Activity中setContentView做了什么

我们在开发中每创建一个Activity的同时会通过setContentView（R.layout.xxx）来为其添加布局文件，那么这个布局文件是如何加载进来的呢？下面简单结合源码（8.0）进行分析。

我们默认写的Activity继承的是AppCompatActivity，跟踪我们的Activity到AppCompatActivity中，代码如下：

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

继续跟踪setContentView(layoutResID)，会进入到AppCompatDelegate中，看到注释说明该方法会取代activity中的setContentView()的调用

   /**
     * Should be called instead of {@link Activity#setContentView(int)}}
     */
    public abstract void setContentView(@LayoutRes int resId);

AppCompatDelegate是一个抽象类，那么我们需要找到该方法的真正实现类，继续跟踪，进入到真正的实现类AppCompatDelegateImpl中

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

首先看一下ensureSubDecor()，此方法是在装载view之前做的一些准备，这里借用一张图方便理解

方法如下，不做深入研究了，感兴趣的朋友可以点击深入查看

private void ensureSubDecor() {
        if (!mSubDecorInstalled) {
            mSubDecor = createSubDecor();

            // If a title was set before we installed the decor, propagate it now
            CharSequence title = getTitle();
            if (!TextUtils.isEmpty(title)) {
                if (mDecorContentParent != null) {
                    mDecorContentParent.setWindowTitle(title);
                } else if (peekSupportActionBar() != null) {
                    peekSupportActionBar().setWindowTitle(title);
                } else if (mTitleView != null) {
                    mTitleView.setText(title);
                }
            }

            applyFixedSizeWindow();

            onSubDecorInstalled(mSubDecor);

            mSubDecorInstalled = true;

            // Invalidate if the panel menu hasn't been created before this.
            // Panel menu invalidation is deferred avoiding application onCreateOptionsMenu
            // being called in the middle of onCreate or similar.
            // A pending invalidation will typically be resolved before the posted message
            // would run normally in order to satisfy instance state restoration.
            PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
            if (!mIsDestroyed && (st == null || st.menu == null)) {
                invalidatePanelMenu(FEATURE_SUPPORT_ACTION_BAR);
            }
        }
    }

思路回到AppCompatDelegateImpl中的setContentView，为了不用翻上去看，这里再贴一遍

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

结合上面DectorView的结构图我们可以很清晰的看到，这里通过
LayoutInflater.from(mContext).inflate(resId, contentParent)
将我们传入的布局id转换成view并添加到DecorView中，到此布局文件就转换成view加载到DecorView中了