Android加载View中Background详解

对大多数android的开发者来说，最经常的操作莫过于对界面进行布局，view中背景图片的加载是最经常做的。但是我们很少关注这个过程，这篇文章主要解析view中背景图片加载的流程。了解view中背景图片的加载（资源的加载）可以让我们对资源加载的过程进行一些优化，另外当需要进行整个应用的换肤时，也可以更得心应手。

view图片的加载，我们最常见的就是通过在xml文件当中进行drawable的设置，然后让android系统帮我们完成，或者手动写代码加载成bitmap，然后加载到view上。这篇文章主要分析android在什么时候以及怎么帮我们完成背景图片的加载的，那么我们就从activity.setcontentview还是layoutinflater.inflate(...)方法开始分析。

不管是从activity.setcontentview(...)还是layoutinflater.inflate(...)方法进行view的初始化，最终都会到达layoutinflater.inflate(xmlpullparser parser, viewgroup root, boolean attachtoroot)这个方法中。在这里我们主要关注view的背景图片加载，对于xml如何解析和加载就放过了。

    public view inflate(xmlpullparser parser, viewgroup root, boolean attachtoroot) {
        synchronized (mconstructorargs) {
            final attributeset attrs = xml.asattributeset(parser);
            context lastcontext = (context)mconstructorargs[0];
            mconstructorargs[0] = mcontext;
            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();
                if (debug) {
                    system.out.println("**************************");
                    system.out.println("creating root view: "
                            + name);
                    system.out.println("**************************");
                }
                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);
                } else {
                    // temp is the root view that was found in the xml
                    view temp;
                    if (tag_1995.equals(name)) {
                        temp = new blinklayout(mcontext, attrs);
                    } else {
                        temp = createviewfromtag(root, name, attrs);
                    }
                    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
                        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);
                        }
                    }
                    if (debug) {
                        system.out.println("-----> start inflating children");
                    }
                     // inflate all children under temp
                    rinflate(parser, temp, attrs, 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.
                    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 (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;
            }
            return result;
        }
    }
上面这么长一串代码，其实思路很清晰，就是针对xml文件进行解析，然后根据xml解析出的每一个节点进行view的初始化，紧接着将view的layout参数设置到view上，然后将view添加到它的父控件上。
为了了解view是怎么被加载出来的，我们只需要了解
 temp = createviewfromtag(root, name, attrs);
跟进去看看。
    /*
     * default visibility so the bridgeinflater can override it.
     */
    view createviewfromtag(view parent, string name, attributeset attrs) {
        if (name.equals("view")) {
            name = attrs.getattributevalue(null, "class");
        }
        if (debug) system.out.println("******** creating view: " + name);
        try {
            view view;
            if (mfactory2 != null) view = mfactory2.oncreateview(parent, name, mcontext, attrs);
            else if (mfactory != null) view = mfactory.oncreateview(name, mcontext, attrs);
            else view = null;
            if (view == null && mprivatefactory != null) {
                view = mprivatefactory.oncreateview(parent, name, mcontext, attrs);
            }
            if (view == null) {
                if (-1 == name.indexof('.')) {
                    view = oncreateview(parent, name, attrs);
                } else {
                    view = createview(name, null, attrs);
                }
            }
            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;
        }
    }

上面代码的重点在于try...catch里的内容。try包起来的东西就是对view进行初始化，注意到上面代码中有几个factory，这些factory可以在view进行初始化，也就是说其实我们可以在这里干预view的初始化。从上面代码我们可以知道，如果我们自定义了一个factory，那么当前要初始化的view会优先被我们自定义的factory初始化，而不通过系统默认的factory初始化。那么如果我们要自定义factory，应该在哪里定义呢？容易想到，factory必须要赶在资源加载前自定义完成，所以我们应该在oncreate(...)的this.setcontentview(...)之前设置layoutinflater.factory。

  getlayoutinflater().setfactory(factory);
接下来我们看到上面函数里面的

  if (-1 == name.indexof('.')) {
        view = oncreateview(parent, name, attrs);
    } else {
        view = createview(name, null, attrs);
    }

