使用IndexedDB存儲圖像和文件

有一天，我們寫了關于如何在localStorage中保存圖像和文件的文章，它是關于我們今天可用的實用主義。 然而，localStorage有一些性能影響 - 我們將在稍后的博客中討論這個問題 - 并且未來期望的方法是使用IndexedDB。 在這里，我將向您介紹如何在IndexedDB中存儲圖像和文件，然后通過ObjectURL呈現它們。

本文是翻譯過來的，原文在這里Storing images and files in IndexedDB

關于作者： Robert Nyman [Editor emeritus]

Technical Evangelist & Editor of Mozilla Hacks. Gives talks & blogs about HTML5, JavaScript & the Open Web. Robert is a strong believer in HTML5 and the Open Web and has been working since 1999 with Front End development for the web - in Sweden and in New York City. He regularly also blogs at http://robertnyman.com and loves to travel and meet people.

使用IndexedDB存儲圖像和文件的常規步驟

首先，我們來談談我們將創建一個IndexedDB數據庫，將文件保存到其中然后將其讀出并顯示在頁面中的步驟：

• 1、創建或打開數據庫
• 2、創建一個objectStore
• 3、將圖像文件檢索為blob
• 4、初始化一個數據庫事物
• 5、保存圖像blob到數據庫中去
• 6、讀出保存的文件并從中創建ObjectURL并將其設置為頁面中圖像元素的src

1、創建或打開數據庫。

// IndexedDB window.indexedDB = window.indexedDB || window.webkitIndexedDB || window.mozIndexedDB || window.OIndexedDB || window.msIndexedDB,IDBTransaction = window.IDBTransaction || window.webkitIDBTransaction || window.OIDBTransaction || window.msIDBTransaction,dbVersion = 1;/* Note: The recommended way to do this is assigning it to window.indexedDB,to avoid potential issues in the global scope when web browsers start removing prefixes in their implementations.You can assign it to a varible, like var indexedDB… but then you have to make sure that the code is contained within a function. */// Create/open database var request = indexedDB.open("elephantFiles", dbVersion);request.onsuccess = function (event) {console.log("Success creating/accessing IndexedDB database");db = request.result;db.onerror = function (event) {console.log("Error creating/accessing IndexedDB database");};// Interim solution for Google Chrome to create an objectStore. Will be deprecatedif (db.setVersion) {if (db.version != dbVersion) {var setVersion = db.setVersion(dbVersion);setVersion.onsuccess = function () {createObjectStore(db);getImageFile();};}else {getImageFile();}}else {getImageFile();} }// For future use. Currently only in latest Firefox versions request.onupgradeneeded = function (event) {createObjectStore(event.target.result); };

使用它的預期方法是在創建數據庫時觸發onupgradeneeded事件或獲取更高版本號。 目前僅在Firefox中支持此功能，但很快將在其他Web瀏覽器中支持。 如果Web瀏覽器不支持此事件，則可以使用已棄用的setVersion方法并連接到其onsuccess事件。

2、創建一個objectStore（如果它尚不存在）

// Create an objectStore console.log("Creating objectStore") dataBase.createObjectStore("elephants");

在這里，您創建一個ObjectStore，您將存儲數據 - 或者在我們的例子中，文件 - 并且一旦創建，您不需要重新創建它，只需更新其內容即可。

3、將圖像文件檢索為blob

// Create XHR var xhr = new XMLHttpRequest(),blob;xhr.open("GET", "elephant.png", true); // Set the responseType to blob xhr.responseType = "blob";xhr.addEventListener("load", function () {if (xhr.status === 200) {console.log("Image retrieved");// File as responseblob = xhr.response;// Put the received blob into IndexedDBputElephantInDb(blob);} }, false); // Send XHR xhr.send();

此代碼直接將文件的內容作為blob獲取。目前只支持Firefox。 收到整個文件后，將blob發送到函數以將其存儲在數據庫中。

4、初始化一個數據庫事物

// Open a transaction to the database var transaction = db.transaction(["elephants"], IDBTransaction.READ_WRITE);

