Android-View 从 xml 文件到 Java 对象的转换过程

Arrom
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View 从 xml 文件到 Java 对象的转换过程

setContentView(R.layout.xxx);

进入AppCompatActivity.java

@Override
public void setContentView(@LayoutRes int layoutResID) {
    getDelegate().setContentView(layoutResID);
}

AppCompatActivity是Activity的子类

每个继承 android.app.Activity 的子类 setContentView(resId) 实现方式都被重载了。我们这里先看最基础的 android.app.Activity Activity.java

public void setContentView(@LayoutRes int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

进入getWindow方法

public Window getWindow() {
    return mWindow;
}

mWindow = new PhoneWindow(this, window, activityConfigCallback);

mWindow唯一子类PhoneWindow

进入PhoneWindow类中的setContentView

@Override
public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        installDecor();
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        mContentParent.removeAllViews();
    }

    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                getContext());
        transitionTo(newScene);
    } else {
        mLayoutInflater.inflate(layoutResID, mContentParent);
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
    mContentParentExplicitlySet = true;
}

mLayoutInflater的赋值语句

@UnsupportedAppUsage
public PhoneWindow(@UiContext Context context) {
    super(context);
    mLayoutInflater = LayoutInflater.from(context);
    mRenderShadowsInCompositor = Settings.Global.getInt(context.getContentResolver(),
            DEVELOPMENT_RENDER_SHADOWS_IN_COMPOSITOR, 1) != 0;
}

Activity.setContentView(resId) 最终还是使用LayoutInflater.from(context).inflate(resId, ……)

xml 转成成 Java 对象是通过 LayoutInflater 的 inflate() 方法来完成的

LayoutInflater对象如何实例化

public abstract class LayoutInflater{
}

抽象类不能实例化,获取LayoutInflater对象的方式

  1. 在 Activity 中通过 getLayoutInflater()
  2. 通过 LayoutInflater.from(context)
  3. context.getSystemService(Context.LAYOUT_INFLATER_SERVICE)

进入from方法

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

是通过服务获取LayoutInflater实例对象。

源码分析

context是一个抽象类，实现类是ContextImpl

进入ContextImpl类的getSystemService方法

public Object getSystemService(String name) {
    if (vmIncorrectContextUseEnabled()) {
        // Check incorrect Context usage.
        if (WINDOW_SERVICE.equals(name) && !isUiContext()) {
            final String errorMessage = "Tried to access visual service "
                    + SystemServiceRegistry.getSystemServiceClassName(name)
                    + " from a non-visual Context:" + getOuterContext();
            final String message = "WindowManager should be accessed from Activity or other "
                    + "visual Context. Use an Activity or a Context created with "
                    + "Context#createWindowContext(int, Bundle), which are adjusted to "
                    + "the configuration and visual bounds of an area on screen.";
            final Exception exception = new IllegalAccessException(errorMessage);
            StrictMode.onIncorrectContextUsed(message, exception);
            Log.e(TAG, errorMessage + " " + message, exception);
        }
    }
    return SystemServiceRegistry.getSystemService(this, name);
}

进入SystemServiceRegistry类的getSystemService方法

public static Object getSystemService(ContextImpl ctx, String name) {
    if (name == null) {
        return null;
    }
    final ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
    if (fetcher == null) {
        if (sEnableServiceNotFoundWtf) {
            Slog.wtf(TAG, "Unknown manager requested: " + name);
        }
        return null;
    }

    final Object ret = fetcher.getService(ctx);
    if (sEnableServiceNotFoundWtf && ret == null) {
        // Some services do return null in certain situations, so don't do WTF for them.
        switch (name) {
            case Context.CONTENT_CAPTURE_MANAGER_SERVICE:
            case Context.APP_PREDICTION_SERVICE:
            case Context.INCREMENTAL_SERVICE:
            case Context.ETHERNET_SERVICE:
                return null;
        }
        Slog.wtf(TAG, "Manager wrapper not available: " + name);
        return null;
    }
    return ret;
}

SystemServiceRegistry主要是用来缓存、注册、获取系统服务的。

我们找到了 Context.LAYOUT_INFLATER_SERVICE 注册代码

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

 private static <T> void registerService(String serviceName, Class<T> serviceClass,
         ServiceFetcher<T> serviceFetcher) {
     SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
     SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
 }

LayoutInflater 的实现类是 PhoneLayoutInflater

LayoutInflater的作用读取xml文件并创建view对象

public View inflate(XmlPullParser parser, @Nullable ViewGroup root) {
    return inflate(parser, root, root != null);
}

进入inflate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) + ")");
    }

    View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
    if (view != null) {
        return view;
    }
    XmlResourceParser parser = res.getLayout(resource);
    try {
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}

通过 res 对象把 resId 指向的 xml 文件转换为 XmlResourceParser 然后执行 inflate(parser, root, attachToRoot) 方法