这段函数就是对view进行初始化，有两种情况，一种是系统自带的view，它在

 if (-1 == name.indexof('.'))
这里面进行初始化，因为如果是系统自带的view，传入的那么一般不带系统的前缀"android.view."。另一个分支初始化的是我们自定义的view。我们跟进oncreateview看看。

  protected view oncreateview(string name, attributeset attrs)
            throws classnotfoundexception {
        return createview(name, "android.view.", attrs);
    }
    public final view createview(string name, string prefix, attributeset attrs)
            throws classnotfoundexception, inflateexception {
        constructor<? extends view> constructor = sconstructormap.get(name);
        class<? extends view> clazz = null;
        try {
            if (constructor == null) {
                // class not found in the cache, see if it's real, and try to add it
                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);
                    }
                }
                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;
            final view view = constructor.newinstance(args);
            if (view instanceof viewstub) {
                // always use ourselves when inflating viewstub later
                final viewstub viewstub = (viewstub) view;
                viewstub.setlayoutinflater(this);
            }
            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;
        }
    }

从oncreateview(...)中我们知道，其实createviewfromtag(...)中对view的初始化最终都是通过createview(...)这个函数进行初始化的，不同只在于系统控件需要通过oncreateview(...)加上前缀，以便类加载器(classloader)正确地通过类所在的包初始化这个类。createview(...)这个函数的思路很清晰，不看catch里面的内容，try里面开头的两个分支就是用来将所要用的类构造函数提取出来，android系统会对使用过的类构造函数进行缓存，因为像textview这些常用的控件可能会被使用很多次。接下来，就是通过类构造函数对view进行初始化了。我们注意到传入构造函数的mconstructorargs是一个包含两个元素的数组。

 final object[] mconstructorargs = new object[2];
那么我们就很清楚了，它就是调用系统控件中对应两个参数的构造函数。为了方便，我们就从最基础的view进行分析。

  public view(context context, attributeset attrs) {
        this(context, attrs, 0);
    }
    public view(context context, attributeset attrs, int defstyle) {
     this(context);
     typedarray a = context.obtainstyledattributes(attrs, com.android.internal.r.styleable.view,
             defstyle, 0);
     drawable background = null;
     int leftpadding = -1;
     int toppadding = -1;
     int rightpadding = -1;
     int bottompadding = -1;
     int startpadding = undefined_padding;
     int endpadding = undefined_padding;
     int padding = -1;
     int viewflagvalues = 0;
     int viewflagmasks = 0;
     boolean setscrollcontainer = false;
     int x = 0;
     int y = 0;
     float tx = 0;
     float ty = 0;
     float rotation = 0;
     float rotationx = 0;
     float rotationy = 0;
     float sx = 1f;
     float sy = 1f;
     boolean transformset = false;
     int scrollbarstyle = scrollbars_inside_overlay;
     int overscrollmode = moverscrollmode;
     boolean initializescrollbars = false;
     boolean leftpaddingdefined = false;
     boolean rightpaddingdefined = false;
     boolean startpaddingdefined = false;
     boolean endpaddingdefined = false;
     final int targetsdkversion = context.getapplicationinfo().targetsdkversion;
     final int n = a.getindexcount();
      for (int i = 0; i < n; i++) {
          int attr = a.getindex(i);
          switch (attr) {
              case com.android.internal.r.styleable.view_background:
                  background = a.getdrawable(attr);
                  break;
              case com.android.internal.r.styleable.view_padding:
                  padding = a.getdimensionpixelsize(attr, -1);
                  muserpaddingleftinitial = padding;
                  muserpaddingrightinitial = padding;
                  leftpaddingdefined = true;
                  rightpaddingdefined = true;
                  break;
   //省略一大串无关的函数
 }

由于我们只关注view中的背景图是怎么加载的，注意这个函数其实就是遍历attributeset attrs这个东西，然后对view的各个属性进行初始化。我们直接进入

 background = a.getdrawable(attr);
这里看看(typedarray.getdrawable)。

    public drawable getdrawable(int index) {
        final typedvalue value = mvalue;
        if (getvalueat(index*assetmanager.style_num_entries, value)) {
            if (false) {
                system.out.println("******************************************************************");
                system.out.println("got drawable resource: type="
                                   + value.type
                                   + " str=" + value.string
                                   + " int=0x" + integer.tohexstring(value.data)
                                   + " cookie=" + value.assetcookie);
                system.out.println("******************************************************************");
            }
            return mresources.loaddrawable(value, value.resourceid);
        }
        return null;
    }

