Android findViewById()执行流程

虽然现在都不手写findViewById，但还是有必要看一下源码流程。

    val tv = findViewById<TextView>(R.id.tv)

AppCompatActivity.findViewById()

    public <T extends View> T findViewById(@IdRes int id) {
        return getDelegate().findViewById(id);
    }

getDelegate()返回AppCompatActivity代理类实例AppCompatDelegateImpl

AppCompatDelegateImpl.findViewById()

    public <T extends View> T findViewById(@IdRes int id) {
        ensureSubDecor();
        return (T) mWindow.findViewById(id);
    }

Window.findViewById()

    public <T extends View> T findViewById(@IdRes int id) {
        return getDecorView().findViewById(id);
    }

Window.getDecorView()是个抽象方法，Window实现类PhoneWindow重写了该方法

PhoneWindow.getDecorView()

    private DecorView mDecor;

    public final View getDecorView() {
        if (mDecor == null || mForceDecorInstall) {
            installDecor();
        }
        return mDecor;
    }

DecorView继承自FrameLayout是个ViewGroup

DecorView.findViewById()--->View.findViewById()

    public final <T extends View> T findViewById(@IdRes int id) {
        if (id == NO_ID) {
            return null;
        }
        return findViewTraversal(id);
    }

ViewGroup重写了findViewTraversal()方法

ViewGroup.findViewTraversal()

    @Override
    protected <T extends View> T findViewTraversal(@IdRes int id) {
        if (id == mID) {
            return (T) this;
        }

        final View[] where = mChildren;
        final int len = mChildrenCount;

        for (int i = 0; i < len; i++) {
            View v = where[i];

            if ((v.mPrivateFlags & PFLAG_IS_ROOT_NAMESPACE) == 0) {
                v = v.findViewById(id);

                if (v != null) {
                    return (T) v;
                }
            }
        }

        return null;
    }

遍历调用子View的findViewById()，继续跟进

View.findViewTraversal()

    protected <T extends View> T findViewTraversal(@IdRes int id) {
        if (id == mID) {
            return (T) this;
        }
        return null;
    }

id == mID则返回此View

流程很简单。findViewById()方法从Window开始，调用顶层View>DecorView.findViewById()。DecorView是ViewGroup，遍历调用子View的findViewById()，深度递归，直到找到id相同的View。