Android LayoutInflater深入分析及应用
layoutinflater解析
前言:
在android中,如果是初级玩家,很可能对layoutinflater不太熟悉,或许只是在fragment的oncreateview()中模式化的使用过而已。但如果稍微有些工作经验的人就知道,这个类有多么重要,它是连接布局xml和java代码的桥梁,我们常常疑惑,为什么android支持在xml书写布局?
我们想到的必然是android内部帮我们解析xml文件,layoutinflater就是帮我们做了这个工作。
首先layoutinflater是一个系统服务,这个我们可以从from方法看出来
/** * obtains the layoutinflater from the given context. */ public static layoutinflater from(context context) { layoutinflater layoutinflater = (layoutinflater) context.getsystemservice(context.layout_inflater_service); if (layoutinflater == null) { throw new assertionerror("layoutinflater not found."); } return layoutinflater; }
通常我们拿到layoutinflater对象之后就会调用其inflate方法进行加载布局,inflate是一个重载方法
public view inflate(int resource, viewgroup root) { return inflate(resource, root, root != null); }
可以看到,我们调用2个参数的方法时候其默认是添加到父布局中的(父布局一般不为空)
public view inflate(int resource, viewgroup root, boolean attachtoroot) { final resources res = getcontext().getresources(); if (debug) { log.d(tag, "inflating from resource: \"" + res.getresourcename(resource) + "\" (" + integer.tohexstring(resource) + ")"); } final xmlresourceparser parser = res.getlayout(resource); try { return inflate(parser, root, attachtoroot); } finally { parser.close(); } }
这个方法中,其实是使用resources将资源id还原为xmlresoourceparser对象,然后调用inflate(xmlpullparser parser, viewgroup root, boolean attachtoroot)方法,解析布局的具体步骤都是在这个方法中实现
public view inflate(xmlpullparser parser, viewgroup root, boolean attachtoroot) { synchronized (mconstructorargs) { trace.tracebegin(trace.trace_tag_view, "inflate"); final attributeset attrs = xml.asattributeset(parser); context lastcontext = (context)mconstructorargs[0]; mconstructorargs[0] = mcontext; view result = root; try { // look for the root node. //1.循环寻找根节点,其实就是节点指针遍历的过程 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!"); } //2.得到节点的名字,用于判断该节点 final string name = parser.getname(); if (debug) { system.out.println("**************************"); system.out.println("creating root view: " + name); system.out.println("**************************"); } //3.对节点名字进行判断,然后是merge就将其添加到父布局中(依据merge的特性必须添加到父布局中) 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, attrs, false, false); } else { //4.创建根据节点创建view // temp is the root view that was found in the xml final view temp = createviewfromtag(root, name, attrs, false); viewgroup.layoutparams params = null; if (root != null) { if (debug) { system.out.println("creating params from root: " + root); } // create layout params that match root, if supplied //5.根据attrs生成布局参数 params = root.generatelayoutparams(attrs); if (!attachtoroot) { // set the layout params for temp if we are not // attaching. (if we are, we use addview, below) //6.如果view不添加到父布局中,那就给其本身设置布局参数 temp.setlayoutparams(params); } } if (debug) { system.out.println("-----> start inflating children"); } // inflate all children under temp // 7.将该节点下的子view全部加载 rinflate(parser, temp, attrs, true, true); if (debug) { system.out.println("-----> done inflating children"); } // we are supposed to attach all the views we found (int temp) // to root. do that now. //8.如果添加到父布局中,直接addview 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. //9.如果不添加到父布局,那么将自己返回 if (root == null || !attachtoroot) { result = temp; } } } catch (xmlpullparserexception e) { inflateexception ex = new inflateexception(e.getmessage()); ex.initcause(e); throw ex; } catch (ioexception e) { inflateexception ex = new inflateexception( parser.getpositiondescription() + ": " + e.getmessage()); ex.initcause(e); throw ex; } finally { // don't retain static reference on context. mconstructorargs[0] = lastcontext; mconstructorargs[1] = null; } trace.traceend(trace.trace_tag_view); return result; } }
重点的步骤我已经加上注释了,核心
1.找到根布局标签
2.创建根节点对应的view
3.创建其子view
我们从这里面可以看出来,子view的解析其实都是rinflate方法,如果xml中有根布局,那么就调用createviewfromtag创建布局中的根view。我们也可以明白merge的原来,因为它直接调用rinflate添加到父view中,看到rinflate(parser, root, attrs, false, false)和rinflate(parser, temp, attrs, true, true)第二个参数区别我们就明白了。
接下来我们看下rinflate如何创建多个布局
void rinflate(xmlpullparser parser, view parent, final attributeset attrs, boolean finishinflate, boolean inheritcontext) throws xmlpullparserexception, ioexception { //获取当前解析器指针所在节点处于布局层次 final int depth = parser.getdepth(); int type; //进行树的深度优先遍历(如果一个节点有子节点将会再次进入rinflate,否则继续循环) 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(); //如果其中有request_focus标签,那就给这个节点view设置焦点 if (tag_request_focus.equals(name)) { parserequestfocus(parser, parent); //如果其中有tag标签,那就给这个节点view设置tag(key,value) } else if (tag_tag.equals(name)) { parseviewtag(parser, parent, attrs); } else if (tag_include.equals(name)) { //如果其中是include标签,如果include标签 if (parser.getdepth() == 0) { throw new inflateexception("<include /> cannot be the root element"); } parseinclude(parser, parent, attrs, inheritcontext); } else if (tag_merge.equals(name)) { throw new inflateexception("<merge /> must be the root element"); } else { //创建该节点代表的view并添加到父view中,此外遍历子节点 final view view = createviewfromtag(parent, name, attrs, inheritcontext); final viewgroup viewgroup = (viewgroup) parent; final viewgroup.layoutparams params = viewgroup.generatelayoutparams(attrs); rinflate(parser, view, attrs, true, true); viewgroup.addview(view, params); } } //代表着一个节点含其子节点遍历结束 if (finishinflate) parent.onfinishinflate(); }