我们发现它调用mresources.loaddrawable(...)，进去看看。

    /*package*/ drawable loaddrawable(typedvalue value, int id)
            throws notfoundexception {
        if (trace_for_preload) {
            // log only framework resources
            if ((id >>> 24) == 0x1) {
                final string name = getresourcename(id);
                if (name != null) android.util.log.d("preloaddrawable", name);
            }
        }
        boolean iscolordrawable = false;
        if (value.type >= typedvalue.type_first_color_int &&
                value.type <= typedvalue.type_last_color_int) {
            iscolordrawable = true;
        }
        final long key = iscolordrawable ? value.data :
                (((long) value.assetcookie) << 32) | value.data;
        drawable dr = getcacheddrawable(iscolordrawable ? mcolordrawablecache : mdrawablecache, key);
        if (dr != null) {
            return dr;
        }
        drawable.constantstate cs = iscolordrawable
                ? spreloadedcolordrawables.get(key)
                : (spreloadeddensity == mconfiguration.densitydpi
                        ? spreloadeddrawables.get(key) : null);
        if (cs != null) {
            dr = cs.newdrawable(this);
        } else {
            if (iscolordrawable) {
                dr = new colordrawable(value.data);
            }
            if (dr == null) {
                if (value.string == null) {
                    throw new notfoundexception(
                            "resource is not a drawable (color or path): " + value);
                }
                string file = value.string.tostring();
                if (trace_for_miss_preload) {
                    // log only framework resources
                    if ((id >>> 24) == 0x1) {
                        final string name = getresourcename(id);
                        if (name != null) android.util.log.d(tag, "loading framework drawable #"
                                + integer.tohexstring(id) + ": " + name
                                + " at " + file);
                    }
                }
                if (debug_load) log.v(tag, "loading drawable for cookie "
                        + value.assetcookie + ": " + file);
                if (file.endswith(".xml")) {
                    try {
                        xmlresourceparser rp = loadxmlresourceparser(
                                file, id, value.assetcookie, "drawable");
                        dr = drawable.createfromxml(this, rp);
                        rp.close();
                    } catch (exception e) {
                        notfoundexception rnf = new notfoundexception(
                            "file " + file + " from drawable resource id #0x"
                            + integer.tohexstring(id));
                        rnf.initcause(e);
                        throw rnf;
                    }
                } else {
                    try {
                        inputstream is = massets.opennonasset(
                                value.assetcookie, file, assetmanager.access_streaming);
        //                system.out.println("opened file " + file + ": " + is);
                        dr = drawable.createfromresourcestream(this, value, is,
                                file, null);
                        is.close();
        //                system.out.println("created stream: " + dr);
                    } catch (exception e) {
                        notfoundexception rnf = new notfoundexception(
                            "file " + file + " from drawable resource id #0x"
                            + integer.tohexstring(id));
                        rnf.initcause(e);
                        throw rnf;
                    }
                }
            }
        }
        if (dr != null) {
            dr.setchangingconfigurations(value.changingconfigurations);
            cs = dr.getconstantstate();
            if (cs != null) {
                if (mpreloading) {
                    if (verifypreloadconfig(value, "drawable")) {
                        if (iscolordrawable) {
                            spreloadedcolordrawables.put(key, cs);
                        } else {
                            spreloadeddrawables.put(key, cs);
                        }
                    }
                } else {
                    synchronized (mtmpvalue) {
                        //log.i(tag, "saving cached drawable @ #" +
                        //        integer.tohexstring(key.intvalue())
                        //        + " in " + this + ": " + cs);
                        if (iscolordrawable) {
                            mcolordrawablecache.put(key, new weakreference<drawable.constantstate>(cs));
                        } else {
                            mdrawablecache.put(key, new weakreference<drawable.constantstate>(cs));
                        }
                    }
                }
            }
        }
        return dr;
    }

就是这个函数了，所有view的背景的加载都在这里了。这个函数的逻辑就比较复杂了，大体说来就是根据背景的类型（纯颜色、定义在xml文件中的，或者是一张静态的背景），如果缓存里面有，就直接用缓存里的。

总结一下，经过上面的分析，我们知道了，android就是在activity.setcontentview(...)中为我们进行资源文件的加载，精确到具体的函数的话，资源文件的加载就是在每一个被初始化的view的构造函数中进行加载的。

以上就是本文的全部内容了，希望对大家能够有所帮助。