View 绘制体系知识梳理(2) setContentView 源码解析

1、概述

在Activity当中，咱们通常都会调用setContentView方法来初始化布局。

2、与ContentView相关的方法

在Activity当中，与ContentView相关的函数有下面这几个，咱们先看一下它们的注释说明：

/**
     * Set the activity content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the activity.
     *
     * @param layoutResID Resource ID to be inflated.
     *
     * @see #setContentView(android.view.View)
     * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
     */
    public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

    /**
     * Set the activity content to an explicit view.  This view is placed
     * directly into the activity's view hierarchy. It can itself be a complex * view hierarchy. When calling this method, the layout parameters of the * specified view are ignored. Both the width and the height of the view are * set by default to {@link ViewGroup.LayoutParams#MATCH_PARENT}. To use * your own layout parameters, invoke * {@link #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)} * instead. * * @param view The desired content to display. * * @see #setContentView(int) * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams) */ public void setContentView(View view) { getWindow().setContentView(view); initWindowDecorActionBar(); } /** * Set the activity content to an explicit view. This view is placed * directly into the activity's view hierarchy.  It can itself be a complex
     * view hierarchy.
     *
     * @param view The desired content to display.
     * @param params Layout parameters for the view.
     *
     * @see #setContentView(android.view.View)
     * @see #setContentView(int)
     */
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        getWindow().setContentView(view, params);
        initWindowDecorActionBar();
    }

    /**
     * Add an additional content view to the activity.  Added after any existing
     * ones in the activity -- existing views are NOT removed.
     *
     * @param view The desired content to display.
     * @param params Layout parameters for the view.
     */
    public void addContentView(View view, ViewGroup.LayoutParams params) {
        getWindow().addContentView(view, params);
        initWindowDecorActionBar();
    }
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经过上面的注释，能够看到这4个方法的用途：

• 第一种：渲染layouResId对应的布局，并将它添加到activity的顶级View中。
• 第二种：将View添加到activity的布局当中，它的默认宽高都是ViewGroup.LayoutParams#MATCH_PARENT。
• 第三种：和上面相同，可是指定了LayoutParams。
• 第四种：将内容添加进去，而且必须指定LayoutParams，已经存在的View不会被移除。

这四种方法其实都是调用了PhoneWindow.java中的方法，经过源码咱们能够发现setContentView(View view, ViewGroup.LayoutParams params)和setContentView(@LayoutRes int layoutResID)的步骤基本上是同样的，只不过是在添加到布局的时候，前者由于已经得到了View的实例，所以用的是addView的方法，然后者由于须要先inflate，因此，使用的是LayoutInflater。

3、setContentView方法

下面咱们以setContentView(@LayoutRes int layoutResID)为例，看一下具体的实现步骤：

3.1 setContentView

@Override
    public void setContentView(int layoutResID) {
        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
        // decor, when theme attributes and the like are crystalized. Do not check the feature
        // before this happens.
        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;
    }
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首先，咱们会判断mContentParent是否为空，经过添加的代码咱们能够知道，这个mContentParent其实就是layoutResId最后渲染出的布局所对应的父容器，当这个ContentParent为空时，调用了installDecor，mContentParent就是在里面初始化的。

3.2 installDecor()

private void installDecor() {
        //若是DecorView不存在，那么先生成它，它实际上是一个FrameLayout。
        if (mDecor == null) {
            mDecor = generateDecor();
        }
        //若是`ContentParent`不存在，那么也生成它，此时传入了前面的`DecorView`
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);
            final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(R.id.decor_content_parent);
            if (decorContentParent != null) {
                mDecorContentParent = decorContentParent;
            }
        }
    }
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咱们能够看到，mDecor是一个FrameLayout，它和mContentParent的关系是经过mContentParent = generateLayout(mDecor)产生。

