iOS点九图NinePatch解析

- (UIImage *)resizableImageWithCapInsets:(UIEdgeInsets)capInsets resizingMode:(UIImageResizingMode)resizingMode;

capInsets: UIEdgeInsets{CGFloat top, left, bottom, right}, 定义了受保护区域，除去受保护区域，剩下则是可拉伸区域；
resizingMode: 图片拉伸模式，两种取值，UIImageResizingModeTile平铺和UIImageResizingModeStretch拉伸；

./aapt s -i top1.9.png -o top1_out.png

*
 * The PNG chunk type is "npTc".
 */
struct Res_png_9patch
{
    Res_png_9patch() : wasDeserialized(false), xDivs(NULL),
                       yDivs(NULL), colors(NULL) { }
    int8_t wasDeserialized;
    int8_t numXDivs;
    int8_t numYDivs;
    int8_t numColors;
    // These tell where the next section of a patch starts.
    // For example, the first patch includes the pixels from
    // 0 to xDivs[0]-1 and the second patch includes the pixels
    // from xDivs[0] to xDivs[1]-1.
    // Note: allocation/free of these pointers is left to the caller.
    int32_t* xDivs;
    int32_t* yDivs;
    int32_t paddingLeft, paddingRight;
    int32_t paddingTop, paddingBottom;

//
//  PNGNinePatch.h
//  podDemo
//
//

#ifndef PNGNinePatch_h
#define PNGNinePatch_h

NS_ASSUME_NONNULL_BEGIN

@interface PNGNinePatch : NSObject

@property (nonatomic, assign) int32_t width;
@property (nonatomic, assign) int32_t height;

@property (nonatomic, assign) int8_t numXDivs;
@property (nonatomic, assign) int8_t numYDivs;
@property (nonatomic, assign) int8_t numColors;

@property (nonatomic, assign) int32_t paddingLeft;
@property (nonatomic, assign) int32_t paddingRight;
@property (nonatomic, assign) int32_t paddingTop;
@property (nonatomic, assign) int32_t paddingBottom;

@property (nonatomic, strong) NSArray<NSNumber *> *xDivsArray;
@property (nonatomic, strong) NSArray<NSNumber *> *yDivsArray;

+ (nullable instancetype)ninePatchWithPNGFileData:(NSData *)data;

/// 获取点九图bitmap中的可拉伸区域，如果返回UIEdgeInsetsZero，则表示没有可以拉伸的区域
/// 点九图可能包含多个不连续的可拉伸区域，本函数只取第一个
- (UIEdgeInsets)resizableCapInsets;

@end

NS_ASSUME_NONNULL_END

#endif /* PNGNinePatch_h */

//  PNGNinePatch.m
//  podDemo_Example
//
//  Created by asterpang on 2023/7/20.
//  Copyright © 2023 asterpang. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "PNGNinePatch.h"

static char bytes[8] = {0};

@implementation PNGNinePatch
// https://dev.exiv2.org/projects/exiv2/wiki/The_Metadata_in_PNG_files
// https://android.googlesource.com/platform/frameworks/base/+/56a2301c7a1169a0692cadaeb48b9a6385d700f5/include/androidfw/ResourceTypes.h

+ (instancetype)ninePatchWithPNGFileData:(NSData *)data
{
    if(data.length < 32) {
        return nil;
    }
    int index = 0;

    // 先判断是否png图片
    if ([[self class] readInt32:data fromIndex:&index] != 0x89504e47 || [[self class] readInt32:data fromIndex:&index] != 0x0D0A1A0A) {
        // 不是png图片，不再处理
        return nil;
    }

    // NinePatch的chunk type标记
    // The PNG chunk type is "npTc"
    const char npTc[4] = {'n', 'p', 'T', 'c'};

    BOOL hasNinePatchChunk = NO;

    int32_t chunk_length = 0;

    while (YES) {
        if(index >= data.length - 8) {
            break;
        }
        // 获取chunk长度
        chunk_length = [[self class] readInt32:data fromIndex:&index];

        // 获取chunk type标记
        [data getBytes:bytes range:NSMakeRange(index, 4)];
        index += 4;

        if (memcmp(bytes, npTc, 4) == 0) {
            // 表示读取到了NinePatch信息,index之后的数据是chunk data
            hasNinePatchChunk = YES;
            break;
        }

