详解Android布局加载流程源码

详解Android布局加载流程源码的攻略如下：

1. 确定需求

在详细讲解Android布局加载流程源码之前，需要先了解本次攻略的目的和需求。本次攻略的目的是通过对Android布局加载流程源码的分析，深入了解Android布局加载的过程和原理，以便进一步优化相关的开发工作。

2. 学习布局加载流程

2.1 布局加载的基本流程

Android布局加载的基本流程分为3个阶段，即解析XML文件、创建View树和测量、布局和绘制View。其中，解析XML文件的过程是由LayoutInflator类实现的，创建View树和测量、布局和绘制View的过程则是由ViewGroup类和View类实现的。

2.2 解析XML文件的源码分析

LayoutInflator类的主要作用是将XML文件解析成View对象，其源码分析过程如下：

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    final LayoutInflaterDelegate delegate = this.mDelegate;
    if (delegate != null) {
        return delegate.onCreateView(null, null, parser);
    }
    synchronized (mConstructorArgs) {
        final AttributeSet attrs = Xml.asAttributeSet(parser);
        Context lastContext = (Context) mConstructorArgs[0];
        mConstructorArgs[0] = mContext;
        View result = root;
        try {
            int type;
            while ((type = parser.next()) != XmlPullParser.START_TAG &&
                   type != XmlPullParser.END_DOCUMENT) {
                // Empty loop.
            }
            if (type != XmlPullParser.START_TAG) {
                throw new InflateException(parser.getPositionDescription()
                        + ": No start tag found!");
            }
            final String name = parser.getName();
            if (TAG_MERGE.equals(name)) {
                if (root == null || !attachToRoot) {
                    throw new InflateException("merge can be used only with a valid "
                            + "ViewGroup root and attachToRoot=true");
                }
                rInflate(parser, root, mContext, attrs, false);
            } else {
                // Temp is the root view that was found in the xml
                View temp = rInflate(parser, root, mContext, attrs, false);
                if (root != null && attachToRoot) {
                    root.addView(temp);
                }
                result = temp;
            }
        } finally {
            mConstructorArgs[0] = lastContext;
        }
        return result;
    }
}

在该源码中，首先获取了解析器的属性集，然后通过一个while循环遍历XML文件，直到找到根元素（即XML文件的第一个元素）。接着，通过调用rInflate()方法创建View树。在创建View树过程中，如果该XML文件根元素设有merge标签，则需要结合父View来创建View树，否则直接为根元素创建View树。最后，如果attachToRoot参数为true，则将创建的View加入到根ViewGroup中。

2.3 创建View树和测量、布局和绘制View的源码分析

创建View树和测量、布局和绘制View的过程由ViewGroup类和View类共同完成，其源码分析过程如下：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    //遍历子View,对View进行测量
    measureChildren(widthMeasureSpec, heightMeasureSpec);

    //设置自身宽高
    int width = 0;
    int height = 0;
    int count = getChildCount();
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
            width += child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin;
            height += child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin;
        }
    }
    setMeasuredDimension(resolveSizeAndState(width, widthMeasureSpec, 0),
            resolveSizeAndState(height, heightMeasureSpec, 0));
}

protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    int childLeft = 0;
    int childTop = 0;

    int count = getChildCount();
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final int childWidth = child.getMeasuredWidth();
            final int childHeight = child.getMeasuredHeight();
            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
            child.layout(childLeft + lp.leftMargin, childTop + lp.topMargin,
                    childLeft + childWidth + lp.rightMargin,
                    childTop + childHeight + lp.bottomMargin);

            childLeft += childWidth + lp.leftMargin + lp.rightMargin;
            childTop += childHeight + lp.topMargin + lp.bottomMargin;
        }
    }
}

protected void onDraw(Canvas canvas) {
    // 遍历子View,对View进行绘制
    super.onDraw(canvas);
    int count = getChildCount();
    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            drawChild(canvas, child, getDrawingTime());
        }
    }
}

在该源码中，onMeasure()方法主要实现了对View的测量，onLayout()方法主要实现了对View的布局，onDraw()方法主要实现了对View的绘制。

3. 举例说明

3.1 自定义ViewGroup的示例

以下是一个自定义LinearLayout的示例，可以通过继承ViewGroup类实现：

public class CustomLinearLayout extends ViewGroup {

    public CustomLinearLayout(Context context) {
        super(context);
    }

    public CustomLinearLayout(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public CustomLinearLayout(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int childLeft = 0;
        int childTop = 0;

        int count = getChildCount();
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
                child.layout(childLeft + lp.leftMargin, childTop + lp.topMargin,
                        childLeft + childWidth + lp.rightMargin,
                        childTop + childHeight + lp.bottomMargin);

                childLeft += childWidth + lp.leftMargin + lp.rightMargin;
                childTop += childHeight + lp.topMargin + lp.bottomMargin;
            }
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        measureChildren(widthMeasureSpec, heightMeasureSpec);

        int maxChildHeight = 0;
        int totalWidth = 0;

        int count = getChildCount();
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
                maxChildHeight = Math.max(maxChildHeight, child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                totalWidth += child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin;
            }
        }

        setMeasuredDimension(
                View.resolveSize(totalWidth, widthMeasureSpec),
                View.resolveSize(maxChildHeight, heightMeasureSpec)
        );
    }

    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new MarginLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }

    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new MarginLayoutParams(getContext(), attrs);
    }

    @Override
    protected LayoutParams generateLayoutParams(LayoutParams p) {
        return new MarginLayoutParams(p);
    }
}

该自定义LinearLayout继承了ViewGroup类，并重写了onLayout()方法和onMeasure()方法。onMeasure()方法的作用是先通过measureChildren()方法测量所有子View的大小，然后确定自身的大小。而onLayout()方法，则是实现对所有子View进行布局，这里实现的方式是按照水平方向排列，每个子View之间保留一定的Margin。

3.2 布局文件的示例

以下是一个简单的布局文件的示例，可以对TextView进行布局：

<com.example.CustomLinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal">

    <TextView
        android:id="@+id/tv_username"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Username:" />

    <EditText
        android:id="@+id/et_username"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:maxLines="1"
        android:ellipsize="end"
        android:inputType="textEmailAddress"
        android:layout_marginLeft="10dp" />

</com.example.CustomLinearLayout>

这个布局文件使用了自定义的CustomLinearLayout作为根ViewGroup，该CustomLinearLayout的方向是水平的，因此它的子View（一个TextView和一个EditText）会按照水平方向进行排列。其中，TextView距下一个View有10dp的Margin。

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