Android学习笔记：浅析setContentView

  每次在Github上看到炫酷效果的demo时，都会想，什么时候自己也能实现这样的效果。要想有所收获，就必须得付出。于是，我决定花时间来学习自定义View，虽说自定义View水很深，但凡事得一步步来，终会有所收获的。要实现自定义View，必须得先了解View的绘制流程，但在学习View的绘制流程之前，先来了解下setContentView这个方法时如何将XML布局加载到当前Activity？

  从Activity源码可以看出，setContentView方法有三个重载方法，分别是：

• setContentView(int layoutResId)：将布局文件加载到当前Activity，并且在Activity充当父布局角色。

• setContentView(View view)：将View加载到当前Activity。当调用这个方法设置View时，自己对View设置布局参数宽高会被忽略，默认的宽高是android.view.ViewGroup.LayoutParams.MATCH_PARENT。如果想使用自己定义的布局参数，可以使用setContentView(View view, ViewGroup.LayoutParams params)这个方法代替。

• setContentView(View view, ViewGroup.LayoutParams params)：将View加载到当前Activity。

从源码来看，这个三个重载方法的实现逻辑都差不多，所以我只分析setContentView(int layoutResId).

Step 1

  进入到Activity源码，找到setContentView(int layoutResId)方法如下：

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

只有两行代码，有木有一种挺简单的感觉。其实不然，往下看就知道有多复杂了。从第2行看起，进入getWindow()这个方法，代码如下

public Window getWindow() {    return mWindow;}

获取Activity当前Window对象，Window是抽象类，不能实例化对象，那么是怎么获取到Window对象？在Activity源码中attach方法有这样的一行代码

mWindow = PolicyManager.makeNewWindow(this);

很明显，这是获取Window对象的，继续往下看。进入PolicyManager类makeNewWindow()方法

public static Window makeNewWindow(Context context) {    return sPolicy.makeNewWindow(context);}

那么sPolicy又是什么呢，代码如下：

private static final String POLICY_IMPL_CLASS_NAME =        "com.android.internal.policy.impl.Policy";private static final IPolicy sPolicy;static {        // Pull in the actual implementation of the policy at run-time    try {        Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);        sPolicy = (IPolicy)policyClass.newInstance();    } catch (ClassNotFoundException ex) {        throw new RuntimeException(            POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);    } catch (InstantiationException ex) {        throw new RuntimeException(        POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);    } catch (IllegalAccessException ex) {        throw new RuntimeException(        POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);    }}

从以上代码可以看出，IPolicy是接口，通过反射机制获取到sPolicy，该接口的实现类是Policy，所以定位到该类makeNewWindow()方法

public Window makeNewWindow(Context context) {    return new PhoneWindow(context);}

终于获取到Window对象，该对象的类型是PhoneWindow。那么PhoneWindow又是啥呢？ 进入PhoneWindow该类一看，原来该类是继承抽象类Window，并重写Window类抽象方法。可以这么理解，Activity的窗口是PhoneWindow。获取到Window对象也就结束了。

Step2

  进入PhoneWindow类setContentView(int layoutResId)方法中

@Overridepublic 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);    }    final Callback cb = getCallback();    if (cb != null && !isDestroyed()) {            cb.onContentChanged();    }}

一开始进入就判断mContentParent(类型是ViewGroup)是否为null，由于一开始加载布局，所以mContentParent为null。于是执行if语句，即执行installDecor()方法，于是进入到该方法代码实现