        // 跳过本chunk(数据长度 chunk_length + CRC 4bytes)
        index += chunk_length + 4;
    }

    PNGNinePatch *ninePatch = nil;

    if(hasNinePatchChunk && chunk_length > 0 && data.length > index + chunk_length) {

        ninePatch = PNGNinePatch.new;

        int8_t wasDeserialized  = [[self class] readInt8:data fromIndex:&index];
        if(wasDeserialized == 0) {
            // nothing to do
        }

        ninePatch.numXDivs = [[self class] readInt8:data fromIndex:&index];
        ninePatch.numYDivs = [[self class] readInt8:data fromIndex:&index];
        ninePatch.numColors = [[self class] readInt8:data fromIndex:&index];

        // skip xDivsOffset/yDivsOffset
        index += 4 + 4;

        ninePatch.paddingLeft = [[self class] readInt32:data fromIndex:&index];
        ninePatch.paddingRight = [[self class] readInt32:data fromIndex:&index];
        ninePatch.paddingTop = [[self class] readInt32:data fromIndex:&index];
        ninePatch.paddingBottom = [[self class] readInt32:data fromIndex:&index];

        // skip colorOffset
        index += 4;

        // now xDivs，即点九图上方黑点标记数组，横向可拉伸区域
        NSMutableArray<NSNumber *> *xDivsArray = NSMutableArray.new;
        for(int count = 0; count < ninePatch.numXDivs; count++) {
            [data getBytes:bytes range:NSMakeRange(index, 4)];
            index += 4;
            int32_t x = ntohl( *(int32_t *)bytes);
            [xDivsArray addObject:@(x)];
        }

        // now yDivs，即点九图左边黑点标记数组，纵向可拉伸区域
        NSMutableArray<NSNumber *> *yDivsArray = NSMutableArray.new;
        for(int count = 0; count < ninePatch.numYDivs; count++) {
            [data getBytes:bytes range:NSMakeRange(index, 4)];
            index += 4;
            int32_t y = ntohl(*(int32_t *)bytes);
            [yDivsArray addObject:@(y)];
        }
        ninePatch.xDivsArray = xDivsArray;
        ninePatch.yDivsArray = yDivsArray;
    }

    return ninePatch;
}

- (UIEdgeInsets)resizableCapInsetsWithImageSize:(CGSize)imageSize
{
    if(self.xDivsArray.count < 2 || self.yDivsArray.count < 2) {
        return UIEdgeInsetsZero;
    }
    // 可以是多段分割，指定拉伸/压缩，不过我们约定需求没那么复杂，只需要拉伸第一段区域
    // 如需多段处理，则更该代码
    int32_t xStart = self.xDivsArray[0].intValue;
    int32_t xEnd = self.xDivsArray[1].intValue;
    int32_t yStart = self.yDivsArray[0].intValue;
    int32_t yEnd = self.yDivsArray[1].intValue;

    if(xEnd < xStart || yEnd < yStart) {
        return UIEdgeInsetsZero;
    }

    UIEdgeInsets insets;
    insets.top = yStart;
    insets.left = xStart;
    insets.bottom = imageSize.height - yEnd;
    insets.right = imageSize.width - xEnd;

    if(insets.bottom < 0 || insets.right < 0) {
        return UIEdgeInsetsZero;
    }

    return insets;
}

+ (int8_t)readInt8:(NSData *)data fromIndex:(int *)index
{
    [data getBytes:bytes range:NSMakeRange(*index, 1)];
    *index += 1;
    return (int8_t)bytes[0];
}

+ (int32_t)readInt32:(NSData *)data fromIndex:(int *)index
{
    [data getBytes:bytes range:NSMakeRange(*index, 4)];
    *index += 4;
    return ntohl(*(int32_t *)bytes);
}

@end

PNGNinePatch *ninePatch = [PNGNinePatch ninePatchWithPNGFileData:imageFileData];
UIEdgeInsets insets = [ninePatch resizableCapInsets];
image = [image resizableImageWithCapInsets:insets resizingMode:UIImageResizingModeStretch];