从0开始写android

系列文章目录

文章目录

一、从0开始实现 onCreate 的setContentView
二、从0 开始实现 onMeasure
三、从0 开始实现 onLayout
四、从0 开始实现 onDraw
总结

前言

接上文，测量完View树的每个节点View的宽和高后，开始布局。

一、ViewRootImpl 的调用栈

ViewRootImpl->performLayout(lp, mWidth, mHeight);

host.layout()

ViewGroup -> layout(int l, int t, int r, int b)

View -> layout(int l, int t, int r, int b)

View ->setFrame(l, t, r, b)

onLayout(changed, l, t, r, b)

DecorView-> onLayout(changed, l, t, r, b)

FrameLayout->onLayout()

layoutChildren()

二、布局每个View

View.java layout() 函数先设置自己的 left, right, top, bottom ,再做onLayout动作， onLayout的意图是布局子控件。 view 已经没有子控件了，所以 view的onLayout 什么也不做。

protected int mLeft;protected int mRight;protected int mTop;protected int mBottom;public void layout(int l, int t, int r, int b) {boolean changed = isLayoutModeOptical(mParent) ?setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {onLayout(changed, l, t, r, b);
}}protected boolean setFrame(int left, int top, int right, int bottom) {...mLeft = left;mTop = top;mRight = right;mBottom = bottom;...
}protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

由于 ViewGroup 的 onLayout 是虚函数, 所以 ViewGroup 需要实现自己的onLayout

   public final void layout(int l, int t, int r, int b) {if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {if (mTransition != null) {mTransition.layoutChange(this);}super.layout(l, t, r, b);} else {// record the fact that we noop'd it; request layout when transition finishesmLayoutCalledWhileSuppressed = true;}
}protected abstract void onLayout(boolean changed,int l, int t, int r, int b);