這篇文章主要介紹Android載入資源的主要流程

我們從一個簡單的函式入口進行分析

getResources().getString(resId);

    public String getString(int id) throws NotFoundException {
        CharSequence res = getText(id);
        if (res != null) {
            return res.toString();
        }
        throw new NotFoundException("String resource ID #0x"
                                    + Integer.toHexString(id));
    }

    public CharSequence getText(int id) throws NotFoundException {
        CharSequence res = mAssets.getResourceText(id);
        if (res != null) {
            return res;
        }
        throw new NotFoundException("String resource ID #0x"
                                    + Integer.toHexString(id));
    }
 

可以看到最後呼叫的是mAssets的getResourceText方法，mAssets為一個AssetManager，那我們進去這個AssetManager裡面的getResourceText看下究竟做了什麼

AssetManager.java：

    /*package*/ final CharSequence getResourceText(int ident) {
        synchronized (this) {
            TypedValue tmpValue = mValue;
            int block = loadResourceValue(ident, (short) 0, tmpValue, true);
            if (block >= 0) {
                if (tmpValue.type == TypedValue.TYPE_STRING) {
                    return mStringBlocks[block].get(tmpValue.data);
                }
                return tmpValue.coerceToString();
            }
        }
        return null;
    }
 

TypedValue.java :

/**
 * Container for a dynamically typed data value.  Primarily used with
 * {@link android.content.res.Resources} for holding resource values.
 */
public class TypedValue {
    ...

    /** The type held by this value, as defined by the constants here.
     *  This tells you how to interpret the other fields in the object. */
    public int type;

    /** If the value holds a string, this is it. */
    public CharSequence string;

    /** Basic data in the value, interpreted according to {@link #type} */
    public int data;

    /** Additional information about where the value came from; only
     *  set for strings. */
    public int assetCookie;

    /** If Value came from a resource, this holds the corresponding resource id. */
    public int resourceId;

    /** If Value came from a resource, these are the configurations for which
     *  its contents can change. */
    public int changingConfigurations = -1;

    /**
     * If the Value came from a resource, this holds the corresponding pixel density.
     * */
    public int density;

    ...
}
 

可以看到這個類主要是用來存和資源相關的資料的，而StringBlock  Conveniences for retrieving data out of a compiled string resource。這個類主要是用來做string類的資源檢索和儲存的。每一個資源表都對應著一個資源type-value的字串資源池，StringBlock就是用來儲存這種字串的。那麼可以知道getResourceText的主要流程是

1）呼叫loadResourceValue函式，把資源資訊載入到TypedValue裡面。

2）loadResourceValue返回的block從StringBlock內去拿內容。那麼這裡面遺留一個問題，這裡面的block是什麼意思呢？

下面我們著重看下loadResourceValue這個函式，恩，這個函式是一個native函式，具體的實現在android_util_AssetManager.cpp裡面，我們去看下

static jint android_content_AssetManager_loadResourceValue(JNIEnv* env, jobject clazz,
                                                           jint ident,
                                                           jshort density,
                                                           jobject outValue,
                                                           jboolean resolve)
{
    if (outValue == NULL) {
         jniThrowNullPointerException(env, "outValue");
         return 0;
    }
    AssetManager* am = assetManagerForJavaObject(env, clazz);
    if (am == NULL) {
        return 0;
    }
    const ResTable& res(am->getResources());

    Res_value value;
    ResTable_config config;
    uint32_t typeSpecFlags;
    ssize_t block = res.getResource(ident, &value, false, density, &typeSpecFlags, &config);
#if THROW_ON_BAD_ID
    if (block == BAD_INDEX) {
        jniThrowException(env, "java/lang/IllegalStateException", "Bad resource!");
        return 0;
    }
#endif
    uint32_t ref = ident;
    if (resolve) {
        block = res.resolveReference(&value, block, &ref, &typeSpecFlags, &config);
#if THROW_ON_BAD_ID
        if (block == BAD_INDEX) {
            jniThrowException(env, "java/lang/IllegalStateException", "Bad resource!");
            return 0;
        }
#endif
    }
    return block >= 0 ? copyValue(env, outValue, &res, value, ref, block, typeSpecFlags, &config) : block;

和這段程式碼相關的類是ResTable，在doc上面是這樣描述的Convenience class for accessing data in a ResTable resource。這個類和資源表是有關係的，具體的關係我們之後再闡明，那麼上面這個函式主要做的事情就是從ResTable中查詢資源，我們接著追一下里面的getResource函式。

