Android LayoutInflater

一.  LayoutInflater简介

1. LayoutInflater的作用：

动态加载xml布局，并且将布局实例化为我们需要的view。

2. LayoutInflater的实例：

LayoutInflater inflater = LayoutInflater.from(context);

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;
}

可以看出实际上是调用context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

而LayoutInflater本身是一个抽象类，它的具体实现类是在ContextImpl的getSystemService方法中被实例化。

public Object getSystemService(String name) {
    return SystemServiceRegistry.getSystemService(this, name);
}

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

由此可以知道，我们用的LayoutInflater实例全部是来自于同一个PhoneLayoutInflater。

3. 常用方式：

直接通过加载xml布局文件，获取我们需要的view：

View view = LayoutInflater.from(mContext).inflate(R.layout.layout_xxx, null);

在setContentView中，也是使用LayoutInflater加载布局：

（这里的root是content，整个窗口的根视图 https://www.cnblogs.com/nemohao/p/11648272.html）

public void setContentView(int resId) {
    ensureSubDecor();
    ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
    contentParent.removeAllViews();
    LayoutInflater.from(mContext).inflate(resId, contentParent);
    mOriginalWindowCallback.onContentChanged();
}

 

二.  LayoutInflater加载过程

通过调用LayoutInflater的inflate方法，经过重载后最终都是这个方法：

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) + ")");
    }

    //XmlResourceParser继承了XmlPullParser和AttributeSet这2个接口
    final XmlResourceParser parser = res.getLayout(resource);
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

然后调用了调用到这个inflate方法：

（可以看出Android中解析xml布局是基于Pull解析的，关于XML的解析，可以参阅 https://www.cnblogs.com/nemohao/p/11649098.html）

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {final Context inflaterContext = mContext;

        //拿到attr
        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;
    }
}

 

可以看出：

如果是merge开头的xml，则调用了rInflate方法(里边其实也是rInflateChildren())，然后finishInflate传入的是false；

不是merge则拿到第一个Tag通过createViewFromTag来创建root，之后再调用rInflateChildren()，finishInflate传入的是true。