目前在做專案時候有這樣的需求：佈局檔案的控制元件型別大致相同，例如某佈局檔案由GridView、ScrollView、TextView、Button四個控制元件組成，但是控制元件的擺放位置不同。因為擺放的方式很多，不可能把所有擺放方式都寫一個佈局檔案，因為這樣不利於迭代開發。這時候就想出能不能把佈局檔案放在伺服器上，當需要某佈局的時候，從伺服器下載佈局檔案儲存到儲存卡上，然後讀取儲存卡上的佈局檔案。

思路大致清除了，那麼現在開始進行可行性分析。

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
	}
 

上面的程式碼太熟悉不過了，建立一個新Android工程，這玩意就已經寫好了！通過setContentView()方法，我們可以設定佈局檔案，但是這裡的引數是R檔案的佈局ID，如果是佈局檔案的路徑就好了，這樣就可以直接讀取SD卡的XML佈局檔案了！按住Ctrl+滑鼠左鍵，點選setContentView()進入：

    /**
     * Set the activity content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the activity.
     *
     * @param layoutResID Resource ID to be inflated.
     * 
     * @see #setContentView(android.view.View)
     * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
     */
    public void setContentView(int layoutResID) {
        getWindow().setContentView(layoutResID);
        initActionBar();
    }
 

從這裡我們可以看到，setContentView()方法呼叫了Window類裡的setContentView()方法，調來調去啊！繼續按住Ctrl+滑鼠左鍵，點選setContentView()進入：

    /**
     * Convenience for
     * {@link #setContentView(View, android.view.ViewGroup.LayoutParams)}
     * to set the screen content from a layout resource.  The resource will be
     * inflated, adding all top-level views to the screen.
     *
     * @param layoutResID Resource ID to be inflated.
     * @see #setContentView(View, android.view.ViewGroup.LayoutParams)
     */
    public abstract void setContentView(int layoutResID);
 

花擦，這裡變成了抽象函式，在哪裡實現啊！不要驚慌，通過原始碼可以得知，Window類是一個抽象類，那麼它的實現應該是在子類中。那麼Window類有哪些子類？

/**
 * Abstract base class for a top-level window look and behavior policy.  An
 * instance of this class should be used as the top-level view added to the
 * window manager. It provides standard UI policies such as a background, title
 * area, default key processing, etc.
 *
 * <p>The only existing implementation of this abstract class is
 * android.policy.PhoneWindow, which you should instantiate when needing a
 * Window.  Eventually that class will be refactored and a factory method
 * added for creating Window instances without knowing about a particular
 * implementation.
 */

這是Window類原始碼的開頭說明，從中可以立馬看到PhoneWindow，對！沒錯就是它！在PhoneWindow中終於找到了它的具體實現！

    @Override
    public void setContentView(int layoutResID) {
        if (mContentParent == null) {
            installDecor();
        } else {
            mContentParent.removeAllViews();
        }
<span style="color:#ff0000;">         mLayoutInflater.inflate(layoutResID, mContentParent);
</span>         final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
    }

從標紅的程式碼，我們可以看到，佈局的ID是用在了該段程式碼，看到了inflate方法，我想大家應該不陌生，在BaseAdpter中經常會用到，用來填充佈局的！繼續按住Ctrl+滑鼠左鍵，點選inflate(....)進入：

    /**
     * Inflate a new view hierarchy from the specified xml resource. Throws
     * {@link InflateException} if there is an error.
     * 
     * @param resource ID for an XML layout resource to load (e.g.,
     *        <code>R.layout.main_page</code>)
     * @param root Optional view to be the parent of the generated hierarchy.
     * @return The root View of the inflated hierarchy. If root was supplied,
     *         this is the root View; otherwise it is the root of the inflated
     *         XML file.
     */
    public View inflate(int resource, ViewGroup root) {
        return inflate(resource, root, root != null);
    }

繼續按住Ctrl+滑鼠左鍵，點選return中的inflate(....)進入：

    /**
     * Inflate a new view hierarchy from the specified xml resource. Throws
     * {@link InflateException} if there is an error.
     * 
     * @param resource ID for an XML layout resource to load (e.g.,
     *        <code>R.layout.main_page</code>)
     * @param root Optional view to be the parent of the generated hierarchy (if
     *        <em>attachToRoot</em> is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if <em>attachToRoot</em> 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(int resource, ViewGroup root, boolean attachToRoot) {
        if (DEBUG) System.out.println("INFLATING from resource: " + resource);
<span style="color:#ff0000;">         XmlResourceParser parser = getContext().getResources().getLayout(resource);
</span>        try {
            return inflate(parser, root, attachToRoot);
        } finally {
            parser.close();
        }
    }

重點來了，看到標紅程式碼，我們可以看到佈局ID被getLayout方法作為引數使用，最後返回了XmlResourceParser這玩意，那麼它是啥？點選它進去看看：

