自定义 View 之 布局(layout)

你家房子装修,你想在一面墙上画一幅画,你告诉工人:“给我画一副长两米高一米的风景画”(measure),工人就该问你了:“画在墙上的什么位置?”,这个位置的确定,就是布局(Layout)过程完成的。

Layout 的过程实际上和 Measure 类似,也是从上到下的去执行布局操作。

在 setContentView 那篇文章中,我们讲了在 RootViewImpl 对象的 performTraversals 方法中,会执行 performXXX 开始测量、布局和绘制过程:

    private void performTraversals() {
        ...
        if (mFirst || windowShouldResize || insetsChanged ||
                viewVisibilityChanged || params != null || mForceNextWindowRelayout) {
            ...
            if (!mStopped || mReportNextDraw) {
                ...
                     // Ask host how big it wants to be
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    ...
                }
            }
        } else {
            ...
        }
        ...
        if (didLayout) {
            performLayout(lp, mWidth, mHeight);
            ...
        }
        ...
        if (!cancelDraw && !newSurface) {
            ...
            performDraw();
        } else {
            ...
        }
        mIsInTraversal = false;
    }

RootViewImpl.performTraversals().performLayout


private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { //省略好几行代码 final View host = mView; //又省略好几行代码 try { host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); //依旧省略好几行代码 } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false; }

首先我们确定 mView 实际上是一个 DecorView,实际上是调用了 DecorView 的 layout 方法,DecorView 及其父类 FrameLayout 都没有 layout,只能看 FrameLayout 的父类 ViewGroup 的 layout 方法了.

ViewGroup.layout

    @Override
    public final void layout(int l, int t, int r, int b) {
        if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
            if (mTransition != null) {
                mTransition.layoutChange(this);
            }
            //【重点】
            super.layout(l, t, r, b);
        } else {
            // record the fact that we noop'd it; request layout when transition finishes
            mLayoutCalledWhileSuppressed = true;
        }
    }

调用了父类 View 的 layout 方法:

View.layout

    public void layout(int l, int t, int r, int b) {
        //省略好多代码
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        //    【重点】,调用 setOpticalFrame 或者 setFrame
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        // 根据 setOpticalFrame 或者 setFrame 的返回值,来确定是否调用onLayout
        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);
            //省略好多代码
        }
        //省略好多代码
    }

那么 setOpticalFrame 和 setFrame 是用来干什么的呢?

View.setOpticalFrame 和 View.setFrame

setOpticalFrame 方法最终会调用 setFrame:

    private boolean setOpticalFrame(int left, int top, int right, int bottom) {
        Insets parentInsets = mParent instanceof View ?
                ((View) mParent).getOpticalInsets() : Insets.NONE;
        Insets childInsets = getOpticalInsets();
        return setFrame(
                left   + parentInsets.left - childInsets.left,
                top    + parentInsets.top  - childInsets.top,
                right  + parentInsets.left + childInsets.right,
                bottom + parentInsets.top  + childInsets.bottom);
    }

看 setFrame:

    protected boolean setFrame(int left, int top, int right, int bottom) {
        boolean changed = false;
         // 省略
        ...
        // 如果存储的 上下左右 值发生改变
        if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
            changed = true;

            // Remember our drawn bit
            int drawn = mPrivateFlags & PFLAG_DRAWN;
            // 保存旧数据
            int oldWidth = mRight - mLeft;
            int oldHeight = mBottom - mTop;
            // 保存新数据
            int newWidth = right - left;
            int newHeight = bottom - top;
            //【宽高】发生变化了吗?
            boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);

            // 是否请求重绘?
            invalidate(sizeChanged);
            //保存新数据
            mLeft = left;
            mTop = top;
            mRight = right;
            mBottom = bottom;
            mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);

            mPrivateFlags |= PFLAG_HAS_BOUNDS;

            //宽高发生变化时调用
            if (sizeChanged) {
                sizeChange(newWidth, newHeight, oldWidth, oldHeight);
            }
            // 还是省略
        }
        return changed;
    }

在 setFrame 方法中,判断我们的 View 是否发生了位置或者大小的变化,如果发生了变化,则强制性重绘。

View.onLayout

在 layout 方法中,我们通过 setFrame 发现位置改变了,调用 onLayout 方法:

    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    }

我们发现该方法为空,为什么呢?

每个布局当中子元素的排序是不同的,所以根据不同的排列形式,需要重写 onLayout 方法,来放置子元素。

我们来看一下 FrameLayout 的 onLayout 方法:

FrameLayout.onLayout

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        layoutChildren(left, top, right, bottom, false /* no force left gravity */);
    }

    void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
        final int count = getChildCount();//获取布局下的子元素个数
        //左边距和右边距
        final int parentLeft = getPaddingLeftWithForeground();
        final int parentRight = right - left - getPaddingRightWithForeground();
        //上边距和下边距
        final int parentTop = getPaddingTopWithForeground();
        final int parentBottom = bottom - top - getPaddingBottomWithForeground();
        //循环遍历整个子元素
        for (int i = 0; i < count; i++) {
            //拿到子元素
            final View child = getChildAt(i);
            //如果该元素是显示的
            if (child.getVisibility() != GONE) {
            //拿到子元素的 LayoutParams
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                //拿到子元素的测量宽高
                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                int childLeft;
                int childTop;

                int gravity = lp.gravity;
                if (gravity == -1) {
                    gravity = DEFAULT_CHILD_GRAVITY;
                }

                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

                //两个switch,针对不同方向的 gravity做处理
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                        lp.leftMargin - lp.rightMargin;
                        break;
                    case Gravity.RIGHT:
                        if (!forceLeftGravity) {
                            childLeft = parentRight - width - lp.rightMargin;
                            break;
                        }
                    case Gravity.LEFT:
                    default:
                        childLeft = parentLeft + lp.leftMargin;
                }

                switch (verticalGravity) {
                    case Gravity.TOP:
                        childTop = parentTop + lp.topMargin;
                        break;
                    case Gravity.CENTER_VERTICAL:
                        childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                        lp.topMargin - lp.bottomMargin;
                        break;
                    case Gravity.BOTTOM:
                        childTop = parentBottom - height - lp.bottomMargin;
                        break;
                    default:
                        childTop = parentTop + lp.topMargin;
                }
                //拿上面switch得到的子元素的上下左右距离开始子元素的布局
                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }

可以看到,在处理子元素的布局时,需要考虑到 padding 值,因为子元素在父布局中的位置受到 padding 值的影响。

更多阅读

Copyright© 2020-2022 li-xyz 冀ICP备2022001112号-1