include、merge、ViewStub分析

include、merge、ViewStub分析

include的使用：

<include
    android:id="@+id/my_layout"
    layout="@layout/include_layout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

源码从Activity的setContentView开始：

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

这块得耐心一丢丢了

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
        //打印inflate的栈
        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 {
            int type;
            //移动type不是开始标记也不是结束标记
            while ((type = parser.next()) != XmlPullParser.START_TAG &&type != XmlPullParser.END_DOCUMENT) {}
            if (type != XmlPullParser.START_TAG) {
                throw new InflateException(parser.getPositionDescription()
                        + ": No start tag found!");
            }
            //得到解析的标签名
            final String name = parser.getName();

            if (DEBUG) {
                System.out.println("**************************");
                System.out.println("Creating root view: " + name);
                System.out.println("**************************");
            }
            //如果此时是merge标签
            if (TAG_MERGE.equals(name)) {
                //当root也就是父view为空或者没有attachToRoot到父view上时抛出异常
                if (root == null || !attachToRoot) {
                    throw new InflateException("<merge /> can be used only with a valid "
                            + "ViewGroup root and attachToRoot=true");
                }
                //两者都满足时进行下一步解析
                rInflate(parser, root, inflaterContext, attrs, false);
            } else {
                //后面是将这个布局添加至viewroot中
                rInflateChildren(parser, temp, attrs, true);
                if (root != null && attachToRoot) {
                    root.addView(temp, params);
                }
            }
        } catch (Exception e) {
            throw ie;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        return result;
    }
}

所以解析的核心还在于：

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) {
        //找到开始节点
        if (type != XmlPullParser.START_TAG) {
            continue;
        }
        //解析节点名称
        final String name = parser.getName();
        //requestFocus标签的特殊处理
        if (TAG_REQUEST_FOCUS.equals(name)) {
            pendingRequestFocus = true;
            consumeChildElements(parser);
        } else if (TAG_TAG.equals(name)) {//对tag标签的处理
            parseViewTag(parser, parent, attrs);
        } else if (TAG_INCLUDE.equals(name)) {//对include的处理
            if (parser.getDepth() == 0) {
                throw new InflateException("<include /> cannot be the root element");
            }
            parseInclude(parser, context, parent, attrs);
        } else if (TAG_MERGE.equals(name)) {//对merge的处理
            throw new InflateException("<merge /> must be the root element");
        } else {
            //对普通布局节点的处理
            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();
    }
}

这里回想一下，最初在LayoutInflater.inflate处理merge标签的时候，也就是开始标签

int type;
while ((type = parser.next()) != XmlPullParser.START_TAG && type != XmlPullParser.END_DOCUMENT) {
}

if (type != XmlPullParser.START_TAG) {
    throw new InflateException(parser.getPositionDescription() + ": No start tag found!");
}

final String name = parser.getName();

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);
}

也就是说如果设置了merge标签，必须要在最开始的时候设置，而且父容器不能为null&&需要绑定到父容器上

当进入rInflate的时候只要碰见有merge标签就报错说明只要是merge下面要是嵌套merge就报错，也就是merge得用到根节点

当然这里也看到了include标签，我们看看include标签处理什么

if (TAG_INCLUDE.equals(name)) {
    if (parser.getDepth() == 0) {
        throw new InflateException("<include /> cannot be the root element");
    }
    parseInclude(parser, context, parent, attrs);//进行include的解析
}

private void parseInclude(XmlPullParser parser, Context context, View parent,
        AttributeSet attrs) throws XmlPullParserException, IOException {
    int type;

    if (parent instanceof ViewGroup) {
        final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
        final int themeResId = ta.getResourceId(0, 0);
        final boolean hasThemeOverride = themeResId != 0;
        if (hasThemeOverride) {
            context = new ContextThemeWrapper(context, themeResId);
        }
        ta.recycle();

        //解析layout节点
        int layout = attrs.getAttributeResourceValue(null, ATTR_LAYOUT, 0);
        if (layout == 0) {
            final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
            if (value == null || value.length() <= 0) {
                throw new InflateException("You must specify a layout in the"
                        + " include tag: <include layout=\"@layout/layoutID\" />");
            }
            layout = context.getResources().getIdentifier(value.substring(1), null, null);
        }