/**
 * The XML parsing interface returned for an XML resource.  This is a standard
 * XmlPullParser interface, as well as an extended AttributeSet interface and
 * an additional close() method on this interface for the client to indicate
 * when it is done reading the resource.
 */
public interface XmlResourceParser extends XmlPullParser, AttributeSet, AutoCloseable {
    /**
     * Close this interface to the resource.  Calls on the interface are no
     * longer value after this call.
     */
    public void close();
}

wow，它是一個介面，而且是繼承自XmlPullParser，那麼XmlPullParser是什麼？檢視它的原始碼，這裡就不貼出原始碼的英文介紹，因為巴拉巴拉太長了！這裡只要知道，它是解析XML檔案要使用到的。到這裡，我想大概有思路了，因為Android是支援解析Xml檔案的，例如遊戲的配置檔案可以用XML檔案完成。如果我們能夠將SD卡上的佈局檔案轉成XmlPullParser的話就應該可以完成這種需求！

最後來看看inflate方法的最終實現程式碼：

/**
     * Inflate a new view hierarchy from the specified XML node. Throws
     * {@link InflateException} if there is an error.
     * <p>
     * <em><strong>Important</strong></em>   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
     *        <em>attachToRoot</em> is true), or else simply an object that
     *        provides a set of LayoutParams values for root of the returned
     *        hierarchy (if <em>attachToRoot</em> 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, 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;
        }
    }

最後通過該方法，就可以返回一個View，該View相當於整個佈局（因為佈局也是View）。通過返回的View，就可以進行findViewById的操作了！

關於從SD卡讀取XML檔案的網上例子有很多，下面專案中用到的方法，用來得到XmlPullParser。

	public XmlPullParser getXmlPullParser(String resource) {

		XmlPullParser parser = Xml.newPullParser();
		try {
			FileInputStream is = new FileInputStream(resource);
			parser.setInput(is, "utf-8");
		} catch (Exception e) {
			e.printStackTrace();
		}
		return parser;
	}

這裡傳入的就是XML在SD卡中的路徑，通過相關操作就可以得到XmlPullParser物件。答題思路清晰明瞭，接下來說說如何做？

首先我們要把Android中的LayoutInflate類拷貝一份到自己工程中，接下來的工作就是照葫蘆畫瓢了！看下圖：

從上圖，可以看到LayoutInflate中的所有inflate方法，可惜就是沒有以String 作為引數的方法，那麼這裡我們就按照原始碼中inflate方法進行增加，如下：

	public View inflate(String resource, ViewGroup root) {
		return inflate(resource, root, root != null);
	}

接下來還有

	public View inflate(String resource, ViewGroup root, boolean attachToRoot) {
		XmlPullParser parser = getXmlPullParser(resource);
		return inflate(parser, root, attachToRoot);
	}

最後就可以調到系統原有的以XmlPullPaser作為引數的inflate方法。那麼這裡就完成了嗎？其實沒有，這裡做的操作僅僅只是讀取佈局檔案操作。裡面的控制元件如何識別，控制元件的屬性怎麼操作，都需要繼續研究。XML檔案裡面有佈局，也有控制元件，若SD卡上的XML檔案有某控制元件，那麼我們就要重寫該控制元件。因為這裡讀取佈局的邏輯是我們自己寫的，那麼相應的View的操作也要自己重寫。具體原因將在後續的某期部落格中進行分析。例如XML檔案中有Button控制元件：

        <Button
            android:id="@+id/btn_back"
            android:layout_width="@dimen/mine_back_btn_width"
            android:layout_height="@dimen/mine_back_btn_height"
            android:layout_gravity="center_vertical"
            android:background="@drawable/mine_btn_back" />

例如id，layout_width，layout_height等屬性，如果我們不重寫Button，那麼這些屬性都是無用的。因為我們沒有進行相應的操作，大家通過閱讀Button原始碼就可以瞭解AttributeSet的相關知識，有時間在後續部落格我也將進行分析。

public class VAButton extends android.widget.Button {


	public VAButton(Context context, AttributeSet attrs) {
		super(context);
		setAttributeSet(attrs);
	}

	@SuppressWarnings("deprecation")
	public void setAttributeSet(AttributeSet attrs) {

		HashMap<String, ParamValue> map = YDResource.getInstance().getViewMap();

		int count = attrs.getAttributeCount();
		for (int i = 0; i < count; i++) {
			ParamValue key = map.get(attrs.getAttributeName(i));
			if (key == null) {
				continue;
			}
			switch (key) {
			case id:
				this.setTag(attrs.getAttributeValue(i));
				break;
			case text:
				String value = YDResource.getInstance().getString(
						attrs.getAttributeValue(i));
				this.setText(value);
				break;
			case ellipsize:
				if (attrs.getAttributeBooleanValue(i, false)) {