要開始向數據庫寫入內容，您需要使用objectStore名稱和要執行的操作類型（在本例中為read和write）啟動事務。

5、保存圖像blob到數據庫中去

// Put the blob into the dabase transaction.objectStore("elephants").put(blob, "image");

一旦事務到位，您將獲得對所需objectStore的引用，然后將您的blob放入其中并為其提供密鑰。

6、讀出保存的文件并從中創建ObjectURL并將其設置為頁面中圖像元素的src

// Retrieve the file that was just stored transaction.objectStore("elephants").get("image").onsuccess = function (event) {var imgFile = event.target.result;console.log("Got elephant!" imgFile);// Get window.URL objectvar URL = window.URL || window.webkitURL;// Create and revoke ObjectURLvar imgURL = URL.createObjectURL(imgFile);// Set img src to ObjectURLvar imgElephant = document.getElementById("elephant");imgElephant.setAttribute("src", imgURL);// Revoking ObjectURLURL.revokeObjectURL(imgURL); };

使用相同的事務來獲取剛剛存儲的圖像文件，然后創建一個objectURL并將其設置為頁面中圖像的src。 例如，這也可以是一個附加到腳本元素的JavaScript文件，然后它將解析JavaScript。

最后完整代碼

(function () {// IndexedDBvar indexedDB = window.indexedDB || window.webkitIndexedDB || window.mozIndexedDB || window.OIndexedDB || window.msIndexedDB,IDBTransaction = window.IDBTransaction || window.webkitIDBTransaction || window.OIDBTransaction || window.msIDBTransaction,dbVersion = 1.0;// Create/open databasevar request = indexedDB.open("elephantFiles", dbVersion),db,createObjectStore = function (dataBase) {// Create an objectStoreconsole.log("Creating objectStore")dataBase.createObjectStore("elephants");},getImageFile = function () {// Create XHRvar xhr = new XMLHttpRequest(),blob;xhr.open("GET", "elephant.png", true);// Set the responseType to blobxhr.responseType = "blob";xhr.addEventListener("load", function () {if (xhr.status === 200) {console.log("Image retrieved");// Blob as responseblob = xhr.response;console.log("Blob:" blob);// Put the received blob into IndexedDBputElephantInDb(blob);}}, false);// Send XHRxhr.send();},putElephantInDb = function (blob) {console.log("Putting elephants in IndexedDB");// Open a transaction to the databasevar transaction = db.transaction(["elephants"], IDBTransaction.READ_WRITE);// Put the blob into the dabasevar put = transaction.objectStore("elephants").put(blob, "image");// Retrieve the file that was just storedtransaction.objectStore("elephants").get("image").onsuccess = function (event) {var imgFile = event.target.result;console.log("Got elephant!" imgFile);// Get window.URL objectvar URL = window.URL || window.webkitURL;// Create and revoke ObjectURLvar imgURL = URL.createObjectURL(imgFile);// Set img src to ObjectURLvar imgElephant = document.getElementById("elephant");imgElephant.setAttribute("src", imgURL);// Revoking ObjectURLURL.revokeObjectURL(imgURL);};};request.onerror = function (event) {console.log("Error creating/accessing IndexedDB database");};request.onsuccess = function (event) {console.log("Success creating/accessing IndexedDB database");db = request.result;db.onerror = function (event) {console.log("Error creating/accessing IndexedDB database");};// Interim solution for Google Chrome to create an objectStore. Will be deprecatedif (db.setVersion) {if (db.version != dbVersion) {var setVersion = db.setVersion(dbVersion);setVersion.onsuccess = function () {createObjectStore(db);getImageFile();};}else {getImageFile();}}else {getImageFile();}}// For future use. Currently only in latest Firefox versionsrequest.onupgradeneeded = function (event) {createObjectStore(event.target.result);}; })();

瀏覽器支持

• URL API支持性

• indexDb

Github源碼

總結

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