ResourceTypes.cpp：

ssize_t ResTable::getResource(uint32_t resID, Res_value* outValue, bool mayBeBag, uint16_t density,
        uint32_t* outSpecFlags, ResTable_config* outConfig) const
{
    if (mError != NO_ERROR) {
        return mError;
    }

    const ssize_t p = getResourcePackageIndex(resID);
    const int t = Res_GETTYPE(resID);
    const int e = Res_GETENTRY(resID);

    if (p < 0) {
        if (Res_GETPACKAGE(resID)+1 == 0) {
            ALOGW("No package identifier when getting value for resource number 0x%08x", resID);
        } else {
            ALOGW("No known package when getting value for resource number 0x%08x", resID);
        }
        return BAD_INDEX;
    }
    if (t < 0) {
        ALOGW("No type identifier when getting value for resource number 0x%08x", resID);
        return BAD_INDEX;
    }

    const Res_value* bestValue = NULL;
    const Package* bestPackage = NULL;
    ResTable_config bestItem;
    memset(&bestItem, 0, sizeof(bestItem)); // make the compiler shut up

    if (outSpecFlags != NULL) *outSpecFlags = 0;

    // Look through all resource packages, starting with the most
    // recently added.
    const PackageGroup* const grp = mPackageGroups[p];
    if (grp == NULL) {
        ALOGW("Bad identifier when getting value for resource number 0x%08x", resID);
        return BAD_INDEX;
    }

    // Allow overriding density
    const ResTable_config* desiredConfig = &mParams;
    ResTable_config* overrideConfig = NULL;
    if (density > 0) {
        overrideConfig = (ResTable_config*) malloc(sizeof(ResTable_config));
        if (overrideConfig == NULL) {
            ALOGE("Couldn't malloc ResTable_config for overrides: %s", strerror(errno));
            return BAD_INDEX;
        }
        memcpy(overrideConfig, &mParams, sizeof(ResTable_config));
        overrideConfig->density = density;
        desiredConfig = overrideConfig;
    }

    ssize_t rc = BAD_VALUE;
    size_t ip = grp->packages.size();
    while (ip > 0) {
        ip--;
        int T = t;
        int E = e;

        const Package* const package = grp->packages[ip];
        if (package->header->resourceIDMap) {
            uint32_t overlayResID = 0x0;
            status_t retval = idmapLookup(package->header->resourceIDMap,
                                          package->header->resourceIDMapSize,
                                          resID, &overlayResID);
            if (retval == NO_ERROR && overlayResID != 0x0) {
                // for this loop iteration, this is the type and entry we really want
                ALOGV("resource map 0x%08x -> 0x%08x\n", resID, overlayResID);
                T = Res_GETTYPE(overlayResID);
                E = Res_GETENTRY(overlayResID);
            } else {
                // resource not present in overlay package, continue with the next package
                continue;
            }
        }

        const ResTable_type* type;
        const ResTable_entry* entry;
        const Type* typeClass;
        ssize_t offset = getEntry(package, T, E, desiredConfig, &type, &entry, &typeClass);
        if (offset <= 0) {
            // No {entry, appropriate config} pair found in package. If this
            // package is an overlay package (ip != 0), this simply means the
            // overlay package did not specify a default.
            // Non-overlay packages are still required to provide a default.
            if (offset < 0 && ip == 0) {
                ALOGW("Failure getting entry for 0x%08x (t=%d e=%d) in package %zd (error %d)\n",
                        resID, T, E, ip, (int)offset);
                rc = offset;
                goto out;
            }
            continue;
        }

        if ((dtohs(entry->flags)&entry->FLAG_COMPLEX) != 0) {
            if (!mayBeBag) {
                ALOGW("Requesting resource %p failed because it is complex\n",
                     (void*)resID);
            }
            continue;
        }

        TABLE_NOISY(aout << "Resource type data: "
              << HexDump(type, dtohl(type->header.size)) << endl);

        if ((size_t)offset > (dtohl(type->header.size)-sizeof(Res_value))) {
            ALOGW("ResTable_item at %d is beyond type chunk data %d",
                 (int)offset, dtohl(type->header.size));
            rc = BAD_TYPE;
            goto out;
        }

        const Res_value* item =
            (const Res_value*)(((const uint8_t*)type) + offset);
        ResTable_config thisConfig;
        thisConfig.copyFromDtoH(type->config);

        if (outSpecFlags != NULL) {
            if (typeClass->typeSpecFlags != NULL) {
                *outSpecFlags |= dtohl(typeClass->typeSpecFlags[E]);
            } else {
                *outSpecFlags = -1;
            }
        }

        if (bestPackage != NULL &&
            (bestItem.isMoreSpecificThan(thisConfig) || bestItem.diff(thisConfig) == 0)) {
            // Discard thisConfig not only if bestItem is more specific, but also if the two configs
            // are identical (diff == 0), or overlay packages will not take effect.
            continue;
        }
        
        bestItem = thisConfig;
        bestValue = item;
        bestPackage = package;
    }