3.3 generateLayout(DecorView decor)

protected ViewGroup generateLayout(DecorView decor) {
        //...首先根据不一样的状况，给`layoutResource`赋予不一样的值.
        View in = mLayoutInflater.inflate(layoutResource, null);
        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        mContentRoot = (ViewGroup) in;
        ViewGroup contentParent = (ViewGroup) findViewById(ID_ANDROID_CONTENT);
        if (contentParent == null) {
            throw new RuntimeException("Window couldn't find content container view");
        }
        //...
        return contentParent;
    }
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在上面赋值的过程当中，咱们主要关注如下几个变量，mContentRoot/mContentParent/mDecorContent：

• mContentRoot必定是mDecor的下一级子容器。
• mContentParent是mDecor当中id为R.id.content的ViewGroup，可是它和mDecor的具体层级关系不肯定，这依赖于mContentRoot是经过哪一个xml渲染出来。
• mContentParent必定是传入的layoutResId进行 inflate完成以后的父容器，它必定不会为空，不然会抛出异常，咱们setContentView(xxx)方法传入的布局，就是它的子View。

// This is the top-level view of the window, containing the window decor.
    private DecorView mDecor;

    // This is the view in which the window contents are placed. It is either
    // mDecor itself, or a child of mDecor where the contents go.
    private ViewGroup mContentParent;
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• mDecorContent则是mDecor当中id为decor_content_parent的ViewGroup，可是也有可能mDecor 当中没有这个id的View，这须要依赖与咱们的mContentRoot是使用了哪一个xml来inflate的。

再回到前面setContentView的地方，继续往下看，当mContentParent不为空的时候，那么会移除它底下的全部子View。 以后会调用mLayoutInflater.inflate(layoutResID, mContentParent);方法，把传入的View添加到mContentParent当中，最后回调一个监听。

final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
    cb.onContentChanged();
}
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3.4 mContentParentExplicitlySet标志位

在setContentView的最后，将mContentParentExplicitlySet这个变量设置为true，这个变量实际上是用在requestFeature当中，也就是说，咱们必须在调用setContentView以前，调用requestFeature，不然就会抛出下面的异常：

@Override
    public boolean requestFeature(int featureId) {
        if (mContentParentExplicitlySet) {
            throw new AndroidRuntimeException("requestFeature() must be called before adding content");
        }
        return super.requestFeature(featureId);
    }
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所以：requestFeature(xxx)必需要在调用setContentView(xxx)以前。

3、addContentView(View view, ViewGroup.LayoutParams params)

下面咱们再来看一下，addContentView方法：

@Override
    public void addContentView(View view, ViewGroup.LayoutParams params) {
        if (mContentParent == null) {
            installDecor();
        }
        mContentParent.addView(view, params);
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }
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能够看到，它和set方法的区别就是，它在添加到mContentParent以前，并无把mContentParent的全部子View都移除，而是将它直接添加进去，经过布局分析软件，能够看到mContentParent的类型为ContentFrameLayout，它实际上是一个FrameLayout，所以，它会覆盖在mContentParent已有子View之上。

4、将添加的布局和Activity的Window关联起来

在上面的分析当中，咱们仅仅是初始化了一个DecorView，并根据设置的Style属性，传入的ContentView来初始化它的子布局，可是这时候它还有真正和Activity的Window关联起来，关联的地方在ActivityThread.java中：

final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
        r = performResumeActivity(token, clearHide, reason);
        if (r != null) {
            if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                View decor = r.window.getDecorView();
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
                if (r.mPreserveWindow) {
                    a.mWindowAdded = true;
                    r.mPreserveWindow = false;
                    // Normally the ViewRoot sets up callbacks with the Activity
                    // in addView->ViewRootImpl#setView. If we are instead reusing
                    // the decor view we have to notify the view root that the
                    // callbacks may have changed.
                    ViewRootImpl impl = decor.getViewRootImpl();
                    if (impl != null) {
                        impl.notifyChildRebuilt();
                    }
                }
                if (a.mVisibleFromClient && !a.mWindowAdded) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                }
            } else if (!willBeVisible) {
                if (localLOGV) Slog.v(
                    TAG, "Launch " + r + " mStartedActivity set");
                r.hideForNow = true;
            }
        } else {

        }
    }
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从源码中能够看到，若是在执行handleResumeActivity时，以前DecorView没有被添加到WindowManager当中时，那么它的第一次添加是在onResume()方法执行完以后添加的。