LayoutInflater源码分析

大家都很熟悉LayoutInflater来添加View。我们看一下它的实现。

public abstract class LayoutInflater {}

首先LayoutInflater是个抽象类,也没有继承其他类.

1.我们可以通过下面两种方式获得LayoutInflater;

LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
或者Activity.getLayoutInflater();

2.一般使用LayoutInflater的方式是以下几种

 public View inflate(int resource, ViewGroup root, boolean attachToRoot)
 public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)
 public View inflate(int resource, ViewGroup root)
 public View inflate(XmlPullParser parser, ViewGroup root)

其中加载xml分R.id.xx和直接载入.xml,但是由于解析xml需要时间，这时如果xml不是预先被编译就不能显示视图报错(如在加载assets文件中的xml文件).使用R.id.xx实际上也是通过XmlPullParser来加载视图的.

ViewGroup相当于父视图,如果有root视图，默认attachToRoot为false,这时会messure父视图的layout来加载子视图.如果root视图为空,会创建视图来加载到viewtree的根部.其实创建的是内部类BlinkLayout，它是继承帧视图，方便上下图层的叠加.在加载BlinkLayout之前还是先创建了

ViewStub，来占空视图.

private static class BlinkLayout extends FrameLayout 

下面我们看下最关键的inflate

  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 {
                // 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, 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 {
                        temp = createViewFromTag(root, name, 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
                    rInflate(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) {
                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;
        }
    }

它在解析xml时候实际是调用了rInflate,来递归xml解析.

 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;
            }

            final String name = parser.getName();
            
            if (TAG_REQUEST_FOCUS.equals(name)) {
                parseRequestFocus(parser, parent);
            } else if (TAG_INCLUDE.equals(name)) {
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                throw new InflateException("<merge /> 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 {
                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);
            }
        }

        if (finishInflate) parent.onFinishInflate();
    }

解析的是一下xml的标签

    private static final String TAG_MERGE = "merge";
    private static final String TAG_INCLUDE = "include";
    private static final String TAG_1995 = "blink";
    private static final String TAG_REQUEST_FOCUS = "requestFocus";

我们在xml中引用其他视图的时候会用到<include>来减少重复视图的编写.

我们在parseInclude中可以看到解析不同的tag执行不同的操作.其中requestFocus标签目前还没有做任何事情.

 private void parseInclude(XmlPullParser parser, View parent, AttributeSet attrs)
            throws XmlPullParserException, IOException {

        int type;

        if (parent instanceof ViewGroup) {
            final int layout = attrs.getAttributeResourceValue(null, "layout", 0);
            if (layout == 0) {
                final String value = attrs.getAttributeValue(null, "layout");
                if (value == null) {
                    throw new InflateException("You must specifiy a layout in the"
                            + " include tag: <include layout="@layout/layoutID" />");
                } else {
                    throw new InflateException("You must specifiy a valid layout "
                            + "reference. The layout ID " + value + " is not valid.");
                }
            } else {
                final XmlResourceParser childParser =
                        getContext().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();

                    if (TAG_MERGE.equals(childName)) {
                        // Inflate all children.
                        rInflate(childParser, parent, childAttrs, false);
                    } else {
                        final View view = createViewFromTag(parent, childName, childAttrs);
                        final ViewGroup group = (ViewGroup) parent;

                        // We try to load the layout params set in the <include /> tag. If
                        // they don't exist, we will rely on the layout params set in the
                        // included XML file.
                        // During a layoutparams generation, a runtime exception is thrown
                        // if either layout_width or layout_height is missing. We catch
                        // this exception and set localParams accordingly: true means we
                        // successfully loaded layout params from the <include /> tag,
                        // false means we need to rely on the included layout params.
                        ViewGroup.LayoutParams params = null;
                        try {
                            params = group.generateLayoutParams(attrs);
                        } catch (RuntimeException e) {
                            params = group.generateLayoutParams(childAttrs);
                        } finally {
                            if (params != null) {
                                view.setLayoutParams(params);
                            }
                        }

                        // Inflate all children.
                        rInflate(childParser, view, childAttrs, true);

                        // Attempt to override the included layout's android:id with the
                        // one set on the <include /> tag itself.
                        TypedArray a = mContext.obtainStyledAttributes(attrs,
                            com.android.internal.R.styleable.View, 0, 0);
                        int id = a.getResourceId(com.android.internal.R.styleable.View_id, View.NO_ID);
                        // While we're at it, let's try to override android:visibility.
                        int visibility = a.getInt(com.android.internal.R.styleable.View_visibility, -1);
                        a.recycle();

                        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");
        }

        final int currentDepth = parser.getDepth();
        while (((type = parser.next()) != XmlPullParser.END_TAG ||
                parser.getDepth() > currentDepth) && type != XmlPullParser.END_DOCUMENT) {
            // Empty
        }
    }