private void installDecor() {    if (mDecor == null) {        mDecor = generateDecor();                    mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);            mDecor.setIsRootNamespace(true);    }    if (mContentParent == null) {        mContentParent = generateLayout(mDecor);        mTitleView =(TextView)findViewById(com.android.internal.R.id.title);        if (mTitleView != null) {            if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE))!= 0) {                View titleContainer = findViewById(com.android.internal.R.id.title_container);                if (titleContainer != null) {                    titleContainer.setVisibility(View.GONE);                } else {                    mTitleView.setVisibility(View.GONE);                }                if (mContentParent instanceof FrameLayout) {                       ((FrameLayout)mContentParent).setForeground(null);                }            } else {                mTitleView.setText(mTitle);            }        } else {            mActionBar = (ActionBarView) findViewById(com.android.internal.R.id.action_bar);            if (mActionBar != null) {                if (mActionBar.getTitle() == null) {                    mActionBar.setWindowTitle(mTitle);                }                final int localFeatures = getLocalFeatures();                if ((localFeatures & (1 << FEATURE_PROGRESS)) != 0) {                    mActionBar.initProgress();                }                if ((localFeatures & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {                    mActionBar.initIndeterminateProgress();                }                // Post the panel invalidate for later; avoid application onCreateOptionsMenu                // being called in the middle of onCreate or similar.                mDecor.post(new Runnable() {                    public void run() {                        if (!isDestroyed()) {                                             invalidatePanelMenu(FEATURE_ACTION_BAR);                        }                    }                });            }        }    }}

这个方法有点长，慢慢来看吧。（PS：说真的，我不是百分之百理解，大概理解流程而已）。刚进入，就判断mDecor（类型为DecorView）是否为null。由于一开始mDecor为null，所以执行if语句，即执行

mDecor = generateDecor();

这里调用generateDecor()方法获取DecorView对象，那么是如何获取呢，进去瞧一瞧

protected DecorView generateDecor() {    return new DecorView(getContext(), -1);}

很容易理解吧，直接new一个对象，就获取到DecorView实例。再回到installDecor()方法中，对于if语句后面的几行代码，主要是对获取到DecorView对象进行设置。接下来看第7行代码，也是if语句，判断mContentParent是否为null，由于一开始进入为null，所以执行if语句，即执行

mContentParent = generateLayout(mDecor);

这里将在之前获取到的mDecor作为参数传递到generateLayout()方法中，然后返回ViewGroup赋值给mContentParent。那么该方法是如何实现呢，进入该方法代码实现