    TABLE_NOISY(printf("Found result: package %p\n", bestPackage));

    if (bestValue) {
        outValue->size = dtohs(bestValue->size);
        outValue->res0 = bestValue->res0;
        outValue->dataType = bestValue->dataType;
        outValue->data = dtohl(bestValue->data);
        if (outConfig != NULL) {
            *outConfig = bestItem;
        }
        TABLE_NOISY(size_t len;
              printf("Found value: pkg=%d, type=%d, str=%s, int=%d\n",
                     bestPackage->header->index,
                     outValue->dataType,
                     outValue->dataType == bestValue->TYPE_STRING
                     ? String8(bestPackage->header->values.stringAt(
                         outValue->data, &len)).string()
                     : "",
                     outValue->data));
        rc = bestPackage->header->index;
        goto out;
    }

out:
    if (overrideConfig != NULL) {
        free(overrideConfig);
    }

    return rc;
}

1）resId的解析。

    #define Res_GETTYPE(id) (((id>>16)&0xFF)-1)
    #define Res_GETENTRY(id) (id&0xFFFF)    
    const ssize_t p = getResourcePackageIndex(resID);
    const int t = Res_GETTYPE(resID);
    const int e = Res_GETENTRY(resID);

2）找到相應的PackageGroup。關於PackageGroup的描述如下

// A group of objects describing a particular resource package.
// The first in 'package' is always the root object (from the resource
// table that defined the package); the ones after are skins on top of it.

可以知道是用來描述一系列的資源資原始檔的，且第一個package是系統的資源。

3）迴圈PackageGroup，判斷是否是overlay包

這裡會去迴圈PackageGroup裡面的每一個Package，首先判斷這個Package是不是overlay package，overlay package的意思是該package是用來覆蓋資源的，如果是overlay package，則對id進行相應的轉換。關於overlay機制可以參考 安卓overlay機制

4）根據資源的type和entry判斷該package內是否有相應的資源。這裡面主要表現在getEntry這個函式裡面，這裡需要了解resources.arsc的資源結構，可以參考 安卓資源打包過程 這篇文章瞭解一二。

5）如果該包內無要找的資源，表現在getEntry的返回值<=0，則查詢下一個包，否則進行資源的最優匹配。每一個資源有相應的配置資訊，配置的內容儲存在ResTable_config結構體內，如果當前匹配到的資源比之前匹配的資源要優，則採用當前匹配的資源，恩，就是下面的這段程式碼了

        if (bestPackage != NULL &&
            (bestItem.isMoreSpecificThan(thisConfig) || bestItem.diff(thisConfig) == 0)) {
            // Discard thisConfig not only if bestItem is more specific, but also if the two configs
            // are identical (diff == 0), or overlay packages will not take effect.
            continue;
        }
        
        bestItem = thisConfig;
        bestValue = item;
        bestPackage = package;

    const Res_value* bestValue = NULL;
    const Package* bestPackage = NULL;
    ResTable_config bestItem;

回到上面的android_content_AssetManager_loadResourceValue 函式，我們獲取了返回值block，那麼這個函式還剩最後一步

    if (resolve) {  
        block = res.resolveReference(&value, block, &ref, &typeSpecFlags, &config);  
    #if THROW_ON_BAD_ID  
        if (block == BAD_INDEX) {  
            jniThrowException(env, "java/lang/IllegalStateException", "Bad resource!");  
            return 0;  
        }  
    #endif  
    }  

ResourceTypes.cpp:

ssize_t ResTable::resolveReference(Res_value* value, ssize_t blockIndex,
        uint32_t* outLastRef, uint32_t* inoutTypeSpecFlags,
        ResTable_config* outConfig) const
{
    int count=0;
    while (blockIndex >= 0 && value->dataType == value->TYPE_REFERENCE
           && value->data != 0 && count < 20) {
        if (outLastRef) *outLastRef = value->data;
        uint32_t lastRef = value->data;
        uint32_t newFlags = 0;
        const ssize_t newIndex = getResource(value->data, value, true, 0, &newFlags,
                outConfig);
        if (newIndex == BAD_INDEX) {
            return BAD_INDEX;
        }
        TABLE_THEME(ALOGI("Resolving reference %p: newIndex=%d, type=0x%x, data=%p\n",
             (void*)lastRef, (int)newIndex, (int)value->dataType, (void*)value->data));
        //printf("Getting reference 0x%08x: newIndex=%d\n", value->data, newIndex);
        if (inoutTypeSpecFlags != NULL) *inoutTypeSpecFlags |= newFlags;
        if (newIndex < 0) {
            // This can fail if the resource being referenced is a style...
            // in this case, just return the reference, and expect the
            // caller to deal with.
            return blockIndex;
        }
        blockIndex = newIndex;
        count++;
    }
    return blockIndex;
}