        // The layout might be referencing a theme attribute.
        if (mTempValue == null) {
            mTempValue = new TypedValue();
        }
        if (layout != 0 && context.getTheme().resolveAttribute(layout, mTempValue, true)) {
            layout = mTempValue.resourceId;
        }

        if (layout == 0) {
            final String value = attrs.getAttributeValue(null, ATTR_LAYOUT);
            throw new InflateException("You must specify a valid layout "
                    + "reference. The layout ID " + value + " is not valid.");
        } else {
            final XmlResourceParser childParser = context.getResources().getLayout(layout);

            try {
                final AttributeSet childAttrs = Xml.asAttributeSet(childParser);

                while ((type = childParser.next()) != XmlPullParser.START_TAG &&
                        type != XmlPullParser.END_DOCUMENT) {
                    // Empty.
                }

                if (type != XmlPullParser.START_TAG) {
                    throw new InflateException(childParser.getPositionDescription() +
                            ": No start tag found!");
                }

                final String childName = childParser.getName();
                //如果是merge的话进入这个流程
                if (TAG_MERGE.equals(childName)) {
                    // The <merge> tag doesn't support android:theme, so
                    // nothing special to do here.
                    rInflate(childParser, parent, context, childAttrs, false);
                } else {
                    //创建子节点，添加到父容器
                    final View view = createViewFromTag(parent, childName,
                            context, childAttrs, hasThemeOverride);
                    final ViewGroup group = (ViewGroup) parent;

                    final TypedArray a = context.obtainStyledAttributes(
                            attrs, R.styleable.Include);
                    final int id = a.getResourceId(R.styleable.Include_id, View.NO_ID);
                    final int visibility = a.getInt(R.styleable.Include_visibility, -1);
                    a.recycle();
                    ViewGroup.LayoutParams params = null;
                    try {
                        params = group.generateLayoutParams(attrs);
                    } catch (RuntimeException e) {
                        // Ignore, just fail over to child attrs.
                    }
                    if (params == null) {
                        params = group.generateLayoutParams(childAttrs);
                    }
                    view.setLayoutParams(params);

                    // rInflate同样调用这个
                    rInflateChildren(childParser, view, childAttrs, true);

                    if (id != View.NO_ID) {
                        view.setId(id);
                    }

                    switch (visibility) {
                        case 0:
                            view.setVisibility(View.VISIBLE);
                            break;
                        case 1:
                            view.setVisibility(View.INVISIBLE);
                            break;
                        case 2:
                            view.setVisibility(View.GONE);
                            break;
                    }

                    group.addView(view);
                }
            } finally {
                childParser.close();
            }
        }
    } else {
        throw new InflateException("<include /> can only be used inside of a ViewGroup");
    }

    LayoutInflater.consumeChildElements(parser);
}

viewstub

这个控件相对上面的就比较简单，直接看下源码

<ViewStub
    android:id="@+id/stub_import"
    android:inflatedId="@+id/panel_import"
    android:layout="@layout/progress_overlay"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom" />

加载布局的两种方法

((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE);
// or
View importPanel = ((ViewStub) findViewById(R.id.stub_import)).inflate();

public void setVisibility(int visibility) {
    if (mInflatedViewRef != null) {//开始为null
        View view = mInflatedViewRef.get();
        if (view != null) {
            view.setVisibility(visibility);
        } else {
            throw new IllegalStateException("setVisibility called on un-referenced view");
        }
    } else {
        super.setVisibility(visibility);
        if (visibility == VISIBLE || visibility == INVISIBLE) {
            //当设置可见的时候就会触动加载，因为此布局在构造中设置GONE
            inflate();
        }
    }
}

public View inflate() {
    final ViewParent viewParent = getParent();//得到Parent

    if (viewParent != null && viewParent instanceof ViewGroup) {
        if (mLayoutResource != 0) {
            final ViewGroup parent = (ViewGroup) viewParent;
            final View view = inflateViewNoAdd(parent);//通过这个进行加载创建view
            replaceSelfWithView(view, parent);

            mInflatedViewRef = new WeakReference<>(view);
            if (mInflateListener != null) {
                mInflateListener.onInflate(this, view);
            }

            return view;
        } else {
            throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
        }
    } else {
        throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
    }
}

所以viewstub的原理就是用到的时候才进行创建view。