protected ViewGroup generateLayout(DecorView decor) {        // Apply data from current theme.        TypedArray a = getWindowStyle();        if (false) {            System.out.println("From style:");            String s = "Attrs:";            for (int i = 0; i < com.android.internal.R.styleable.Window.length; i++) {                s = s + " " + Integer.toHexString(com.android.internal.R.styleable.Window[i]) + "="                        + a.getString(i);            }            System.out.println(s);        }        mIsFloating = a.getBoolean(com.android.internal.R.styleable.Window_windowIsFloating, false);        int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)                & (~getForcedWindowFlags());        if (mIsFloating) {            setLayout(WRAP_CONTENT, WRAP_CONTENT);            setFlags(0, flagsToUpdate);        } else {            setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);        }        if (a.getBoolean(com.android.internal.R.styleable.Window_windowNoTitle, false)) {            requestFeature(FEATURE_NO_TITLE);        } else if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionBar, false)) {            // Don't allow an action bar if there is no title.            requestFeature(FEATURE_ACTION_BAR);        }        if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionBarOverlay, false)) {            requestFeature(FEATURE_ACTION_BAR_OVERLAY);        }        if (a.getBoolean(com.android.internal.R.styleable.Window_windowActionModeOverlay, false)) {            requestFeature(FEATURE_ACTION_MODE_OVERLAY);        }        if (a.getBoolean(com.android.internal.R.styleable.Window_windowFullscreen, false)) {            setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN&(~getForcedWindowFlags()));        }        if (a.getBoolean(com.android.internal.R.styleable.Window_windowShowWallpaper, false)) {            setFlags(FLAG_SHOW_WALLPAPER, FLAG_SHOW_WALLPAPER&(~getForcedWindowFlags()));        }        if (a.getBoolean(com.android.internal.R.styleable.Window_windowEnableSplitTouch,                getContext().getApplicationInfo().targetSdkVersion                        >= android.os.Build.VERSION_CODES.HONEYCOMB)) {            setFlags(FLAG_SPLIT_TOUCH, FLAG_SPLIT_TOUCH&(~getForcedWindowFlags()));        }        a.getValue(com.android.internal.R.styleable.Window_windowMinWidthMajor, mMinWidthMajor);        a.getValue(com.android.internal.R.styleable.Window_windowMinWidthMinor, mMinWidthMinor);        if (getContext().getApplicationInfo().targetSdkVersion                < android.os.Build.VERSION_CODES.HONEYCOMB) {            addFlags(WindowManager.LayoutParams.FLAG_NEEDS_MENU_KEY);        }        if (mAlwaysReadCloseOnTouchAttr || getContext().getApplicationInfo().targetSdkVersion                >= android.os.Build.VERSION_CODES.HONEYCOMB) {            if (a.getBoolean(                    com.android.internal.R.styleable.Window_windowCloseOnTouchOutside,                    false)) {                setCloseOnTouchOutsideIfNotSet(true);            }        }        WindowManager.LayoutParams params = getAttributes();        if (!hasSoftInputMode()) {            params.softInputMode = a.getInt(                    com.android.internal.R.styleable.Window_windowSoftInputMode,                    params.softInputMode);        }        if (a.getBoolean(com.android.internal.R.styleable.Window_backgroundDimEnabled,                mIsFloating)) {            /* All dialogs should have the window dimmed */            if ((getForcedWindowFlags()&WindowManager.LayoutParams.FLAG_DIM_BEHIND) == 0) {                params.flags |= WindowManager.LayoutParams.FLAG_DIM_BEHIND;            }            params.dimAmount = a.getFloat(                    android.R.styleable.Window_backgroundDimAmount, 0.5f);        }        if (params.windowAnimations == 0) {            params.windowAnimations = a.getResourceId(                    com.android.internal.R.styleable.Window_windowAnimationStyle, 0);        }        // The rest are only done if this window is not embedded; otherwise,        // the values are inherited from our container.        if (getContainer() == null) {            if (mBackgroundDrawable == null) {                if (mBackgroundResource == 0) {                    mBackgroundResource = a.getResourceId(                            com.android.internal.R.styleable.Window_windowBackground, 0);                }                if (mFrameResource == 0) {                    mFrameResource = a.getResourceId(com.android.internal.R.styleable.Window_windowFrame, 0);                }                if (false) {                    System.out.println("Background: "                            + Integer.toHexString(mBackgroundResource) + " Frame: "                            + Integer.toHexString(mFrameResource));                }            }            mTextColor = a.getColor(com.android.internal.R.styleable.Window_textColor, 0xFF000000);        }        // Inflate the window decor.        int layoutResource;        int features = getLocalFeatures();        // System.out.println("Features: 0x" + Integer.toHexString(features));        if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {            if (mIsFloating) {                TypedValue res = new TypedValue();                getContext().getTheme().resolveAttribute(                        com.android.internal.R.attr.dialogTitleIconsDecorLayout, res, true);                layoutResource = res.resourceId;            } else {                layoutResource = com.android.internal.R.layout.screen_title_icons;            }            // XXX Remove this once action bar supports these features.            removeFeature(FEATURE_ACTION_BAR);            // System.out.println("Title Icons!");        } else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0                && (features & (1 << FEATURE_ACTION_BAR)) == 0) {            // Special case for a window with only a progress bar (and title).            // XXX Need to have a no-title version of embedded windows.            layoutResource = com.android.internal.R.layout.screen_progress;            // System.out.println("Progress!");        } else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {            // Special case for a window with a custom title.            // If the window is floating, we need a dialog layout            if (mIsFloating) {                TypedValue res = new TypedValue();                getContext().getTheme().resolveAttribute(                        com.android.internal.R.attr.dialogCustomTitleDecorLayout, res, true);                layoutResource = res.resourceId;            } else {                layoutResource = com.android.internal.R.layout.screen_custom_title;            }            // XXX Remove this once action bar supports these features.            removeFeature(FEATURE_ACTION_BAR);        } else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {            // If no other features and not embedded, only need a title.            // If the window is floating, we need a dialog layout            if (mIsFloating) {                TypedValue res = new TypedValue();                getContext().getTheme().resolveAttribute(                        com.android.internal.R.attr.dialogTitleDecorLayout, res, true);                layoutResource = res.resourceId;            } else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {                if ((features & (1 << FEATURE_ACTION_BAR_OVERLAY)) != 0) {                    layoutResource = com.android.internal.R.layout.screen_action_bar_overlay;                } else {                    layoutResource = com.android.internal.R.layout.screen_action_bar;                }            } else {                layoutResource = com.android.internal.R.layout.screen_title;            }            // System.out.println("Title!");        } else {            // Embedded, so no decoration is needed.            layoutResource = com.android.internal.R.layout.screen_simple;            // System.out.println("Simple!");        }        mDecor.startChanging();        View in = mLayoutInflater.inflate(layoutResource, null);        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);        if (contentParent == null) {            throw new RuntimeException("Window couldn't find content container view");        }        if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {            ProgressBar progress = getCircularProgressBar(false);            if (progress != null) {                progress.setIndeterminate(true);            }        }        // Remaining setup -- of background and title -- that only applies        // to top-level windows.        if (getContainer() == null) {            Drawable drawable = mBackgroundDrawable;            if (mBackgroundResource != 0) {                drawable = getContext().getResources().getDrawable(mBackgroundResource);            }            mDecor.setWindowBackground(drawable);            drawable = null;            if (mFrameResource != 0) {                drawable = getContext().getResources().getDrawable(mFrameResource);            }            mDecor.setWindowFrame(drawable);            // System.out.println("Text=" + Integer.toHexString(mTextColor) +            // " Sel=" + Integer.toHexString(mTextSelectedColor) +            // " Title=" + Integer.toHexString(mTitleColor));            if (mTitleColor == 0) {                mTitleColor = mTextColor;            }            if (mTitle != null) {                setTitle(mTitle);            }            setTitleColor(mTitleColor);        }        mDecor.finishChanging();        return contentParent;    }

