View知识点 -- LayoutInflate过程以及View绘制过程

1 LayoutInflate

Inflate的作用:将xml布局文件解析为对应的View对象

LayoutInflater layoutInflater = LayoutInflater.from(this);  
View buttonLayout = layoutInflater.inflate(R.layout.button_layout, null);  
View知识点 -- LayoutInflate过程以及View绘制过程_第1张图片
参数与返回值.png
View知识点 -- LayoutInflate过程以及View绘制过程_第2张图片
总体流程.png

xml布局文件是由树结构的多个子view组合而成,可想而知,解析过程肯定涉及到逐层递归。
源码如下:

    /**
     * Inflate a new view hierarchy from the specified XML node. Throws
     * {@link InflateException} if there is an error.
     * 

* Important For performance * reasons, view inflation relies heavily on pre-processing of XML files * that is done at build time. Therefore, it is not currently possible to * use LayoutInflater with an XmlPullParser over a plain XML file at runtime. * * @param parser XML dom node containing the description of the view * hierarchy. * @param root Optional view to be the parent of the generated hierarchy (if * attachToRoot is true), or else simply an object that * provides a set of LayoutParams values for root of the returned * hierarchy (if attachToRoot is false.) * @param attachToRoot Whether the inflated hierarchy should be attached to * the root parameter? If false, root is only used to create the * correct subclass of LayoutParams for the root view in the XML. * @return The root View of the inflated hierarchy. If root was supplied and * attachToRoot is true, this is root; otherwise it is the root of * the inflated XML file. */ public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) { synchronized (mConstructorArgs) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate"); final Context inflaterContext = mContext; final AttributeSet attrs = Xml.asAttributeSet(parser); Context lastContext = (Context) mConstructorArgs[0]; mConstructorArgs[0] = inflaterContext; View result = root; try { // Look for the root node. int type; while ((type = parser.next()) != XmlPullParser.START_TAG && type != XmlPullParser.END_DOCUMENT) { // Empty } if (type != XmlPullParser.START_TAG) { throw new InflateException(parser.getPositionDescription() + ": No start tag found!"); } final String name = parser.getName(); //如果是头结点为Merge标签 if (TAG_MERGE.equals(name)) { if (root == null || !attachToRoot) { throw new InflateException(" can be used only with a valid " + "ViewGroup root and attachToRoot=true"); } rInflate(parser, root, inflaterContext, attrs, false); } else //不是merge标签 { // Temp is the root view that was found in the xml final View temp = createViewFromTag(root, name, inflaterContext, attrs); ViewGroup.LayoutParams params = null; if (root != null) { // Create layout params that match root, if supplied params = root.generateLayoutParams(attrs); if (!attachToRoot) { // Set the layout params for temp if we are not // attaching. (If we are, we use addView, below) temp.setLayoutParams(params); } } // Inflate all children under temp against its context. rInflateChildren(parser, temp, attrs, true); // We are supposed to attach all the views we found (int temp) // to root. Do that now. if (root != null && attachToRoot) { root.addView(temp, params); } // Decide whether to return the root that was passed in or the top view found in xml. if (root == null || !attachToRoot) { result = temp; } } } ……//catch操作已省略 return result; } }

可以看到,LayoutInflater使用pull解析布局文件。先创建出根布局的View实例,然后递归查找这个View的子View,递归完成后将这个View添加到父布局。
其中,createViewFromTag方法,通过节点名以及属性,创建出view。createViewFromTag内部会调用createView()方法,最终通过反射创建出View实例并返回。
然后调用rInflateChildren()方法,最终会调用到rInflate()方法,其中又会递归调用rInflateChildren()。


/**
     * Recursive method used to descend down the xml hierarchy and instantiate
     * views, instantiate their children, and then call onFinishInflate().
     * 

* Note: Default visibility so the BridgeInflater can * override it. */ void rInflate(XmlPullParser parser, View parent, Context context, AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException { final int depth = parser.getDepth(); int type; while (((type = parser.next()) != XmlPullParser.END_TAG || parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) { if (type != XmlPullParser.START_TAG) { continue; } final String name = parser.getName(); if (TAG_REQUEST_FOCUS.equals(name)) { parseRequestFocus(parser, parent); } else if (TAG_TAG.equals(name)) { parseViewTag(parser, parent, attrs); } else if (TAG_INCLUDE.equals(name)) { if (parser.getDepth() == 0) { throw new InflateException(" cannot be the root element"); } parseInclude(parser, context, parent, attrs); } else if (TAG_MERGE.equals(name)) { throw new InflateException(" must be the root element"); } else { final View view = createViewFromTag(parent, name, context, attrs); final ViewGroup viewGroup = (ViewGroup) parent; final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs); rInflateChildren(parser, view, attrs, true); viewGroup.addView(view, params); } } if (finishInflate) { parent.onFinishInflate(); } }

把整个布局文件都解析完成后就形成了一个完整的DOM结构,最终会把最顶层的根布局返回,至此inflate()过程全部结束。

2 View绘制流程

2.1 onMeasure()

public class MyView extends View {  
    ......  
     
//传入的宽高为父视图对子视图提供的参考大小
    @Override  
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {  
        setMeasuredDimension(200, 200);  
    }  
}  
  • 作用:测量视图的大小。
  • 视图大小的控制是由父视图、布局文件、以及视图本身共同完成的,父视图会提供给子视图参考的大小,而开发人员可以在XML文件中指定视图的大小,然后视图本身会对最终的大小进行拍板。
  • setMeasuredDimension()方法调用之后,我们才能使用getMeasuredWidth()和getMeasuredHeight()来获取视图测量出的宽高,以此之前调用这两个方法得到的值都会是0。
  • 以下例子中,视图大小始终会是200*200。

2.2 onLayout()

//参数为父布局提供的参考位置
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
    }
  • 作用:父布局用来确定子View的摆放位置。相对于父布局的位置。
  • 若在onLayout()中需要使用子view的宽高,则需要在onMeasure()中先调用MeasureChild()进行测量。
  • onLayout()之后,就可以调用getWidth()和getHeight()方法来获取宽高。
    getWidth()为视图右边坐标减去左边坐标;getMeasuredWidth()为setMeasuredDimension()来设置的。
public class SimpleLayout extends ViewGroup {  
  
    public SimpleLayout(Context context, AttributeSet attrs) {  
        super(context, attrs);  
    }  
  
    @Override  
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {  
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);  
        if (getChildCount() > 0) {  
            View childView = getChildAt(0);  
            measureChild(childView, widthMeasureSpec, heightMeasureSpec);  
        }  
    }  
  
    @Override  
    protected void onLayout(boolean changed, int l, int t, int r, int b) {  
        if (getChildCount() > 0) {  
            View childView = getChildAt(0);  
            childView.layout(0, 0, childView.getMeasuredWidth(), childView.getMeasuredHeight());  
        }  
    }  
  
}  

2.3 onDraw()

  • 作用:真正对视图进行绘制。
  • onDraw()函数中会传入Canvas画布对象,通过Paint对象在画布上进行绘制。
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
    }

3 View重绘函数

invalidate():重新执行draw方法,不执行measure和layout。在UI线程调用。
postInvalifate():在非UI线程中使用。
requestLayout():重新走一遍整个绘制流程。

4 Check异常点

1 根布局是Merge标签,却没有设置添加到父View
2 解析到include标签,但是parser深度为0,即include为根节点
3 解析子View的时候解析到Merge标签

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