					this.setFocusable(true);
					this.setFocusableInTouchMode(true);
					this.setSingleLine(true);
					this.setEllipsize(TruncateAt.MARQUEE);
					this.setMarqueeRepeatLimit(1000);
					this.setSingleLine();
					this.setHorizontallyScrolling(true);
					this.requestFocus();
				}
				break;
			case fadingEdge:
				this.setHorizontalFadingEdgeEnabled(attrs
						.getAttributeBooleanValue(i, false));
				break;
			case scrollHorizontally:
				this.setHorizontallyScrolling(attrs.getAttributeBooleanValue(i,
						false));
				break;
			case textColor:
				this.setTextColor(YDResource.getInstance().getIntColor(
						attrs.getAttributeValue(i)));
				break;
			case textSize:
				String val1 = attrs.getAttributeValue(i);
				if (!TextUtils.isEmpty(val1)) {
					this.setTextSize(YDResource.getInstance()
							.calculateRealSize(val1));
				}
				break;
			case visibility:
				String val2 = attrs.getAttributeValue(i);
				if (!TextUtils.isEmpty(val2)) {
					if (val2.equals("invisible")) {
						this.setVisibility(View.INVISIBLE);
					} else if (val2.equalsIgnoreCase("gone")) {
						this.setVisibility(View.GONE);
					}
				}
				break;
			case background:
				String backgroundString = attrs.getAttributeValue(i);
				if (backgroundString.startsWith("#")) {
					this.setBackgroundColor(YDResource.getInstance()
							.getIntColor(attrs.getAttributeValue(i)));
				} else {
					if (backgroundString.startsWith("@drawable/")) {
						backgroundString = backgroundString.substring(10);
					}
					String rootpath = getContext().getFilesDir().toString();
					StringBuilder sb = new StringBuilder();
					sb.append(rootpath).append("/").append(backgroundString)
							.append(".png");

					Bitmap bm = BitmapFactory.decodeFile(sb.toString());
					setBackgroundDrawable(new BitmapDrawable(bm));
				}
				break;
			case textStyle:
				if ("bold".equalsIgnoreCase(attrs.getAttributeValue(i)))
					this.setTypeface(Typeface.defaultFromStyle(Typeface.BOLD));
				break;
			case style:
				String style = attrs.getAttributeValue(i);
				style = style.substring(style.indexOf("/") + 1);

				Log.i("button", "設定屬性值");
				int id = YDResource.getInstance().getIdentifier(
						"R.style." + style);
				this.setTextAppearance(getContext(), id);
				break;
			case src:

				break;
			case contentDescription:
				String content = attrs.getAttributeValue(i);
				this.setContentDescription(content);
				break;
			case gravity:
				this.setGravity(Gravity.CENTER_HORIZONTAL);
				break;
			default:
				break;
			}
		}
	}

}

通過上面程式碼，我們可以知道，這裡的操作都是對各種屬性標籤利用程式碼來進行設定。如果有些複雜的屬性不想寫了，其實最後還是可以通過程式碼來完成的。這裡就是你的佈局需要多少控制元件屬性，就要在程式碼中完成多少屬性的設定。佈局控制元件的程式碼，以FrameLayout為例：

public class VAFrameLayout extends android.widget.FrameLayout {



	public VAFrameLayout(Context context, AttributeSet attrs) {
		super(context);
		setLayoutParams(generateLayoutParams(attrs));
	}

	@SuppressWarnings({ "unchecked", "deprecation" })
	@Override
	public LayoutParams generateLayoutParams(AttributeSet attrs) {
		// TODO Auto-generated method stub
		LayoutParams params = this.generateDefaultLayoutParams();
		HashMap<String, ParamValue> map = YDResource.getInstance()
				.getLayoutMap();
		params.width = -2;
		params.height = -2;
		int count = attrs.getAttributeCount();
		for (int i = 0; i < count; i++) {
			String name = attrs.getAttributeName(i);
			ParamValue key = map.get(name);
			if (key == null) {
				continue;
			}
			switch (key) {
			case layout_width:
				String width = attrs.getAttributeValue(i);
				if (width.startsWith("f") || width.startsWith("m")) {
					params.width = LayoutParams.MATCH_PARENT;
					break;
				}
				if (width.startsWith("w")) {
					params.width = LayoutParams.WRAP_CONTENT;
					break;
				}
				params.width = YDResource.getInstance()
						.calculateRealSize(width);
				break;
			case layout_height:
				String height = attrs.getAttributeValue(i);
				if (height.startsWith("f") || height.startsWith("m")) {
					params.width = -1;
					break;
				}
				if (height.startsWith("w")) {
					params.width = -2;
					break;
				}
				params.height = YDResource.getInstance().calculateRealSize(
						height);
				break;
			case layout_gravity:

				params.gravity = YDResource.getInstance().getGravity(
						attrs.getAttributeValue(i));

				break;
			case layout_marginLeft:
				params.leftMargin = YDResource.getInstance().calculateRealSize(
						attrs.getAttributeValue(i));
				break;
			case layout_margin:
				int tm = YDResource.getInstance().calculateRealSize(
						attrs.getAttributeValue(i));
				params.setMargins(tm, tm, tm, tm);
				break;

			default:
				break;
			}
		}
		return params;
	}

}

好了，今天的分析就到這裡！

..................................................................................................................................

文章中肯定有寫的不到位的，歡迎大家給出更好的思路。需要原始碼的可以留言，目前這部分操作已經打包成jar，可以在專案中使用。