这个方法真的很长，我根据自己的理解总结下这个方法的主要功能：
- 设置窗口属性
- 设置对应布局文件layoutResource
- 加载布局文件layoutResource
- 将加载获取到View添加到mDecor
- 设置窗口背景

由于个人能力有限，所以只能简单地分析下。定位到代码

View in = mLayoutInflater.inflate(layoutResource, null);

根据之前各种条件获取到对应布局文件id，即layoutResource，再调用LayoutInflater.inflate加载布局获取到对应View（如果想理解LayoutInflater原理，可以参考这篇博客Android LayoutInflater原理分析，带你一步步深入了解View(一)）。接着再看

decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));

把上一步加载布局文件获取到的View添加到mDecor，接着

ViewGroup contentParent =(ViewGroup)findViewById(ID_ANDROID_CONTENT);

通过findViewById**(ID_ANDROID_CONTENT = com.android.internal.R.id.content)获取到contentParent，最终将**contentParent作为返回值，赋值给mContentParent（记得id），这样就获取到mContentParent。再回到installDecor()方法中，剩下的逻辑就自己看吧。再回到setContentView(int layoutResId)，有一行关键的代码

mLayoutInflater.inflate(layoutResID, mContentParent);

这代码挺熟悉吧，这行代码说明加载XML布局到当前Activity还有一个父布局，那就是mContentParent，它是一个FrameLayout布局，资源id为R.id.content。到此也就了解了XML布局是怎样加载到当前Activity的流程。

  以上就是自己对该知识点的理解，把它记录下来作为笔记。若哪里有错误，欢迎指出，大家一起学习，共同进步！

参考博客链接