page({
    oncanvasready() {
        const self = this;
        my.createselectorquery().select('#canvas').node().exec((res) => {
            const canvas = res[0].node;
            const ctx = canvas.getcontext('2d');
            self.canvas = canvas;
            self.ctx = ctx;
            self.draw();
        });
    },
    draw() {
        const ctx = this.ctx;
        // 创建离屏 canvas
        const offcanvas = my.createoffscreencanvas();
        const offctx = offcanvas.getcontext('2d');
        // 用 离屏canvas 创建 image
        const image = offcanvas.createimage();
        image.onload = function() {
            offcanvas.width = image.width;
            offcanvas.height = image.height;
            // 将 image 画到 离屏canvas 上
            offctx.drawimage(image, 0, 0, offcanvas.width, offcanvas.height);
            // 将 离屏canvas 中的部分内容画到 canvas 上
            ctx.drawimage(offcanvas, 50, 50, 100, 100, 0, 0, 100, 100);
        }
        image.src = 'https://img.alicdn.com/tfs/tb1gvvmj2bntkjjy0fdxxcppvxa-520-280.jpg';
    },
});