基于Android-29 

1.通過decodeFile獲取bitmap

 decodeFile(String pathName) 

進(jìn)行方法重載的調(diào)用 

decodeFile(String pathName, Options opts) 

然后,會(huì)根據(jù)文件路徑pathName,創(chuàng)建一個(gè)fileInputStream 最終去調(diào)用decodeStream

 stream = new FileInputStream(pathName);
 bm = decodeStream(stream, null, opts);

重點(diǎn)出現(xiàn)了,decodeStream,我們先暫停一下,看一下其他獲取bitmap的方法

2.通過decodeFileDescriptor取bitmap

decodeFileDescriptor(FileDescriptor fd)

接下來會(huì)同樣進(jìn)行方法重載的調(diào)用

 decodeFileDescriptor(FileDescriptor fd, Rect outPadding, Options opts)

在這里面我們先摘取關(guān)鍵代碼看一下

if (nativeIsSeekable(fd)) {
    bm = nativeDecodeFileDescriptor(fd, outPadding, opts,
                                    Options.nativeInBitmap(opts),
                                    Options.nativeColorSpace(opts)); 
                                                    //同樣有重點(diǎn),步驟1,native方法
} else {
    FileInputStream fis = new FileInputStream(fd);
    try {
        bm = decodeStreamInternal(fis, outPadding, opts); //同樣有重點(diǎn),步驟2
    } finally {
        try {
            fis.close();
        } catch (Throwable t) {/* ignore */}
    }
}
setDensityFromOptions(bm, opts);//同樣有重點(diǎn),步驟3

這里是通 過``nativeDecodeFileDescriptor或者decodeStreamInternal步驟獲取到bitmap,然后再通過步驟3給bitmap設(shè)置desity`,

關(guān)鍵方法 nativeDecodeFileDescriptor 與 ecodeStreamInternal

3.通過decodeByteArray獲取bitmap

decodeByteArray(byte[] data, int offset, int length)

同樣的會(huì)進(jìn)行方法重載調(diào)用

decodeByteArray(byte[] data, int offset, int length, Options opts)

在這里面我們摘取主要代碼看一下

bm = nativeDecodeByteArray(data, offset, length, opts,
                    Options.nativeInBitmap(opts),
                    Options.nativeColorSpace(opts));//步驟1,調(diào)用native方法獲取bitmap

if (bm == null && opts != null && opts.inBitmap != null) {
    throw new IllegalArgumentException("Problem decoding into existing bitmap");
}
setDensityFromOptions(bm, opts);//步驟2,給目標(biāo)bitmap設(shè)置desity

這個(gè)方法里面就比較直接,調(diào)用nativeDecodeByteArray方法獲取到bitmap后,根據(jù)options給bitmap設(shè)置Desity屬性

4.通過decodeResource獲取bitmap

decodeResource(Resources res, int id)

同樣進(jìn)行方法重載

decodeResource(Resources res, int id, Options opts) 

這里面有一處主要的代碼

final TypedValue value = new TypedValue();
InputStream is = null; 
is = res.openRawResource(id, value);//獲取到inputStream,注意

bm = decodeResourceStream(res, value, is, null, opts);
//調(diào)用這個(gè),此時(shí)value.density已經(jīng)有值,會(huì)將資源文件對應(yīng)目錄的dpi設(shè)置給value.density后面會(huì)使用

那decodeResourceStream(res, value, is, null, opts);是什么呢,同樣是查看源碼

public static Bitmap decodeResourceStream(@Nullable Resources res, @Nullable TypedValue value,
            @Nullable InputStream is, @Nullable Rect pad, @Nullable Options opts) {
       
        if (opts == null) {
            opts = new Options();
        }

        if (opts.inDensity == 0 && value != null) {
            final int density = value.density;
            if (density == TypedValue.DENSITY_DEFAULT) {
                opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
            } else if (density != TypedValue.DENSITY_NONE) {
                opts.inDensity = density;
            }
        } //這一部分主要是設(shè)置desity來讓bitmap所使用
        
        if (opts.inTargetDensity == 0 && res != null) {
            opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
        }//這一部分主要是設(shè)置目標(biāo)desity來讓bitmap所使用
        
        return decodeStream(is, pad, opts);//又到了decodeStream,似曾相識(shí)的感覺
    }

5.通過decodeStream獲取bitmap

decodeStream(InputStream is)

方法重載調(diào)用

decodeStream(@Nullable InputStream is, @Nullable Rect outPadding,
            @Nullable Options opts) 

那我們就仔細(xì)的看一下這個(gè)就去的具體實(shí)現(xiàn)

if (is instanceof AssetManager.AssetInputStream) {
    final long asset = ((AssetManager.AssetInputStream) is).getNativeAsset();
    bm = nativeDecodeAsset(asset, outPadding, opts,                 Options.nativeInBitmap(opts),                           Options.nativeColorSpace(opts));//方式一,通過asset資源獲取的輸入流
} else {
    bm = decodeStreamInternal(is, outPadding, opts); //方式二,其他情況得到的輸入流
}

if (bm == null && opts != null && opts.inBitmap != null) {
    throw new IllegalArgumentException("Problem decoding into existing bitmap");
}
setDensityFromOptions(bm, opts);//設(shè)置desity

方式一已經(jīng)調(diào)用了native方法,看一下方式二的具體實(shí)現(xiàn)

decodeStreamInternal(@NonNull InputStream is,
            @Nullable Rect outPadding, @Nullable Options opts) 

byte [] tempStorage = null;
if (opts != null) tempStorage = opts.inTempStorage;
if (tempStorage == null) tempStorage = new byte[DECODE_BUFFER_SIZE];
return nativeDecodeStream(is, tempStorage, outPadding, opts,
                          Options.nativeInBitmap(opts),
                          Options.nativeColorSpace(opts));

方式二 也是走到了native方法里面.

我們在進(jìn)入native方法之前先來進(jìn)行一個(gè)簡單地總結(jié):

1. 通過decodeFile獲取bitmap會(huì)最終走到decodeStream
2. 通過decodeFileDescriptor獲取bitmap,最終會(huì)走到nativeDecodeFileDescriptor或者decodeStreamInternal
3. 通過decodeByteArray獲取bitmap,最終會(huì)走到nativeDecodeByteArray
4. 通過decodeResource獲取bitmap,最終會(huì)走到decodeStream
5. 通過decodeStream獲取bitmap,最終會(huì)走到nativeDecodeAsset或者decodeStreamInternal
6. decodeStreamInternal方式最終會(huì)走到nativeDecodeStream

綜上所述,最終獲取bitmap的方法都會(huì)調(diào)用以下四種方式之一來進(jìn)行:

• nativeDecodeFileDescriptor
• nativeDecodeByteArray
• nativeDecodeAsset
• nativeDecodeStream

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