从上面可以看到,所以创建view都将会交给createviewfromtag(view parent, string name, attributeset attrs, boolean inheritcontext)中,我们可以看下该方法如何创建view
view createviewfromtag(view parent, string name, attributeset attrs, boolean inheritcontext) { if (name.equals("view")) { name = attrs.getattributevalue(null, "class"); } context viewcontext; if (parent != null && inheritcontext) { viewcontext = parent.getcontext(); } else { viewcontext = mcontext; } // apply a theme wrapper, if requested. final typedarray ta = viewcontext.obtainstyledattributes(attrs, attrs_theme); final int themeresid = ta.getresourceid(0, 0); if (themeresid != 0) { viewcontext = new contextthemewrapper(viewcontext, themeresid); } ta.recycle(); if (name.equals(tag_1995)) { // let's party like it's 1995! return new blinklayout(viewcontext, attrs); } if (debug) system.out.println("******** creating view: " + name); try { view view; if (mfactory2 != null) { view = mfactory2.oncreateview(parent, name, viewcontext, attrs); } else if (mfactory != null) { view = mfactory.oncreateview(name, viewcontext, attrs); } else { view = null; } if (view == null && mprivatefactory != null) { view = mprivatefactory.oncreateview(parent, name, viewcontext, attrs); } if (view == null) { final object lastcontext = mconstructorargs[0]; mconstructorargs[0] = viewcontext; try { if (-1 == name.indexof('.')) { view = oncreateview(parent, name, attrs); } else { view = createview(name, null, attrs); } } finally { mconstructorargs[0] = lastcontext; } } if (debug) system.out.println("created view is: " + view); return view; } catch (inflateexception e) { throw e; } catch (classnotfoundexception e) { inflateexception ie = new inflateexception(attrs.getpositiondescription() + ": error inflating class " + name); ie.initcause(e); throw ie; } catch (exception e) { inflateexception ie = new inflateexception(attrs.getpositiondescription() + ": error inflating class " + name); ie.initcause(e); throw ie; } }
其实很简单,就是4个降级处理
if(factory2!=null){ factory2.oncreateview(); }else if(factory!=null){ factory.oncreateview(); }else if(mprivatefactory!=null){ mprivatefactory.oncreateview(); }else{ oncreateview() }
其他的oncreateview我们不去设置的话为null,我们看下自己的oncreateview(),其实这个方法会调用createview()
public final view createview(string name, string prefix, attributeset attrs) throws classnotfoundexception, inflateexception { //从构造器map(缓存)中获取需要的构造器 constructor<? extends view> constructor = sconstructormap.get(name); class<? extends view> clazz = null; try { trace.tracebegin(trace.trace_tag_view, name); if (constructor == null) { // class not found in the cache, see if it's real, and try to add it //如果缓存中没有需要的构造器,那就通过classloader加载需要的类 clazz = mcontext.getclassloader().loadclass( prefix != null ? (prefix + name) : name).assubclass(view.class); if (mfilter != null && clazz != null) { boolean allowed = mfilter.onloadclass(clazz); if (!allowed) { failnotallowed(name, prefix, attrs); } } //将使用过的构造器缓存 constructor = clazz.getconstructor(mconstructorsignature); sconstructormap.put(name, constructor); } else { // if we have a filter, apply it to cached constructor if (mfilter != null) { // have we seen this name before? boolean allowedstate = mfiltermap.get(name); if (allowedstate == null) { // new class -- remember whether it is allowed clazz = mcontext.getclassloader().loadclass( prefix != null ? (prefix + name) : name).assubclass(view.class); boolean allowed = clazz != null && mfilter.onloadclass(clazz); mfiltermap.put(name, allowed); if (!allowed) { failnotallowed(name, prefix, attrs); } } else if (allowedstate.equals(boolean.false)) { failnotallowed(name, prefix, attrs); } } } object[] args = mconstructorargs; args[1] = attrs; constructor.setaccessible(true); //通过反射获取需要的实例对象 final view view = constructor.newinstance(args); if (view instanceof viewstub) { // use the same context when inflating viewstub later. final viewstub viewstub = (viewstub) view; //viewstub将创建一个属于自己的layoutinflater,因为它需要在不同的时机去inflate viewstub.setlayoutinflater(cloneincontext((context) args[0])); } return view; } catch (nosuchmethodexception e) { inflateexception ie = new inflateexception(attrs.getpositiondescription() + ": error inflating class " + (prefix != null ? (prefix + name) : name)); ie.initcause(e); throw ie; } catch (classcastexception e) { // if loaded class is not a view subclass inflateexception ie = new inflateexception(attrs.getpositiondescription() + ": class is not a view " + (prefix != null ? (prefix + name) : name)); ie.initcause(e); throw ie; } catch (classnotfoundexception e) { // if loadclass fails, we should propagate the exception. throw e; } catch (exception e) { inflateexception ie = new inflateexception(attrs.getpositiondescription() + ": error inflating class " + (clazz == null ? "<unknown>" : clazz.getname())); ie.initcause(e); throw ie; } finally { trace.traceend(trace.trace_tag_view); } }
大体步骤就是,
1.从缓存中获取特定view构造器,如果没有,则加载对应的类,并缓存该构造器,
2.利用构造器反射构造对应的view
3.如果是viewstub则复制一个layoutinflater对象传递给它
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