进入inflate方法

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 {
            advanceToRootNode(parser);
            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)) {
              //xml根标签是merge，则root不能为空，attachToRoot必须是true
                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 {
                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);
                    }
                    params = root.generateLayoutParams(attrs);
                    if (!attachToRoot) {
                        temp.setLayoutParams(params);
                    }
                }

                if (DEBUG) {
                    System.out.println("-----> start inflating children");
                }

                rInflateChildren(parser, temp, attrs, true);

                if (DEBUG) {
                    System.out.println("-----> done inflating children");
                }
                if (root != null && attachToRoot) {
                    root.addView(temp, params);
                }

                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(
                    getParserStateDescription(inflaterContext, attrs)
                    + ": " + 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;
    }
}

进入rInflate方法

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();
    }
}

这个方法会执行的操作

  1. 遍历该节点的子节点
  2. 子节点有 "requestFocus"、"tag"、""、"include"
  3. 子节点不能是 "merge"
  4. 子节点的其他情况，则是各种 View 的标签
  5. View 标签和 "include" 标签会创建 View 对象
  6. 遍历结束以后执行 parent.onFinishInflate()

如果子节点是 include 则执行 parseInclude() ,parseInclude() 的源码和 inflate(parser, root, attachToRoot) 类似，都是读取xml对应的文件，转换成 XmlResourceParser 然后遍历里的标签

创建View的代码：

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);

createViewFromTag 负责创建View对象

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

进入createViewFromTag方法

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();
    }

    try {
        View view = tryCreateView(parent, name, context, attrs);

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

        return view;
    } catch (InflateException e) {
        throw e;

    } catch (ClassNotFoundException e) {
        final InflateException ie = new InflateException(
                getParserStateDescription(context, attrs)
                + ": Error inflating class " + name, e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;

    } catch (Exception e) {
        final InflateException ie = new InflateException(
                getParserStateDescription(context, attrs)
                + ": Error inflating class " + name, e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    }
}

进入tryCreateView方法

public final View tryCreateView(@Nullable View parent, @NonNull String name,
    @NonNull Context context,
    @NonNull AttributeSet attrs) {
    if (name.equals(TAG_1995)) {
        // Let's party like it's 1995!
        return new BlinkLayout(context, attrs);
    }

    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);
    }

    return view;
}

如果 name属性里面含有 . 表示这是一个自定义 View，系统自带 View 我们可以省略类的路径，而自定义 View 则不能省略。

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

onCreateView和createView最后都会走到createView方法中

public final View createView(@NonNull Context viewContext, @NonNull String name,
        @Nullable String prefix, @Nullable AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
    Objects.requireNonNull(viewContext);
    Objects.requireNonNull(name);
    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) {
            clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                    mContext.getClassLoader()).asSubclass(View.class);

            if (mFilter != null && clazz != null) {
                boolean allowed = mFilter.onLoadClass(clazz);
                if (!allowed) {
                    failNotAllowed(name, prefix, viewContext, attrs);
                }
            }
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            sConstructorMap.put(name, constructor);
        } else {
            if (mFilter != null) {
                Boolean allowedState = mFilterMap.get(name);
                if (allowedState == null) {
                    clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                            mContext.getClassLoader()).asSubclass(View.class);

                    boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                    mFilterMap.put(name, allowed);
                    if (!allowed) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                } else if (allowedState.equals(Boolean.FALSE)) {
                    failNotAllowed(name, prefix, viewContext, attrs);
                }
            }
        }

        Object lastContext = mConstructorArgs[0];
        mConstructorArgs[0] = viewContext;
        Object[] args = mConstructorArgs;
        args[1] = attrs;

        try {
            //反射创建View
            final View view = constructor.newInstance(args);
            if (view instanceof ViewStub) {
                final ViewStub viewStub = (ViewStub) view;
                viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
            }
            return view;
        } finally {
            mConstructorArgs[0] = lastContext;
        }
    } catch (NoSuchMethodException e) {
        final InflateException ie = new InflateException(
                getParserStateDescription(viewContext, attrs)
                + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;

    } catch (ClassCastException e) {
        final InflateException ie = new InflateException(
                getParserStateDescription(viewContext, attrs)
                + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } catch (ClassNotFoundException e) {
        throw e;
    } catch (Exception e) {
        final InflateException ie = new InflateException(
                getParserStateDescription(viewContext, attrs) + ": Error inflating class "
                        + (clazz == null ? "<unknown>" : clazz.getName()), e);
        ie.setStackTrace(EMPTY_STACK_TRACE);
        throw ie;
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

最后发现是通过反射创建view对象。

PhoneLayoutInflater分析

上面我知道LayoutInflater的实现类是PhoneLayoutInflater，我们看一下这个类

public class PhoneLayoutInflater extends LayoutInflater {
    private static final String[] sClassPrefixList = {
        "android.widget.",
        "android.webkit.",
        "android.app."
    };
    public PhoneLayoutInflater(Context context) {
        super(context);
    }

    protected PhoneLayoutInflater(LayoutInflater original, Context newContext) {
        super(original, newContext);
    }
    @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);
    }

    public LayoutInflater cloneInContext(Context newContext) {
        return new PhoneLayoutInflater(this, newContext);
    }
}

我们在看看LayoutInflater中的onCreateView方法

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

对于系统内置的 View，会依次在 View 的标签前面加上"android.widget."、"android.webkit.","android.app." 、"android.view." 然后通过反射的方法创建 View。

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