
    final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
            boolean finishInflate) throws XmlPullParserException, IOException {
        rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
    }

rInflaterChildren方法用于迭代加载内部孩子，下面看rInflater方法：

 /**
     * Recursive method used to descend down the xml hierarchy and instantiate
     * views, instantiate their children, and then call onFinishInflate().
     * <p>
     * <strong>Note:</strong> Default visibility so the BridgeInflater can
     * override it.
     */
    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)) {
                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 (finishInflate) {
            parent.onFinishInflate();
        }
    }

可以看到会调用createViewFromTag创建标签对应的View，再将其添加到parent中，然后再以View为父视图调用rInflateChildren方法，如此反复，直到最终解析出整个XML文档。
再看merge标签的操作，因为是merge，所以默认root是其父布局，所以直接调用了rInflate方法。

总结

针对LayoutInflater的inflate方法，需要知道各种方法中传参所带来的影响，以及最终的View。具体规则再总结一次：
1. 如果root为null，那么返回值是XML的根结点对应的View
2. 如果root不为null，attachToRoot为false，那么返回值是XML的根结点对应的View，但是View的LayoutParams是root生成的
3. 如果root不为null，且attachToRoot为true，那么返回值是root，XML的根结点对应的View被添加到root中